Hubbry Logo
Fish scaleFish scaleMain
Open search
Fish scale
Community hub
Fish scale
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Fish scale
Fish scale
Fish scale
View on Wikipedia
from Wikipedia

Cycloid scales cover these teleost fish (rohu)

A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term scale derives from the Old French escale, meaning a shell pod or husk.[1]

Scales vary enormously in size, shape, structure, and extent, ranging from strong and rigid armour plates in fishes such as shrimpfishes and boxfishes, to microscopic or absent in fishes such as eels and anglerfishes. The morphology of a scale can be used to identify the species of fish it came from. Scales originated within the jawless ostracoderms, ancestors to all jawed fishes today. Most bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars. Cartilaginous fishes (sharks and rays) are covered with placoid scales. Some species are covered instead by scutes, and others have no outer covering on part or all of the skin.

Fish scales are part of the fish's integumentary system, and are produced from the mesoderm layer of the dermis, which distinguishes them from reptile scales.[2][3] The same genes involved in tooth and hair development in mammals are also involved in scale development. The placoid scales of cartilaginous fishes are also called dermal denticles and are structurally homologous with vertebrate teeth. Most fish are also covered in a layer of mucus or slime which can protect against pathogens such as bacteria, fungi, and viruses, and reduce surface resistance when the fish swims.

Thelodont scales

[edit]
Left to right: denticles of Paralogania, Shielia taiti, Lanarkia horrida

The bony scales of thelodonts, the most abundant form of fossil fish, are well understood. The scales were formed and shed throughout the organisms' lifetimes, and quickly separated after their death.[4]

Bone, a tissue that is both resistant to mechanical damage and relatively prone to fossilization, often preserves internal detail, which allows the histology and growth of the scales to be studied in detail. The scales comprise a non-growing "crown" composed of dentine, with a sometimes-ornamented enameloid upper surface and an aspidine base.[5] Its growing base is made of cell-free bone, which sometimes developed anchorage structures to fix it in the side of the fish.[6] Beyond that, there appear to be five types of bone growth, which may represent five natural groupings within the thelodonts—or a spectrum ranging between the end members meta- (or ortho-) dentine and mesodentine tissues.[7] Each of the five scale morphs appears to resemble the scales of more derived groupings of fish, suggesting that thelodont groups may have been stem groups to succeeding clades of fish.[6]

However, using scale morphology alone to distinguish species has some pitfalls. Within each organism, scale shape varies hugely according to body area,[8] with intermediate forms appearing between different areas—and to make matters worse, scale morphology may not even be constant within one area. To confuse things further, scale morphologies are not unique to taxa, and may be indistinguishable on the same area of two different species.[9]

The morphology and histology of thelodonts provides the main tool for quantifying their diversity and distinguishing between species, although ultimately using such convergent traits is prone to errors. Nonetheless, a framework comprising three groups has been proposed based upon scale morphology and histology.[7] Comparisons to modern shark species have shown that thelodont scales were functionally similar to those of modern cartilaginous fish, and likewise has allowed an extensive comparison between ecological niches.[10]

Cosmoid scales

[edit]
Queensland lungfish

Cosmoid scales are found only on ancient lobe-finned fishes, including some of the earliest lungfishes (subclass Dipnoi), and in Crossopterygii, including the living coelacanth in a modified form (see elasmoid scales, below). They were probably derived from a fusion of placoid-ganoid scales. The inner part of the scales is made of dense lamellar bone called isopedine. On top of this lies a layer of spongy or vascular bone supplied with blood vessels, followed by a complex dentine-like layer called cosmine with a superficial outer coating of vitrodentine. The upper surface is keratin. Cosmoid scales increase in size through the growth of the lamellar bone layer.[11]

Elasmoid scales

[edit]
Lobe-finned fishes, like this preserved coelacanth, have elasmoid scales.

Elasmoid scales are thin, imbricated scales composed of a layer of dense, lamellar collagen bone called isopedine, above which is a layer of tubercles usually composed of bone, as in Eusthenopteron. The layer of dentine that was present in the first lobe-finned fish is usually reduced, as in the extant coelacanth, or entirely absent, as in extant lungfish and in the Devonian Eusthenopteron.[12] Elasmoid scales have appeared several times over the course of fish evolution. They are present in some lobe-finned fishes, such as all extant and some extinct lungfishes, as well as the coelacanths which have modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales. They are also present in some tetrapodomorphs like Eusthenopteron, amiids, and teleosts, whose cycloid and ctenoid scales represent the least mineralized elasmoid scales.

The zebrafish elasmoid scales are used in the lab to study bone mineralization process, and can be cultured (kept) outside of the organism.[13][14]

Ganoid scales

[edit]
The scales of this spotted gar appear glassy due to ganoine.
Mineral texture of ganoine layers in the scales of an alligator gar

Ganoid scales are found in the sturgeons, paddlefishes, gars, bowfin, and bichirs. They are derived from cosmoid scales and often have serrated edges. They are covered with a layer of hard enamel-like dentine in the place of cosmine, and a layer of inorganic bone salt called ganoine in place of vitrodentine.

Ganoine is a characteristic component of ganoid scales. It is a glassy, often multi-layered mineralized tissue that covers the scales, as well as the cranial bones and fin rays in some non-teleost ray-finned fishes,[15] such as gars, bichirs, and coelacanths.[16][17] It is composed of rod-like apatite crystallites.[18] Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis.[17] While often considered a synapomorphic character of ray-finned fishes, ganoine or ganoine-like tissues are also found on the extinct acanthodii.[17] It has been suggested ganoine is homologous to tooth enamel in vertebrates[15] or even considered a type of enamel.[18]



Amblypterus striatus
 Ganoid scales of the extinct Carboniferous fish, Amblypterus striatus. (a) shows the outer surface of four of the scales, and (b) shows the inner surface of two of the scales. Each of the rhomboidal-shaped ganoid scales of Amblypterus has a ridge on the inner surface which is produced at one end into a projecting peg which fits into a notch in the next scale, similar to the manner in which tiles are pegged together on the roof of a house.

Most ganoid scales are rhomboidal (diamond-shaped) and connected by peg-and-socket joints. They are usually thick and fit together more like a jigsaw rather than overlapping like other scales.[19] In this way, ganoid scales are nearly impenetrable and are excellent protection against predation.

Geometrically laid out ganoid scales on a bichir

In sturgeons, the scales are greatly enlarged into armour plates along the sides and back, while in the bowfin the scales are greatly reduced in thickness to resemble cycloid scales.

  • Earrings made from the ganoid scales of an alligator gar
    Earrings made from the ganoid scales of an alligator gar
  • Fossil of a primitive rayfin with ganoid scales
    Fossil of a primitive rayfin with ganoid scales
  • Ganoid scales on a fossilised Lepidotes, circa. 130 mya
    Ganoid scales on a fossilised Lepidotes, circa. 130 mya

Native Americans and people of the Caribbean used the tough ganoid scales of the alligator gar for arrow heads, breastplates, and as shielding to cover plows. In current times jewellery is made from these scales.[20]

Leptoid scales

[edit]

Leptoid (bony-ridge) scales are found on higher-order bony fish, the teleosts (the more derived clade of ray-finned fishes). The outer part of these scales fan out with bony ridges while the inner part is criss-crossed with fibrous connective tissue. Leptoid scales are thinner and more translucent than other types of scales, and lack the hardened enamel-like or dentine layers. Unlike ganoid scales, further scales are added in concentric layers as the fish grows.[21]

Leptoid scales overlap in a head-to-tail configuration, like roof tiles, making them more flexible than cosmoid and ganoid scales. This arrangement allows a smoother flow of water over the body, and reduces drag.[22] The scales of some species exhibit bands of uneven seasonal growth called annuli (singular annulus). These bands can be used to age the fish.

Leptoid scales come in two forms: cycloid (smooth) and ctenoid (comb-like).[23]

Cycloid scales

[edit]

Cycloid (circular) scales have a smooth texture and are uniform, with a smooth outer edge or margin. They are most common on fish with soft fin rays, such as salmon and carp.

Asian arowana have large cycloid scales arranged on the fish in a mosaic of raised ribs (left). The scales themselves are covered with a delicate net pattern (right).[24][25]
Cycloid (circular) scales
The cycloid scale of a carp has a smooth outer edge (at top of image).
This Poropuntius huguenini is a carp-like fish with circular cycloid scales that are smooth to the touch.
Cycloid (circular) scales are usually found on carp-like or salmon-like fishes.

Ctenoid scales

[edit]

Ctenoid (toothed) scales are like cycloid scales, except they have small teeth or spinules called ctenii along their outer or posterior edges. Because of these teeth, the scales have a rough texture. They are usually found on fishes with spiny fin rays, such as the perch-like fishes. These scales contain almost no bone, being composed of a surface layer containing hydroxyapatite and calcium carbonate and a deeper layer composed mostly of collagen. The enamel of the other scale types is reduced to superficial ridges and ctenii.

Ctenoid (toothed) scales
The ctenoid scale of a perch has a toothed outer edge (at top of image).
This dottyback is a perch-like fish with toothed ctenoid scales that are rough to the touch.
The size of the teeth on ctenoid scales can vary with position, as these scales from the rattail Cetonurus crassiceps show.
Ctenoid scales from a perch vary from the medial (middle of the fish), to dorsal (top), to caudal (tail end) scales.
Crazy fish have cycloid scales on the belly but ctenoid scales elsewhere.[26]
Ctenoid (toothed) scales are usually found on perch-like fishes.
 

Ctenoid scales, similar to other epidermal structures, originate from placodes and distinctive cellular differentiation makes them exclusive from other structures that arise from the integument.[27] Development starts near the caudal fin, along the lateral line of the fish.[28] The development process begins with an accumulation of fibroblasts between the epidermis and dermis.[27] Collagen fibrils begin to organize themselves in the dermal layer, which leads to the initiation of mineralization.[27] The circumference of the scales grows first, followed by thickness when overlapping layers mineralize together.[27]

Ctenoid scales can be further subdivided into three types:

  • Crenate scales, where the margin of the scale bears indentations and projections.
  • Spinoid scales, where the scale bears spines that are continuous with the scale itself.
  • True ctenoid scales, where the spines on the scale are distinct structures.

Most ray-finned fishes have ctenoid scales. Some species of flatfishes have ctenoid scales on the eyed side and cycloid scales on the blind side, while other species have ctenoid scales in males and cycloid scales in females.

Reflection

[edit]
The herring's reflectors are nearly vertical for camouflage from the side.
The deep sea hatchetfish has scales which reflect blue light.
The scales of a typical teleost fish, like this Atlantic herring, are silvered.

Many teleost fish are covered with highly reflective scales which function as small mirrors and give the appearance of silvered glass. Reflection through silvering is widespread or dominant in fish of the open sea, especially those that live in the top 100 metres. A transparency effect can be achieved by silvering to make an animal's body highly reflective. At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side.[29]

The marine hatchetfish is extremely flattened laterally (side to side), leaving the body just millimetres thick, and the body is so silvery as to resemble aluminium foil. The mirrors consist of microscopic structures similar to those used to provide structural coloration: stacks of between 5 and 10 crystals of guanine spaced about ¼ of a wavelength apart to interfere constructively and achieve nearly 100 per cent reflection. In the deep waters that the hatchetfish lives in, only blue light with a wavelength of 500 nanometres percolates down and needs to be reflected, so mirrors 125 nanometres apart provide good camouflage.[29]

Most fish in the upper ocean are camouflaged by silvering. In fish such as the herring, which lives in shallower water, the mirrors must reflect a mixture of wavelengths, and the fish accordingly has crystal stacks with a range of different spacings. A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally. The overall mirror effect is achieved with many small reflectors, all oriented vertically.[29]

Fish scales with these properties are used in some cosmetics, since they can give a shimmering effect to makeup and lipstick.[30]

Placoid scales

[edit]
Not to be confused with Sharkskin.
Placoid scales as viewed through an electron microscope. Also called dermal denticles, these are structurally homologous with vertebrate teeth.

Placoid (pointed, tooth-shaped) scales are found in the cartilaginous fishes: sharks, rays. They are also called dermal denticles. Placoid scales are structurally homologous with vertebrate teeth ("denticle" translates to "small tooth"), having a central pulp cavity supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the dermis. The outermost layer is composed of vitrodentine, a largely inorganic enamel-like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.

Similar scales can also be found under the head of the denticle herring. The amount of scale coverage is much less in rays.

Rhomboidal scales with the properties of both placoid and ganoid scales are suspected to exist in modern jawed fish ancestors: jawless ostracoderms and then jawed placoderms.

Shark skin

[edit]
Cartilaginous fishes, like this tiger shark, have placoid scales (dermal denticles).

Shark skin is almost entirely covered by small placoid scales. The scales are supported by spines, which feel rough when stroked in a backward direction, but when flattened by the forward movement of water, create tiny vortices that reduce hydrodynamic drag and reduce turbulence, making swimming both more efficient and quieter compared to that of bony fishes.[31] It also serves a role in anti-fouling by exhibiting the lotus effect.[32]

All denticles are composed of an interior pulp cavity with a nervous and arterial supply rooted in the dermis to supply the denticle with mucus.[33] Denticles contain riblet structures that protrude from the surface of the scale; under a microscope this riblet can look like a hook or ridges coming out of the scale. The overall shape of the protrusion from the denticle is dependent on the type of shark and can be generally described with two appearances.[34] The first is a scale in which ridges are placed laterally down the shark and parallel with the flow of the water. The second form is a smooth scale with what looks like a hooked riblet curling out of the surface aiming towards the posterior side of the shark.[34] Both riblet shapes assist in creating a turbulent boundary layer forcing the laminar flow farther away from the sharks skin.[35]

Unlike bony fish, sharks have a complicated dermal corset made of flexible collagenous fibers arranged as a helical network surrounding their body. The corset works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy.[36] Depending on the position of these placoid scales on the body, they can be flexible and can be passively erected, allowing them to change their angle of attack. These scales also have riblets which are aligned in the direction of flow, these riblets reduce the drag force acting on the shark skin by pushing the vortex further away from the skin surface, inhibiting any high-velocity cross-stream flow.[37]

Scale morphology

[edit]

The general anatomy of the scales varies, but all of them can be divided into three parts: the crown, the neck and the base. The scale pliability is related to the size of the base of the scale. The scales with higher flexibility have a smaller base, and thus are less rigidly attached to the stratum laxum. On the crown of the fast-swimming sharks there are a series of parallel riblets or ridges which run from an anterior to posterior direction.[38]

Analyzing the three components of the scale it can be concluded that the base of the denticle does not come into contact with any portion of the fluid flow.[39] The crown and the neck of the denticles however play a key role and are responsible for creating the turbulent vortices and eddies found near the skin's surface.[39] Because denticles come in so many different shapes and sizes, it can be expected that not all shapes will produce the same type of turbulent flow. During a recent research experiment biomimetic samples of shark denticles with a crescent like microstructure were tested in a water tank using a traction table as a slide. The experiment showed that the surface with denticles experienced a 10% drag reduction overall versus the smooth sample. The reason for this drag reduction was that the turbulent vortices became trapped between the denticles, creating a 'cushion like' barrier against the laminar flow.[40] This same type of experiment was performed by another research group which implemented more variation in their biomimetic sample. The second group arrived at the same conclusion as the first. However, because their experiment contained more variation within the samples they were able to achieve a high degree of experimental accuracy. In conclusion, they stated that more practical shapes were more durable than ones with intricate ridge-lines. The practical shapes were low profile and contained trapezoidal or semi-circular trough-like cross sections, and were less effective but nonetheless reduced drag by 6 or 7%.[41]

Drag reduction

[edit]
Effects of turbulent flow on boundary layer
Diagram of the side profile of a shark denticle showing a vortex in the wake downstream of the denticle

Sharks decrease drag and overall cost of transport (COT) through multiple different avenues. Pressure drag is created from the pressure difference between the anterior and posterior sides of the shark due to the amount of volume that is pushed past the shark to propel itself forward.[42] This type of drag is also directly proportional to the laminar flow. When the laminar flow increases around the fish the pressure drag does as well.[43] Frictional drag is a result of the interaction between the fluid against the shark's skin and can vary depending on how the boundary layer changes against the surface of the fish.[42]

The riblets impede the cross-stream translation of the streamwise vortices in the viscous sublayer. The mechanism is complex and not yet understood fully. Basically, the riblets inhibit the vortex formation near the surface because the vortex cannot fit in the valleys formed by the riblets. This pushes the vortex further up from the surface, interacting only with the riblet tips, not causing any high-velocity flow in the valleys. Since this high-velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too.[38]

Recent research has shown that there is a pre and post-breakdown regime in the near-wall boundary layer where the sublayer thickens at a declining rate and then abruptly undergoes a breakdown into turbulent vortices before finally collapsing. This system is completely self-regulating and mediates the growth and decay cycle; the vortices accumulate during the growth period and are abruptly liquidated into Strouhal arrays of hairpin vortices lifting off the wall. Lifting vortices are what push the boundary layer out and away from the surface of the shark which results in reducing the overall drag experienced by the fish.[44]

Technical application

[edit]

The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was in making hand-grips for swords. The rough texture of the skin is also used in Japanese cuisine to make graters called oroshiki, by attaching pieces of shark skin to wooden boards. The small size of the scales grates the food very finely.

Barnacle growth on boat hull

In the marine industry, fouling is the process by which something in the water becomes encrusted with sea life such as barnacles and algae. When ships' hulls are fouled, they are much less efficient (because they are rougher), and they are expensive and time-consuming to clean. Therefore, inexpensive and environmentally safe anti-fouling surfaces are in very high demand to increase the efficiency of shipping, fishing, and naval fleets, among other applications. Dermal denticles are a promising area of research for this type of application due to the fact that sharks are among the only fish without build up or growth on their scales. Studies by the U.S. Navy have shown that if a biomimetic material can be engineered, it could potentially lead to fuel cost savings for military vessels of up to 45%.[45]

There are many examples of biomimetic materials and surfaces based on the structure of aquatic organisms, including sharks. Such applications intend to enable more efficient movement through fluid mediums such as air, water, and oil.

Surfaces that mimic the skin of sharks have also been used in order to keep microorganisms and algae from coating the hulls of submarines and ships. One variety is traded as "sharklet".[46][47]

A lot of the new methods for replicating shark skin involve the use of polydimethylsiloxane (PDMS) for creating a mold. Usually the process involves taking a flat piece of shark skin, covering it with the PDMS to form a mold and pouring PDMS into that mold again to get a shark skin replica. This method has been used to create a biomimetic surface which has superhydrophobic properties, exhibiting the lotus effect.[46] One study found that these biomimetic surfaces reduced drag by up to 9%,[37] while with flapping motion drag reduction reached 12.3%.[48]

Denticles also provide drag reduction on objects where the main form of drag is caused by turbulent flow at the surface. A large portion of the total drag on long objects with relatively flat sides usually comes from turbulence at the wall, so riblets will have an appreciable effect. Along with marine applications, the aerospace industry can benefit greatly from these biomimetic designs. Other applications include pipes, where they score the insides to a riblet-like roughness and have discovered a 5% drag reduction, and a few percent reduction is claimed with competitive swimwear.[49]

Parametric modeling has been done on shark denticles with a wide range of design variations such as low and high-profile vortex generators.[50] Through this method, the most thorough characterization has been completed for symmetrical two-dimensional riblets with sawtooth, scalloped and blade cross sections.[49] These biomimetic models were designed and analyzed to see the effects of applying the denticle-like structures to the wings of various airplanes. During the simulation, it was noted that the sample altered how the low and high angles of attack reacted. Both the geometry of the denticles and their arrangement have a profound effect on the aerodynamic response of the aerofoils. Out of both the low and high-profile samples tested, the low-profile vortex generators outperformed the current smooth wing structures by 323%. This increase in performance is due to a separation bubble in the denticle's wake and stream-wise vortices that replenish momentum lost in the boundary layer due to skin friction.[50]

Scutes

[edit]
Pineconefish are covered in scutes.

Scutes are similar to scales and serve the same function. Unlike the scales of fish, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface. Forming in the living dermis, the scutes produce a horny outer layer, that is superficially similar to that of scales.

Scute comes from Latin for shield, and can take the form of:

  • an external shield-like bony plate, or
  • a modified, thickened scale that often is keeled or spiny, or
  • a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle forming caudal keels, or along the ventral profile.

Some fish, such as pineconefish, are completely or partially covered in scutes. River herrings and threadfins have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some jacks have a row of scutes following the lateral line on either side.

Scale development

[edit]

Scales typically appear late in the development of fish. In the case of zebrafish, it takes 30 days after fertilization before the different layers needed to start forming the scales have differentiated and become organized. For this it is necessary that consolidation of the mesenchyme occurs, then morphogenesis is induced, and finally the process of differentiation or late metamorphosis occurs.[51][52]

  • Mesenchyme consolidation: The consolidation or structuring of the mesenchyme originates during the development of the dermis. This process depends on whether the fish is cartilaginous or bony. For cartilaginous fish the structuring originates through the formation of two layers. The first is superficial and wide and the second is thin and compact. These two layers are separated by mesenchymal cells. Bony fish generate an acellular substrate organized by perpendicularly by collagen fibers. Subsequently, for both fish the fibroblasts elongate. These penetrate the compact layer of the mesenchyme, which consolidates prior to the formation of the scale, in order to initiate the dermal plate.[51][52][53]
  • Morphogenesis induction: The morphogenesis is due to the formation of the epidermal papilla, which is generated by joining the epidermis and dermis through a process of invagination. Morphogenesis begins at the time when fibroblasts are relocated to the upper part of the compact mesenchyme. Throughout this process, the basal cells of the epithelium form a delimiting layer, which is located in the upper part of the mesenchyme. Subsequently, these cells will differentiate in the area where the scale primordium will arise.[51][52][53]
  • Differentiation or late metamorphosis: This differentiation is generated by two different forms according to the type of scale being formed. The formation of elasmoid scales (cycloids and ctenoids) occurs through the formation of a space between the matrix of the epidermal papilla. This space contains collagen fibers. Around this space elasmoblasts differentiate and are responsible for generating the necessary material for the formation of the scale. Subsequently, matrix mineralization occurs, allowing the scale to acquire the rigid characteristic that identifies them.[51][52][53]

Unlike elasmoid scales, ganoid scales are composed of mineralized and non-mineralized collagen in different regions. The formation of these occurs through the entry of the surface cells of the mesenchyme into the matrix, the latter is composed of collagen fibers and is located around the vascular capillaries, thus giving rise to vascular cavities. At this point, elasmoblasts are replaced by osteoblasts, thus forming bone. The patches of the matrix of the scale that are not ossified are composed of compacted collagen that allow it to maintain the union with the mesenchyme. This are known as Sharpey fibers.[51][52][53]

One of the genes that regulate the development of scale formation in fish is the sonic hedgehog (shh) gene, which by means of the (shh) protein, involved in organogenesis and in the process of cellular communication, enable the formation of the scales.[54][55] The apolipoprotein E (ApoE), that allows the transport and metabolism of triglycerides and cholesterol, has an interaction with shh, because ApoE provides cholesterol to the shh signaling pathway. It has been shown that during the process of cell differentiation and interaction, the level of ApoE transcription is high, which has led to the conclusion that this protein is important for the late development of scales.[54][55]

Modified scales

[edit]
Lateral line
Cycloid scales of a common roach. Modified scales along the lateral line are visible in the lower half.
Closeup of a modified cycloid scale from the lateral line of a wrasse
Surgeonfish
Surgeonfish (left) have a sharp, scalpel-like modified scale on either side just before the tail. Closeup (right).

Different groups of fish have evolved a number of modified scales to serve various functions.

  • Almost all fishes have a lateral line, a system of mechanoreceptors that detect water movements. In bony fishes, the scales along the lateral line have central pores that allow water to contact the sensory cells.
  • The dorsal fin spines of dogfish sharks and chimaeras, the stinging tail spines of stingrays, and the "saw" teeth of sawfishes and sawsharks are fused and modified placoid scales.
  • Surgeonfish have a scalpel-like blade, which is a modified scale, on either side of the caudal peduncle.[56]
  • Some herrings, anchovies, and halfbeaks have deciduous scales, which are easily shed and aid in escaping predators.
  • Male Percina darters have a row of enlarged caducous scales between the pelvic fins and the anus.
  • Porcupine fishes have scales modified into large external spines.
  • By contrast, pufferfish have thinner, more hidden spines than porcupine fish, which become visible only when the fish puffs up. Unlike the porcupine fish, these spines are not modified scales, but develop under the control of the same network of genes that produce feathers and hairs in other vertebrates.[57][58]

Fish without scales

[edit]
  • Mandarinfish lack scales and protect themselves with a layer of smelly and bitter slime.
    Mandarinfish lack scales and protect themselves with a layer of smelly and bitter slime.

Fish without scales usually evolve alternatives to the protection scales can provide, such as tough leathery skin or bony plates.

  • Jawless fish (lampreys and hagfishes) have smooth skin without scales and without dermal bone.[59] Lampreys get some protection from a tough leathery skin. Hagfish exude copious quantities of slime or mucus if they are threatened.[60] They can tie themselves in an overhand knot, scraping off the slime as they go and freeing themselves from a predator.[61]
  • Most eels are scaleless, though some species are covered with tiny smooth cycloid scales.
  • Most catfish lack scales, though several families have body armour in the form of dermal plates or some sort of scute.[62]
  • Mandarinfish lack scales and have a layer of smelly and bitter slime which blocks out disease and probably discourages predators, implying their bright coloration is aposematic.[63]
  • Anglerfish have loose, thin skin often covered with fine forked dermal prickles or tubercles, but they do not have regular scales. They rely on camouflage to avoid the attention of predators, while their loose skin makes it difficult for predators to grab them.

Many groups of bony fishes, including pipefish, seahorses, boxfish, poachers, and several families of sticklebacks, have developed external bony plates, structurally resembling placoid scales, as protective armour against predators.

  • Seahorses lack scales but have thin skin stretched over a bony plate armour arranged in rings through the length of their bodies.
  • In boxfish, the plates fuse together to form a rigid shell or exoskeleton enclosing the entire body. These bony plates are not modified scales but skin that has been ossified. Because of this heavy armour boxfish are limited to slow movements, but few other fish are able to eat the adults.
Eels seem scaleless, but some species are covered with tiny smooth cycloid scales.
  • Boxfish have plates of ossified skin fused together to form a rigid shell.
    Boxfish have plates of ossified skin fused together to form a rigid shell.
  • Seahorses have thin skin stretched over bony plates arranged in rings.
    Seahorses have thin skin stretched over bony plates arranged in rings.

Some fish, such as hoki and swordfish, are born with scales but shed them as they grow.

Filefish have rough non-overlapping scales with small spikes, which is why they are called filefish. Some filefish appear scaleless because their scales are so small.

Prominent scaling appears on tuna only along the lateral line and in the corselet, a protective band of thickened and enlarged scales in the shoulder region. Over most of their body tuna have scales so small that to casual inspection they seems scaleless.[64]

  • Some filefish appear scaleless because their scales are so small.
    Some filefish appear scaleless because their scales are so small.
  • To casual examination tuna seem largely free of scales, but they are not.
    To casual examination tuna seem largely free of scales, but they are not.

Lepidophagy

[edit]
Dorsal view of right-bending (left) and left-bending (right) jaw morphs adapted for eating fish scales[65]

Lepidophagy is a specialised feeding behaviour in fish that involves eating the scales of other fish.[66] Lepidophagy has independently evolved in at least five freshwater families and seven marine families.[67]

Fish scales can be nutritious, containing a dermal portion and a layer of protein-rich mucus apart from the layers of keratin and enamel. They are a rich source of calcium phosphate.[67] However, the energy expended to make a strike versus the amount of scales consumed per strike puts a limit on the size of lepidophagous fish, and they are usually much smaller than their prey.[67] Scale eating behaviour usually evolves because of lack of food and extreme environmental conditions. The eating of scales and the skin surrounding the scales provides protein rich nutrients that may not be available elsewhere in the niche.[68]

See also

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fish scale

View on Grokipedia
from Grokipedia
Fish scales are small, rigid dermal plates that grow out of the skin of most species, serving as a primary protective barrier against predators, abrasion, and environmental stressors while also facilitating flexibility and hydrodynamic during . These structures have evolved under selective pressures to balance toughness, lightness, and mobility, with modern exhibiting four main types: cosmoid, placoid, ganoid, and elasmoid scales. Elasmoid scales, predominant in ray-finned fishes, represent the most advanced form and are characterized by their thin, overlapping arrangement that allows for body articulation without compromising defense. Structurally, elasmoid scales typically consist of a hard outer limiting layer made of calcium-deficient hydroxyapatite crystals with minimal collagen, overlaid on a thicker elasmodine layer formed by orthogonally arranged type I collagen fibrils in a Bouligand plywood configuration, which provides graded mineralization and energy dissipation during impacts. This hierarchical composition—primarily hydroxyapatite nanocrystals embedded in collagen matrices—endows scales with remarkable mechanical properties, such as high puncture resistance (up to 1.2 GPa Young's modulus in species like Arapaima gigas) and the ability to slide and rotate under stress to prevent cracking. In species like carp (Cyprinus carpio) and tarpon (Megalops atlanticus), scales feature three distinct layers: a rigid external mineralized zone, a flexible collagenous middle, and a thin basal plate, optimizing protection against bites or strikes. Beyond protection, fish scales play roles in osmoregulation, sensory perception, and camouflage, with their iridescent or pigmented surfaces aiding in species-specific signaling. Notably, scales exhibit rapid regeneration when damaged, driven by proliferation and sequential mineralization, restoring functionality within weeks to months—though regenerated scales often lack the full hierarchical of original ones, prioritizing immediate toughness over long-term optimization. This regenerative capacity, temperature-dependent (faster and stronger at 20°C than 10°C), underscores the adaptive of fish .

Functions of Fish Scales

Protection and Armor

Fish scales primarily function as a defensive barrier, shielding the underlying and tissues from physical damage inflicted by predators, environmental abrasion, and other hazards. In many species, scales form a flexible yet robust armor through their imbricated , where posterior edges overlap anterior ones, creating a continuous protective covering that allows mobility while resisting penetration. This design is evident in ganoid scales, which feature a hard, enamel-like outer layer of ganoine—composed of crystals—providing rigidity and high hardness up to 2500 MPa, atop a compliant bony base of mineralized . The ganoine layer effectively deflects sharp impacts, such as predator teeth, by causing fractures in the attacking structures at forces as low as 500 N per tooth. In armored , such as those in the family , modified scales called scutes exemplify enhanced penetration resistance. These obliquely aligned, pentagonal scutes consist of multiple layers including superficial and basal lamellar plates, a mid-plate of woven and secondary osteons, and denticles capped with hard enameloid and , which collectively prevent deep bites from predators like . Experimental puncture tests on similar elasmoid scales from reveal that the mineralized surface layer (~40–50 μm thick) and underlying twisted Bouligand structure dissipate energy through fiber stretching, rotation, and . This multilayered architecture not only halts penetration by spines or teeth but also supports rapid remodeling via woven for sustained defense. Fossil records from the seas, approximately 375 million years ago, illustrate the evolutionary refinement of scales for survival amid intense predation. Early jawed fishes like the lobe-finned Holoptychius bergmanni possessed heavy, interlocking scale armor that protected against large-toothed predators in a "fish-eat-fish" , as evidenced by well-preserved and fossils from the Canadian . Placoderms, dominant vertebrates, further demonstrate this trend with thick, overlapping dermal plates serving as exoskeletal shields, highlighting scales' role in enabling diversification and dominance in predatory marine environments. Scale thickness and overlap patterns critically influence force distribution, mitigating localized damage during impacts. Thicker scales, such as those in (up to 300 μm basal plate), combined with overlaps of 25–50%, spread applied forces across multiple units via hinge-like sliding and connective tissues, increasing puncture resistance threefold when three scales overlap compared to a single one. In numerical simulations of impacts up to 100 J, higher overlap ratios (e.g., 40%) distribute loads over wider areas, elevating peak resistance to 6.0 kN for conical indenters while preserving flexibility, thus optimizing protection without excessive rigidity. This comparative mechanics underscores scales' adaptive balance between armor and locomotion.

Hydrodynamics and Movement

Fish scales play a crucial role in minimizing hydrodynamic drag during by directing flow and reducing through their microscopic surface features and imbricated arrangements. In ctenoid scales, common in many fishes, the posterior comb-like projections (ctenii) and underlying microstructured surfaces act as riblet-like channels that align streamwise vortices, promoting streaky flow patterns within the and delaying the transition to . This mechanism stabilizes over the body, contributing to overall drag reduction in exposed to moderate flow speeds. Cycloid scales, prevalent in softer-skinned fishes like salmonids, feature smooth, rounded edges with precise overlap angles—typically around 50-70% coverage—that create a continuous, low-profile surface. These overlaps minimize protrusions that could induce , channeling water smoothly posteriorward and reducing by up to 10% in fast-swimming species, as estimated from analyses. The angled imbrication further enhances this effect by generating low-momentum streaks that suppress cross-flow instabilities. In evolutionary adaptations of high-speed predators like tunas (genus ), small, fan-shaped scales are embedded in a flexible , forming oblique arrays at approximately 5° to the streamwise direction. This configuration produces velocity streaks via vortex stretching, reducing during sustained cruising and burst accelerations; biomimetic models inspired by this structure achieve up to 7% drag reduction at speeds of 2.5 m/s, reflecting natural efficiencies that enable speeds exceeding 10 m/s. During rapid maneuvers, the scales' minimal protrusion maintains a near-smooth profile, avoiding added resistance from scale erection or sloughing. Biomechanical models of scale-dermis composites reveal how flexibility facilitates body contouring. Using finite element homogenization on imbricated arrays, such as those in (Morone saxatilis), simulations show that scales rotate longitudinally during bending, enabling up to 20-30° with minimal energy loss while nonlinearly at higher angles to prevent excessive deformation. This tunable compliance—driven by low rotational at scale attachments—allows precise undulation for turns and , balancing hydrodynamic with maneuverability without compromising the protective layering.

Sensory and Camouflage Roles

Fish scales play a crucial role in sensory perception by integrating neuromasts, the mechanosensory organs of the system, which detect water movements and vibrations essential for , prey detection, and predator avoidance. In many bony fishes, canal neuromasts are embedded within specialized lateral line scales that form ossified canals along the body trunk, allowing these structures to function as accelerometers sensitive to low-frequency stimuli (0–200 Hz) through pressure differentials at canal pores. These scales overlap and align parallel to the skin surface, with neuromasts innervated by the posterior lateral line nerve, enhancing the system's ability to sense hydrodynamic trails from nearby objects or conspecifics. Superficial neuromasts, sometimes referred to as pit organs, may also occur on scale surfaces in certain , providing additional sensitivity to surface vibrations without enclosure in canals. Beyond sensory functions, fish scales contribute to through produced by iridescent crystals arranged in iridophores within the scale's dermal layers. These thin, platelet-like crystals (~100 nm thick) create multilayer reflectors that generate interference patterns, producing silvery or metallic hues that blend with light-scattered aquatic environments for . In flatfishes like flounders, this mechanism enables rapid adaptive color shifts by modulating crystal orientation or spacing, allowing the scales to match substrate patterns and reduce visibility to predators through dynamic . Such structural properties, distinct from -based coloration, provide angle-dependent color changes that enhance overall body without relying on metabolic energy for production. Scale patterns further aid camouflage via disruptive coloration, where bold contrasts and edges on the scales break up the fish's outline to mimic surrounding reef structures or substrates. In parrotfishes (family Scaridae), juvenile scales exhibit high-contrast bands and spots that disrupt body form, providing effective background matching against coral habitats and reducing predation risk during vulnerable life stages. These patterns, formed by the arrangement and pigmentation of overlapping scales, serve dual roles in and signaling but primarily function to deceive visual predators by aligning with environmental textures. The molecular basis of rapid camouflage in fish scales involves chromatophores—pigment cells embedded in the dermal layers adjacent to scale margins—that enable quick shifts in body coloration through translocation. In fishes, melanophores and other chromatophores contain pigment granules (e.g., melanosomes) that disperse or aggregate via microtubule-based motors like and , triggered by hormones such as or α-MSH for aggregation and dispersion, respectively, allowing color changes within minutes. This actin-myosin mediated mechanism at the cell periphery facilitates synchronized responses across scale-associated skin regions, integrating with structural elements for comprehensive adaptation to environmental cues.

Evolutionary Origins

In Early Jawless Fish

The emergence of scale-like structures in early jawless fish, particularly within the thelodonts, marks a pivotal development in dermal armor during the era. Thelodonts, an extinct group of jawless s closely related to the ancestry of jawed fishes, possessed scales composed of odontodes—small, conical structures primarily made of dentine—that first appeared in the Upper Ordovician and proliferated through the Silurian period. These odontodes formed the basic units of the dermal skeleton, featuring a mineralized crown of dentine covered by a thin enameloid layer, which provided rudimentary protection against abrasion and predators. Fossil evidence from articulated specimens, such as those of Loganellia scotica from the Lower of , reveals that these scales were arranged in multiple rows, often five distinct types (rostral, cephalo-pectoral, postpectoral, precaudal, and pinnal), achieving partial body coverage rather than full enclosure. In Loganellia, scales covered the trunk and tail regions extensively but left areas like the orbital and branchial zones less protected or naked, suggesting an for specific ecological niches such as soft-substrate dwelling. This partial squamation, estimated at around 70-80% body coverage in some thelodonts, balanced defense with flexibility for movement. Functionally, these structures in agnathans represented a shift from pharyngeal elements like gill rakers, which filtered food, to external that offered against ectoparasites and physical while potentially aiding in hydrodynamic . In thelodonts, odontode-based scales evolved to serve dual roles in defense and sensory functions, with specialized forms in the head and trunk regions enhancing survival in diverse aquatic environments. Phylogenetically, thelodont odontodes occupy a basal position among vertebrate integumentary structures, serving as precursors to the more integrated scales of jawed fish (gnathostomes). The phylogenetic position of thelodonts remains debated, with recent studies supporting their placement as stem-gnathostomes rather than true agnathans. This evolutionary linkage underscores thelodonts' role in bridging agnathan simplicity to the diverse dermal armors of later s.

Transitions to Jawed Vertebrates

The transition from jawless to jawed vertebrates during the early Paleozoic, particularly in the Devonian period around 400 million years ago, marked a pivotal diversification of dermal scales amid intensifying aquatic predation pressures. The emergence of gnathostomes introduced efficient biting mechanisms, exerting selective pressure on prey species and driving adaptive radiations in protective integumentary structures. Fossil evidence from bite marks on jawless fish indicates that this predation targeted soft-bodied forms, favoring the evolution of robust, enamelled scales that enhanced armor against attacks from early predators like placoderms and acanthodians. In acanthodians, primitive jawed fishes, scales evolved through the replacement of thelodont-like odontodes—small, tooth-like dermal denticles from jawless ancestors—with ganoid-like enamelled structures. These odontodes, characterized by orthodentine crowns on bony bases, clustered and fused into rhombic scales featuring superficial mesodentine or orthodentine layers capped by enameloid or ganoine-like tissue, providing a smoother, more continuous protective surface. This transition reflected odontogenic and osteogenic developmental shifts, enabling larger body sizes and better resistance to abrasion and penetration in predator-prey interactions. Early sarcopterygians, lobe-finned gnathostomes, developed cosmoid scales as a multilayered for enhanced , consisting of an outer cosmine layer (enamel over dentine with interconnected pore canals for sensory or metabolic functions), a middle vascular spongy layer, basal lamellar (isopedine), and pulp cavities for nutrient supply. This complex , seen in fossils like Porolepis and Dipterus, created thick, rhombic scales that offered superior armor against predation compared to thinner thelodont precursors, while the pore-canal system may have supported ion regulation and repair. Fossils of , a late sarcopterygian, illustrate scale vascularization facilitating growth, with regular body scales showing a thin elasmoid-like structure of parallel-fibered over woven-fibered bases and a plywood-like basal plate with preserved vascular canals in the isopedine layer. These features enabled incremental circumferential and superficial accretion, allowing scales to expand with the fish's somatic growth while maintaining protective integrity in a high-predation environment.

Types of Scales in Extinct and Primitive Fish

Thelodont Scales

Thelodont scales represent the earliest known analogs to vertebrate scales, appearing in extinct jawless fishes of the class Thelodonti during the Paleozoic era. These odontode-like structures consisted of a dentine core overlaid by an enameloid cap on the crown, with a basal layer of aspidin, an acellular bone tissue that lacked the cellular components of true bone. The enameloid provided a hard, wear-resistant surface, while the dentine core featured tubular or branching structures for structural support, and the aspidin base anchored the scale to the dermis without vascular canals in the neck region. This composition rendered the scales robust yet lightweight, adapted for superficial dermal integration rather than deep embedding. In terms of distribution, thelodonts exhibited extensive scale coverage over the entire body, including the head and trunk regions. This arrangement varied by species and body region, with distinct morphotypes such as rostral scales on the head, cephalo-pectoral types in transitional areas, and trunk scales along the body, often preserved as isolated microfossils due to their small size and post-mortem dispersal. Scale sizes ranged from 0.5 to 5 mm in length, with crown widths commonly between 0.2 and 1.5 mm, allowing for morphological diversity including ridged, thorn-like, or smooth forms tailored to specific anatomical positions. Fossil records document over 50 genera of thelodonts encompassing approximately 147 species, spanning from the Upper to the Upper , roughly 458 to 359 million years ago. This diversity reflects adaptations across marine environments, with scales often dominating assemblages in and siliciclastic deposits. Paleoenvironmentally, these scales likely served to resist abrasion from sediments in benthic habitats, where many thelodont species inhabited hard substrates, reefs, or sandy-muddy bottoms as demersal detritivores. Abrasion-resistant morphotypes, featuring ridged crowns with spacings of 35–80 μm, suggest protection against scraping during bottom-dwelling activities like in crevices or over rough seafloors. Such features positioned thelodont scales as evolutionary precursors to the more complex cosmoid scales of early jawed vertebrates.

Cosmoid Scales

Cosmoid scales represent an intermediate evolutionary stage in the development of fish integumentary structures, primarily associated with ancient lobe-finned fishes (sarcopterygians) from the Devonian period. These scales are characterized by a complex, multi-layered composition that provided robust protection, distinguishing them from simpler denticles in earlier jawless fish. They are documented in fossils of early sarcopterygians, such as the dipnoans and actinistians, and served as precursors to later scale types in bony vertebrates. The structure of cosmoid scales consists of four distinct layers, starting from the outermost: a thin, hard enamel-like layer known as vitrodentine, which offers a smooth, resistant surface; beneath this lies cosmine, a porous form of dentine featuring a network of pore-canals that likely facilitated sensory functions; this is followed by a layer of vascular for structural support and nutrient distribution; and the innermost basal layer, isopedine, composed of dense lamellar that anchors the scale to the . Unlike ganoid scales in ray-finned fishes, cosmoid scales lack ganoine, emphasizing their specialization in lobe-finned lineages. This layered architecture enhanced durability while maintaining flexibility. In terms of morphology, cosmoid scales are typically rhombic in , with a diagonal long axis oriented obliquely to the fish's body, allowing for overlapping coverage along the flanks. They articulate via a peg-and-socket mechanism, where a broad-based peg on the posterior edge of one scale fits into a socket on the anterior edge of the adjacent scale, ensuring a tight, interlocking dermal armor without gaps. examples, such as those from the sarcopterygian Dipterus, illustrate this design, with scales measuring up to several millimeters in thickness and exhibiting ornamentation from the cosmine layer. Cosmoid scales grew through superficial accretion, where new material was added to the outer layers over time, enabling the scales to enlarge proportionally with the without periodic shedding or resorption. This growth pattern is evident in the incremental layering observed in fossil specimens. Although fully cosmoid scales became extinct with the decline of many primitive sarcopterygian groups, vestigial remnants persist in modern coelacanths () and lungfishes (Dipnoi), where scales retain cosmine-like features or reduced pore systems, reflecting their evolutionary legacy amid the dominance of fishes with simpler elasmoid scales.

Scales in Cartilaginous Fish

Placoid Scale Structure

Placoid scales, also known as dermal denticles, are characteristic of cartilaginous fishes such as and rays, exhibiting a tooth-like morphology that provides structural integrity and protection. Each scale consists of a basal plate embedded in the , supporting a protruding spine composed of an enameloid crown, a dentine body, and an inner pulp cavity. The enameloid, a hard, translucent outer layer akin to , covers the crown and is secreted by the , while the underlying dentine forms the bulk of the spine with its calcified, canaliculated structure for strength. The pulp cavity, located at the core, contains vascular connective tissue, blood vessels, nerves, lymph channels, and odontoblasts responsible for dentine formation, ensuring nourishment and sensory functions. The basal plate is typically - or rhomboid-shaped, composed of a cement-like bony material that anchors the scale to the via fibers, with small apertures allowing access to the pulp cavity. Unlike overlapping scales in bony fishes, placoid scales are arranged without overlap, protruding individually through the , which enables independent replacement throughout the fish's life as older scales are shed and new ones form in the gaps. This non-overlapping facilitates localized regeneration and to wear, with scales erupting fully formed and not enlarging post-maturity. Morphological variations occur across , reflecting ecological adaptations; in many , the spine is trident-shaped with backward-directed projections that contribute to texture and roughness, while in rays, scales tend to be flatter with reduced spines to suit their benthic lifestyle. Size typically ranges from 0.03 to 0.1 cm in crown length for most , though larger forms up to 1 cm occur in certain deep-water , with areal densities varying from 400 to 2,000 scales per cm², equating to millions per square meter on the body surface. For instance, the exhibits densities up to 2,000 per cm² in regions with smaller denticles.

Specialized Forms in Sharks and Rays

In sharks, dermal denticles exhibit specialized hydrodynamic adaptations, with their angled crowns oriented to promote unidirectional flow over the body surface, thereby reducing and drag during high-speed . This structure, as the riblet-like arrangement of denticles channels in a streamlined manner. In rays and skates, placoid scales are adapted for benthic and , often embedded more deeply into the flexible skin of the pectoral fins, which function as wings for and maneuvering. These scales show reduced density across the expansive wing surfaces compared to the body, with concentrations primarily along the anterior margins of the pectoral fins, facilitating greater flexibility and minimizing resistance during slow, flapping motions essential for bottom-dwelling lifestyles. Sexual dimorphism in denticle morphology is prominent in many cartilaginous fishes, particularly in relation to reproductive behaviors such as biting and clasping during copulation. In like the lesser-spotted catshark (Scyliorhinus canicula), mature females possess longer and wider denticles (e.g., up to 374 µm in length on the pectoral fin) in vulnerable areas like the pectoral fins and pelvic girdle to provide enhanced protection against -inflicted damage, while s exhibit higher denticle density (e.g., 40/mm² on the pectoral fin) potentially aiding grip during mating. Similar patterns occur across , where dimorphic denticle traits correlate with intraspecific and .

Scales in Bony Fish

Ganoid Scales

Ganoid scales are diamond-shaped or rhombic coverings that form a rigid, jointed armor on the bodies of certain primitive bony , consisting of a superficial layer of ganoine—a hypermineralized enamel-like tissue composed primarily of —overlying a basal plate of , often with an intermediate layer of dentine. This structure provides robust protection against predators and environmental hazards, with the scales interlocking via peg-and-socket articulations that limit flexibility while maintaining overall body integrity. Ganoid scales evolved as a modification of earlier cosmoid scales, retaining key histological features from ancestral forms. These scales are characteristic of non-teleost bony fishes, including sturgeons (Acipenseridae), gars (Lepisosteidae), bowfins (Amiidae), and bichirs (Polypteridae), where they appear as thick, enamel-surfaced plates. In sturgeons and gars, the scales are often enlarged into scutes that can reach thicknesses of up to 1 cm in larger individuals, offering armor-like defense but restricting movement compared to more pliable scale types. The ganoine layer imparts a glossy, tooth-like hardness, enhancing durability in these ancient lineages. Ganoid scales display annual growth rings, analogous to those in trees, formed by seasonal deposition of bone and ganoine layers, which fisheries biologists use to estimate age and growth history. In long-lived such as sturgeons and gars, these rings enable age assessments exceeding 50 years, informing and conservation efforts. Evolutionarily, ganoid scales persist as a primitive trait from Paleozoic osteichthyan ancestors, such as those in the and Permian periods, differing markedly from the thin, overlapping elasmoid scales that predominate in advanced fishes for improved hydrodynamics and agility.

Elasmoid and Leptoid Scales

Elasmoid scales represent the most common scale type among modern ray-finned fishes (), particularly teleosts, where they form thin, overlapping dermal plates that prioritize flexibility over rigid armor. These scales consist of two primary layers: an outer areolar layer composed of partially mineralized fibers arranged in a plywood-like structure, and an inner fibrillary plate primarily made of unmineralized or lightly calcified , providing tensile strength and elasticity. Unlike ganoid scales, elasmoid scales lack an enamel-like ganoine layer, allowing for greater dermal integration and periodic shedding or regeneration. This layered composition enables the scales to bend without fracturing, offering against abrasion and minor predation while maintaining body contour during movement. Within elasmoid scales, the leptoid subtype predominates in advanced teleosts and is distinguished by its thin profile and posterior overlap, further subdivided into and ctenoid forms based on edge morphology. scales feature smooth, rounded posterior margins with concentric growth rings (circuli), as exemplified in salmonids such as species, where the sleek design minimizes water resistance to support high-speed, streamlined swimming in open water. In contrast, ctenoid scales possess comb-like projections (ctenii) along the posterior edge, characteristic of perciform fishes like perches (Perca spp.), which enhance surface grip for improved maneuverability and stability during bursts of acceleration or interaction with substrates. These structural variations correlate with ecological niches, with forms favoring fast cruisers and ctenoid aiding agile predators. The vibrant observed in many elasmoid-scaled fishes results from platelets stacked in iridophore cells beneath the scales, creating multilayer reflectors that selectively scatter light via . These platelets, typically 5–20 μm in diameter and 0.1 μm thick, exhibit a high (n ≈ 1.83). This mechanism generates angle-dependent hues, such as the metallic blues and greens in like the (Paracheirodon innesi), enhancing and signaling without pigments. Elasmoid scales, encompassing leptoid variants, cover the vast majority (approximately 96%) of extant species, predominantly teleosts, which comprise over 33,000 species (out of approximately 35,000 total species as of 2024). Their nature and low mass facilitate high mobility, allowing rapid evasion and efficient propulsion in diverse aquatic environments, from coral reefs to pelagic zones. This prevalence underscores their evolutionary success in balancing defense with locomotor demands.

Specialized and Modified Scales

Scutes

Scutes represent enlarged, non-overlapping bony plates located primarily on the trunk or head of various fish species, functioning as specialized dermal armor for targeted protection against predators and environmental hazards. Unlike typical overlapping scales, scutes form rigid, plate-like structures that cover specific body regions, enhancing structural integrity without compromising mobility in non-armored areas. In catfishes, particularly armored species within the Loricariidae family such as the common pleco (Hypostomus plecostomus), thoracic scutes along the ventral and lateral surfaces provide robust anti-predator defense by creating a hardened barrier that impedes penetration by predators like larger fish or birds. These scutes consist of superficial and basal bony plates formed by lamellar and zonal bone, with a mid-plate layer of secondary osteons and woven bone; denticles are connected to the scutes via ligaments for added protection. A notable example occurs in sturgeons, such as the (Acipenser transmontanus), which possess up to 50 scutes per side along the lateral row, contributing to abrasion resistance in turbulent riverine habitats where the fish navigate gravelly substrates. These scutes exhibit a pentagonal arrangement with concentric patterns, optimizing durability against mechanical wear. Developmentally, scutes originate from independent centers within the dermal , derived from cells that differentiate into osteoblasts, distinguishing them from true scales that form through more integrated epidermal-dermal interactions.

Modified Sensory and Protective Scales

In certain fish species, scales along the are specialized for enhanced mechanosensory functions, featuring enlarged structures that house canal systems to detect subtle water movements and pressure gradients. These scales, particularly evident in bony fishes like the (Danio rerio), consist of 3–5 specialized scales in the anterior trunk that form the canal network through during development, allowing neuromasts within the canals to sense hydrodynamic stimuli such as flow direction and vibrations for navigation and predator avoidance. This modification improves sensitivity to low-frequency pressures compared to superficial neuromasts, enabling precise detection in turbulent environments. Some fish exhibit spiny fins adapted for defense through delivery, as seen in lionfish (Pterois spp.), where dorsal and fin spines contain glandular apparatus. The comprises high-molecular-weight proteins (50–800 kDa), including for tissue , a pain-producing factor, and capillary permeability factors that induce and cardiovascular effects upon . These protein toxins, primarily peptides around 4.6–4.7 kDa in mass, provide potent ichthyotoxic and cytolytic protection against predators. Adhesive modifications occur in clingfishes (Gobiesox spp.), where disc-like structures formed by modified pelvic fins create a mechanism for attachment to irregular surfaces. This disc generates sub-ambient pressures up to 0.2–0.5 , supported by hierarchical microvilli and papillae on the disc margin that seal against rough substrates, achieving forces 80–230 times the fish's body weight. The structure relies on a combination of , from fibrillar extrusions, and non- secretions to maintain grip in high-flow intertidal zones without chemical bonding. In elongated fish like eels ( spp.), evolutionary adaptations involve partial scale reduction or embedding, resulting in a semi-nude compensated by a thickened epidermal layer for protection. This slime coat, rich in and glycoproteins, serves as a barrier against pathogens, parasites, and abrasion, mimicking the protective role of scales while facilitating burrowing and escape behaviors. The mucus thickness, often several cell layers deep, reduces friction and enhances osmotic regulation in scaleless regions.

Development and Growth

Embryonic Formation

The embryonic formation of scales involves intricate interactions between the ectodermal and the mesodermal , leading to the development of scale primordia. In bony such as (Danio rerio), a common , these primordia emerge as dermal condensations near the epidermal-dermal boundary, initiated by signaling cues from the overlying . Scale development begins around 12 days post-fertilization (dpf) in the caudal peduncle region, where basal epidermal cells differentiate first and induce dermal fibroblasts to form papillae-like structures. This process is driven by mesoderm-derived progenitors in the , challenging earlier assumptions of contributions in teleosts. Key genetic regulators orchestrate the patterning and initiation of these primordia. The eda gene, encoding ectodysplasin-A, is essential for scale placode formation; mutants like nkt exhibit complete absence of scales due to disrupted epidermal-dermal signaling. Similarly, fgf (fibroblast growth factor) pathways, particularly fgf20a and fgf8a, promote dermal condensation and scale outgrowth, with overexpression leading to enlarged scale sheets and inhibition arresting squamation. The shh (sonic hedgehog) gene supports epidermal morphogenesis and osteoblast differentiation, requiring upstream eda and Wnt/β-catenin activity; its repression impairs scale invagination. These genes interact in a network where Wnt/β-catenin initiates broad patterning, refined by Eda and Fgf for precise primordia spacing. Scale primordia form sequentially, starting in the caudal fin and progressing rostrally along the body axis, ensuring orderly coverage. This wave-like progression is coordinated by traveling signaling fronts, such as Eda/ activity, which activate target genes including wnt, shh, and fgf in a spatiotemporal manner. In comparative embryology, differences exist between scale types. Elasmoid scales in bony arise from mesodermal cells, as confirmed by lineage tracing in and medaka showing origins without involvement. In contrast, placoid scales (dermal denticles) in cartilaginous like the skate (Leucoraja erinacea) derive from trunk cells, which migrate to form odontoblasts in the denticle primordia during early embryonic stages. These distinct origins reflect evolutionary divergences in integumentary skeleton development.

Post-Embryonic Expansion and Renewal

After hatching, fish scales undergo post-embryonic expansion primarily through marginal accretion, where new material is added at the scale's periphery in proportion to the fish's overall somatic growth. This process results in the formation of annular growth rings, known as annuli, which resemble the daily increments observed in otoliths and serve as a reliable indicator for age determination. Each annulus typically represents one year of growth, with wider bands forming during periods of rapid somatic expansion and narrower ones during slower phases, allowing researchers to back-calculate historical body from scale measurements. The von Bertalanffy growth model, commonly applied to interpret these annuli, describes at age tt as Lt=L∞(1−e−k(t−t0))L_t = L_\infty (1 - e^{-k(t - t_0)}), where L∞L_\infty is the asymptotic maximum , kk is the growth , and t0t_0 is the theoretical age at zero ; this model integrates scale data to estimate population-level growth parameters in like salmonids and perciforms. Scale regeneration in teleosts occurs rapidly following or loss, typically achieving full replacement within 2-4 weeks through proliferation of epidermal cells that migrate into the scale and differentiate into scale-forming osteoblasts. This process restores both structural integrity and protective function, with the regenerated scale initially thinner and more flexible but mineralizing to match ontogenetic scales over time; for instance, in (Carassius auratus), area growth shifts from rapid expansion to linear weight increase by 28 days post-removal. Seasonal variations significantly influence scale growth rates, with faster annular expansion during warmer months due to elevated metabolic rates and food availability, leading to broader summer rings, and slower winter growth forming distinct annuli boundaries. This pattern is evident in temperate species, where scale circuli spacing narrows in colder periods, reflecting reduced somatic growth; for example, in sunfish (Lepomis macrochirus), annuli form annually as a result of these temperature-driven pauses. Such variations not only aid in precise aging but also provide insights into environmental impacts on fish populations.

Scales in Scale-Less Fish and Ecological Interactions

Fish Lacking Scales

Cyclostomes, including lampreys and , represent a basal group of vertebrates that lack dermal scales entirely, a condition resulting from secondary evolutionary loss from scaled ancestors such as thelodonts, an extinct group of jawless fish characterized by small, placoid-like scales covering their bodies. Instead of scales, these fish possess a thick, elastic reinforced by a dense network of fibers, which provides mechanical resilience against abrasion and penetration. Their primary protective adaptation is the production of abundant from specialized glands, which forms a slippery barrier that deters predators, inhibits parasite attachment, and facilitates escape through entanglement of attackers. This mucous layer also aids in and , compensating for the absence of scaled armor in their soft-bodied, eel-like forms. Among fishes, several lineages exhibit scale reduction or complete absence as a derived trait, often linked to specific ecological demands. A prominent example is the naked carp (Gymnocypris przewalskii), a cyprinid endemic to Lake in , which has evolved scaleless skin to enhance cutaneous and in the lake's fluctuating brackish-to-freshwater conditions. The naked allows direct exposure of epithelial cells to the environment, facilitating of ions like sodium and via specialized transporters, which is crucial for maintaining in this high-altitude, saline-alkaline where gill-based regulation alone is insufficient. Transcriptomic studies reveal upregulated genes for ion channels and aquaporins in the skin of these fish, underscoring the adaptive role of scalelessness in reducing osmoregulatory costs compared to scaled relatives in pure freshwater rivers. The genetic underpinnings of scaleless phenotypes in teleosts frequently involve mutations in the ectodysplasin-A (eda) gene, a key regulator of ectodermal appendage formation that, when disrupted, leads to reduced or absent scales across multiple families including , Adrianichthyidae, and Gasterosteidae. In (Danio rerio), loss-of-function eda alleles result in viable adults with sparse or no scales due to disrupted epidermal-dermal signaling that fails to initiate scale primordia formation. Similarly, in medaka (Oryzias latipes), eda mutations at the rs-3 locus cause near-complete scale loss by impairing ectodysplasin receptor interactions essential for placode organization. In high-altitude cyprinids like schizothoracines, adaptive eda variants, including single polymorphisms and small deletions, correlate with progressive scale regression in over 50 across 11 genera, suggesting of scalelessness in response to environmental pressures. Scalelessness confers advantages in certain lifestyles, particularly for burrowing where reduced body mass and enhanced slipperiness improve substrate penetration. of the family Cobitidae, such as the weather loach (Misgurnus anguillicaudatus), typically have embedded or vestigial scales, resulting in a lightweight, -rich that minimizes during nocturnal burrowing into sediments for and predator avoidance. The viscoelastic properties of their epidermal , rich in glycoproteins, create a low-drag interface with or , allowing efficient movement without abrasion to the delicate . This adaptation not only reduces energetic costs associated with locomotion in confined spaces but also enhances sensory through direct tactile feedback from the exposed .

Lepidophagy and Scale Consumption

Lepidophagy refers to the specialized feeding strategy in which certain species consume the scales of other as a primary or significant dietary component. This behavior has evolved independently in at least five freshwater and seven marine families, providing access to a nutrient-rich resource that is otherwise difficult for predators to exploit without inflicting fatal damage on the prey. Scale-eating specialists, such as the cichlids in the genus Perissodus from , exhibit remarkable adaptations for plucking scales from prey. These possess asymmetric mouths and heads, with left- or right-mouthed individuals specializing in attacking the opposite flank of prey to efficiently remove scales using recurved teeth. This morphological asymmetry is genetically determined and maintained through , where the relative abundance of left- and right-mouthed morphs balances due to prey avoidance behaviors. Fish scales offer substantial nutritional value, particularly high levels of calcium and protein derived from their collagenous structure. In scale-eating piranhas like Catoprion mento, scales form an important proportion of the diet, providing essential minerals and energy with calorific content estimated at 8-10 kJ per gram. Analysis of various scales reveals calcium concentrations ranging from 3,247 to 7,930 mg per 100 g, underscoring their role as a calcium-rich source. The protein content, primarily from , supports growth and tissue repair in lepidophagous species. Prey fish have evolved defensive responses to lepidophagous attacks, including behavioral maneuvers to protect vulnerable areas. In species like (Chaetodon spp.), attacked individuals may erect ctenoid scales or shed them to deter further predation, minimizing injury while allowing regeneration. These mechanisms, combined with rapid evasion, reduce the success rate of scale-plucking attempts. The interaction between scale-eaters and their prey exemplifies an , where prey populations develop increased scale toughness over generations in response to predation pressure. In cichlids, this has led to coevolutionary dynamics, with prey evolving thicker or more adherent scales and predators refining their for efficient scale removal. Such adaptations highlight the selective pressures driving specialization in lepidophagy.

Human Applications and Biomimicry

Drag Reduction Technologies

Riblet patterns, engineered to mimic the placoid denticles found on shark skin, consist of longitudinal micro-grooves aligned with fluid flow to minimize in turbulent boundary layers by channeling low-momentum streaks away from the surface. These structures disrupt the formation of turbulent eddies, reducing without significantly increasing form drag. The development of riblet technology originated from investigations in the , which analyzed denticle morphology and conducted wind tunnel tests on synthetic replicas, confirming their potential for drag in applications. Subsequent optimizations led to practical implementations, with tests showing drag reductions of 5-8% under optimal conditions, such as when riblet spacing matches the local . A notable example is 3M's riblet films applied to Olympic racing swimsuits in , where they achieved approximately 3-4% drag reduction for athletes, enhancing swimming performance. In engineering applications, riblets have been integrated into aircraft wings through initiatives like Speedo-F1 collaborations, which tested biomimetic surfaces for improved , and onto ship hulls to lower consumption in marine transport. These surfaces typically feature microstructures 50-100 μm in height and spacing, scaled to the of the operating fluid. Despite their efficacy, riblet applications in marine settings face limitations from , where algal and microbial growth clogs the grooves; this requires supplementary or self-cleaning coatings to preserve drag-reducing properties over time.

Other Engineering Inspirations

Biomimicry of fish scales extends beyond hydrodynamics into protective materials, drawing from the robust, overlapping structure of elasmoid scales found in fish like the (). These scales feature a hard, mineralized outer layer atop a flexible base, enabling energy dissipation through deformation and sliding during impacts. Engineers have replicated this in composite armors for applications, such as , using 3D-printed or layered ceramics and polymers with interlocking plates embedded in compliant matrices. The overlapping mechanics distribute loads across multiple elements, significantly enhancing puncture resistance—up to 10 times greater than equivalent soft structures—and energy absorption, with optimized designs achieving over 200% improvement compared to rigid lattices under low-velocity impacts. Studies on scale-reinforced composites have shown improved impact resistance and reduced back-face deformation at higher volume fractions. This approach balances flexibility for mobility with protection. The iridescent coloration of scales in fish, resulting from multilayer reflectors of platelets, has inspired optical coatings that exploit structural interference for light management. These biomimetic nanostructures mimic the scales' periodic layering to minimize surface reflections, achieving anti-reflective effects. In applications, such coatings enhance capture by reducing losses from , with fish-scale-inspired ZnO morphologies demonstrating multifunctional properties including UV resistance and hydrophobicity alongside optical tuning. Research highlights how these designs, fabricated via templating or , improve light transmittance by emulating the scales' chaotic yet efficient reflector architecture, potentially boosting panel efficiency without traditional dielectric layers. Self-healing materials draw inspiration from the regenerative capacity of fish scales, which renew through epidermal-dermal interactions involving remodeling and mineralization. This biological process has guided the development of matrices incorporating fish-derived or nanoparticles, enabling autonomous repair via microcapsule rupture or dynamic bonds. For instance, scaffolds blending decellularized fish scales with exhibit enhanced osteogenic activity, supporting applications in biomedical composites. These materials prioritize , with teleost-inspired designs showing improved in networks that heal cracks through hydration-induced reconfiguration. Post-2020 advances include 3D-printed structures emulating scute-like scales for , providing programmable stiffness and adaptability. Drawing from fish scale hierarchies, these prosthetics feature modular, overlapping elements that adjust rigidity via pneumatic or phase changes, enhancing grip and impact resistance in soft robots. A 2024 design, inspired by and fish scales, achieves concurrent actuation and sensing for variable compliance, with printed lattices absorbing impacts and an apparent bending modulus change of up to 53 times between soft and stiff states—as of 2024. Such innovations, often using multi- , outperform uniform prosthetics in energy efficiency and durability.

References

  1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7322541/
  2. https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2021.649456/full
  3. https://www.ck12.org/flexi/life-science/fish/what-are-the-roles-of-scales-on-fish/
  4. https://pmc.ncbi.nlm.nih.gov/articles/PMC5221522/
  5. https://pmc.ncbi.nlm.nih.gov/articles/PMC11376312/
  6. https://www.cell.com/matter/fulltext/S2590-2385(20)30244-7
  7. https://drexel.edu/news/archive/2013/march/fossil-species-from-fish-eat-fish-world
  8. https://www.annualreviews.org/doi/pdf/10.1146/annurev-earth-040809-152507
  9. https://www.colorado.edu/lab/barthelat/sites/default/files/attached-files/ab2021.pdf
  10. https://www.mdpi.com/2673-4605/20/1/4
  11. https://www.nature.com/articles/s41598-020-71434-8
  12. https://pubs.aip.org/aip/adv/article/12/3/035218/2819432/Dual-coupling-drag-reduction-inspired-by-tuna-skin
  13. https://www.sciencedirect.com/science/article/pii/S0020768313003843
  14. https://doi.org/10.1121/10.0022565
  15. https://doi.org/10.1057/s41599-023-01854-0
  16. https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/parrotfish
  17. https://doi.org/10.1016/bs.ircmb.2015.09.005
  18. https://www.americanscientist.org/article/new-ideas-about-old-sharks
  19. https://doi.org/10.1371/journal.pone.0172781
  20. https://onlinelibrary.wiley.com/doi/10.1111/pala.12347
  21. https://pmc.ncbi.nlm.nih.gov/articles/PMC6939932/
  22. https://www.scup.com/doi/10.1111/j.1502-3931.2009.00206.x
  23. https://pmc.ncbi.nlm.nih.gov/articles/PMC2736117/
  24. https://doi.org/10.1111/j.1469-7580.2009.01046.x
  25. https://www.researchgate.net/publication/324080928_On_cosmine_Its_origins_biology_and_implications_for_sarcopterygian_interrelationships
  26. https://www.sciencedirect.com/science/article/pii/S1631068315000469
  27. https://doi.org/10.4202/app.2009.1109
  28. https://doi.org/10.3176/earth.2009.4.08
  29. https://pmc.ncbi.nlm.nih.gov/articles/PMC6109381/
  30. https://pmc.ncbi.nlm.nih.gov/articles/PMC5328365/
  31. http://palaeos.com/vertebrates/thelodonti/thelodonti.html
  32. https://australian.museum/learn/animals/fishes/cosmoid-scales/
  33. https://www.brainkart.com/article/Scales---Integumentary-skeleton-of-Fishes_22036/
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC3621673/
  35. https://australian.museum/learn/animals/fishes/placoid-scales/
  36. https://www.mvpozarcollege.edu.in/wp-content/uploads/2025/01/PLACOID-SCALES-FINAL.pdf
  37. https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.908237/full
  38. https://www.mdpi.com/1424-2818/15/11/1105
  39. https://www.thoughtco.com/placoid-scales-definition-2291736
  40. https://pmc.ncbi.nlm.nih.gov/articles/PMC6631580/
  41. https://www.sciencedirect.com/science/article/pii/S2405451816300484
  42. https://repository.library.noaa.gov/view/noaa/3837/noaa_3837_DS1.pdf
  43. https://pmc.ncbi.nlm.nih.gov/articles/PMC3792157/
  44. https://fishionary.fisheries.org/ganoid-scales/
  45. https://australian.museum/learn/animals/fishes/ganoid-scales/
  46. https://pmc.ncbi.nlm.nih.gov/articles/PMC10239528/
  47. https://un.uobasrah.edu.iq/lectures/16204.pdf
  48. https://www.researchgate.net/publication/232680330_Age_growth_and_reproduction_of_spotted_gar_Lepisosteus_oculatus_Lepisosteidae_from_the_Lake_Pontchartrain_Estuary_Louisiana
  49. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1439-0426.2012.01990.x
  50. https://australian.museum/learn/animals/fishes/cycloid-and-ctenoid-scales/
  51. https://fishionary.fisheries.org/ctenoid-scales/
  52. https://pmc.ncbi.nlm.nih.gov/articles/PMC4345507/
  53. https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.200901437
  54. https://fishkeepingnews.com/2025/03/04/260-new-freshwater-fish-species-discovered-in-2024/
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC11015963/
  56. https://manoa.hawaii.edu/exploringourfluidearth/biological/fish/structure-and-function-fish
  57. https://www.fws.gov/story/sturgeon-snoots-and-scutes
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC10372632/
  59. https://pubmed.ncbi.nlm.nih.gov/24836859/
  60. https://onlinelibrary.wiley.com/doi/10.1111/dgd.12196
  61. https://pmc.ncbi.nlm.nih.gov/articles/PMC10423815/
  62. https://www.ncbi.nlm.nih.gov/books/NBK482204/
  63. https://www.tandfonline.com/doi/full/10.1080/15569543.2025.2538137
  64. https://royalsocietypublishing.org/doi/10.1098/rsbl.2013.0234
  65. https://pmc.ncbi.nlm.nih.gov/articles/PMC3645053/
  66. https://escholarship.org/content/qt77c500qh/qt77c500qh_noSplash_71e3939ed8acc0d7d48b2060ff3736a9.pdf
  67. https://pmc.ncbi.nlm.nih.gov/articles/PMC4310352/
  68. https://pmc.ncbi.nlm.nih.gov/articles/PMC5740887/
  69. https://pubmed.ncbi.nlm.nih.gov/8876820/
  70. https://www.sciencedirect.com/science/article/pii/S0012160617305821
  71. https://pmc.ncbi.nlm.nih.gov/articles/PMC1467640/
  72. https://www.sciencedirect.com/science/article/pii/S0960982213002637
  73. https://elifesciences.org/articles/37001
  74. https://pmc.ncbi.nlm.nih.gov/articles/PMC10560567/
  75. https://www.nature.com/articles/ncomms2643
  76. https://www.pnas.org/doi/10.1073/pnas.1713827114
  77. https://pubmed.ncbi.nlm.nih.gov/7926859/
  78. https://www.canr.msu.edu/resources/determining_the_age_of_fish_e1774
  79. https://academic.oup.com/icesjms/article/66/10/2165/679802
  80. https://link.springer.com/article/10.1111/j.1444-2906.2007.01300.x
  81. https://www.sciencedirect.com/science/article/pii/0300962993902623
  82. https://dep.nj.gov/wp-content/uploads/njfw/fish-scale-tale.pdf
  83. https://www.researchgate.net/profile/Philippe-Janvier/publication/300551099_Fossil_hagfishes_fossil_cyclostomes_and_the_lost_world_of_ostracoderms/links/57713d5108ae842225ac0d4d/Fossil-hagfishes-fossil-cyclostomes-and-the-lost-world-of-ostracoderms.pdf
  84. http://home.miracosta.edu/kmeldahl/articles/slime.pdf
  85. https://pmc.ncbi.nlm.nih.gov/articles/PMC10153176/
  86. https://www.zoology.ubc.ca/~woodcm/Woodblog/wp-content/uploads/2016/07/Wang-et-al-2003-CBP-A.pdf
  87. https://pmc.ncbi.nlm.nih.gov/articles/PMC11837439/
  88. https://www.nature.com/articles/srep09780
  89. https://pmc.ncbi.nlm.nih.gov/articles/PMC2542418/
  90. https://www.sciencedirect.com/science/article/pii/S0960982201003244
  91. https://www.researchgate.net/publication/327934822_Adaptive_Evolution_of_the_Eda_Gene_and_Scales_Loss_in_Schizothoracine_Fishes_in_Response_to_Uplift_of_the_Tibetan_Plateau
  92. https://www.omicsonline.org/open-access-pdfs/exploring-the-secrets-of-the-dojo-loach-misgurnus-anguillicaudatus-biology-behavior-and-ecological-significance.pdf
  93. https://www.researchgate.net/publication/368046849_Viscoelastic_characterization_of_the_mucus_from_the_skin_of_loach
  94. https://ornamentalfish.org/wp-content/uploads/Loaches.pdf
  95. https://journals.biologists.com/jeb/article/208/24/4757/15917/Functional-morphology-of-feeding-in-the-scale
  96. https://bmcbiol.biomedcentral.com/articles/10.1186/1741-7007-8-8
  97. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0044670
  98. https://www.fisheriesjournal.com/archives/2021/vol9issue3/PartA/9-2-42-318.pdf
  99. https://pmc.ncbi.nlm.nih.gov/articles/PMC9780470/
  100. https://www.researchgate.net/publication/325061221_Ecology_shapes_the_evolutionary_trade-off_between_predator_avoidance_and_defence_in_coral_reef_butterflyfishes
  101. https://pmc.ncbi.nlm.nih.gov/articles/PMC7097200/
  102. https://royalsocietypublishing.org/doi/10.1098/rsta.2010.0201
  103. https://ntrs.nasa.gov/citations/19880005573
  104. https://ntrs.nasa.gov/search.jsp?R=19860013418
  105. https://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2024_04_18_Aeroshark.pdf
  106. https://www.jafmonline.net/article_626_cfd16cf3b5b62dc57381bf6032ffa045.pdf
  107. https://pmc.ncbi.nlm.nih.gov/articles/PMC11852495/
  108. https://pmc.ncbi.nlm.nih.gov/articles/PMC10975718/
  109. https://dspace.mit.edu/bitstream/handle/1721.1/74456/813318838-MIT.pdf?sequence=2&isAllowed=y
  110. https://www.mdpi.com/2073-4360/15/7/1614
  111. https://www.nature.com/articles/am2015133
  112. https://discovery.researcher.life/article/silvery-fish-skin-as-an-example-of-a-chaotic-reflector/f8d90723e8b33531a0fdc24cb6143214
  113. https://www.mdpi.com/2073-4360/14/13/2532
  114. https://www.science.org/doi/10.1126/scirobotics.adl0307
  115. https://pmc.ncbi.nlm.nih.gov/articles/PMC12292793/
Add your contribution
Related Hubs
User Avatar
No comments yet.