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Fins typically function as foils that provide lift or thrust, or provide the ability to steer or stabilize motion in water or air.

A fin is a thin appendage or component attached to a larger body or structure.[1] Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fins are also used to increase surface areas for heat transfer purposes, or simply as ornamentation.[2][3]

Fins first evolved on fish as a means of locomotion. Fish fins are used to generate thrust and control the subsequent motion. Fish and other aquatic animals, such as cetaceans, actively propel and steer themselves with pectoral and tail fins. As they swim, they use other fins, such as dorsal and anal fins, to achieve stability and refine their maneuvering.[4][5]

The fins on the tails of cetaceans, ichthyosaurs, metriorhynchids, mosasaurs and plesiosaurs are called flukes.

Thrust generation

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Foil shaped fins generate thrust when moved, the lift of the fin sets water or air in motion and pushes the fin in the opposite direction. Aquatic animals get significant thrust by moving fins back and forth in water. Often the tail fin is used, but some aquatic animals generate thrust from pectoral fins.[4] Fins can also generate thrust if they are rotated in air or water. Turbines and propellers (and sometimes fans and pumps) use a number of rotating fins, also called foils, wings, arms or blades. Propellers use the fins to translate torquing force to lateral thrust, thus propelling an aircraft or ship.[6] Turbines work in reverse, using the lift of the blades to generate torque and power from moving gases or water.[7]

Moving fins can provide thrust
Fish get thrust moving vertical tail fins from side to side.
Cetaceans get thrust moving horizontal tail fins up and down.
Stingrays get thrust from large pectoral fins.
Ship propeller
Compressor fins (blades)
Cavitation damage is evident on this propeller.
Drawing by Dr Tony Ayling
Finlets may influence the way a vortex develops around the tail fin.

Cavitation can be a problem with high power applications, resulting in damage to propellers or turbines, as well as noise and loss of power.[8] Cavitation occurs when negative pressure causes bubbles (cavities) to form in a liquid, which then promptly and violently collapse. It can cause significant damage and wear.[8] Cavitation damage can also occur to the tail fins of powerful swimming marine animals, such as dolphins and tuna. Cavitation is more likely to occur near the surface of the ocean, where the ambient water pressure is relatively low. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful.[9] Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.[9]

Scombrid fishes (tuna, mackerel and bonito) are particularly high-performance swimmers. Along the margin at the rear of their bodies is a line of small rayless, non-retractable fins, known as finlets. There has been much speculation about the function of these finlets. Research done in 2000 and 2001 by Nauen and Lauder indicated that "the finlets have a hydrodynamic effect on local flow during steady swimming" and that "the most posterior finlet is oriented to redirect flow into the developing tail vortex, which may increase thrust produced by the tail of swimming mackerel".[10][11][12]

Fish use multiple fins, so it is possible that a given fin can have a hydrodynamic interaction with another fin. In particular, the fins immediately upstream of the caudal (tail) fin may be proximate fins that can directly affect the flow dynamics at the caudal fin. In 2011, researchers using volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes". They found that "continuous tail beats resulted in the formation of a linked chain of vortex rings" and that "the dorsal and anal fin wakes are rapidly entrained by the caudal fin wake, approximately within the timeframe of a subsequent tail beat".[13]

Motion control

[edit]
Fins are used by aquatic animals, such as this orca, to generate thrust and control the subsequent motion.[14][15]

Once motion has been established, the motion itself can be controlled with the use of other fins.[4][16][17] Boats control direction (yaw) with fin-like rudders, and roll with stabilizer and keel fins.[16] Airplanes achieve similar results with small specialised fins that change the shape of their wings and tail fins.[17]

Specialised fins are used to control motion
Fish, boats and airplanes need control of three degrees of rotational freedom.[18][19][20]
The dorsal fin of a white shark contain dermal fibers that work "like riggings that stabilize a ship's mast", and stiffen dynamically as the shark swims faster to control roll and yaw.[21]
Caudal fin of a great white shark
A rudder corrects yaw.
A fin keel limits roll and sideways drift.
Ship stabilising fins reduce roll.
Ailerons control roll.
Elevators control pitch.
The rudder controls yaw.

Stabilising fins are used as fletching on arrows and some darts,[22] and at the rear of some bombs, missiles, rockets and self-propelled torpedoes.[23][24] These are typically planar and shaped like small wings, although grid fins are sometimes used.[25] Static fins have also been used for one satellite, GOCE.

Static tail fins are used as stabilizers
Fletching on an arrow
Asymmetric stabilizing fins impart spin to this Soviet artillery rocket
Conventional "planar" fins on a RIM-7 Sea Sparrow missile

Temperature regulation

[edit]

Engineering fins are also used as heat transfer fins to regulate temperature in heat sinks or fin radiators.[26][27]

Fins can regulate temperature
Motorbikes use fins to cool the engine.[28]
Oil heaters convect with fins.
Sailfish raise their dorsal fin to cool down or to herd schooling fish.[29][30]

Ornamentation and other uses

[edit]

In biology, fins can have an adaptive significance as sexual ornaments. During courtship, the female cichlid, Pelvicachromis taeniatus, displays a large and visually arresting purple pelvic fin. "The researchers found that males clearly preferred females with a larger pelvic fin and that pelvic fins grew in a more disproportionate way than other fins on female fish."[31][32]

Ornamentation
During courtship, the female cichlid, Pelvicachromis taeniatus, displays her visually arresting purple pelvic fin.
Spinosaurus may have used its dorsal fin (sail) as a courtship display.[33]: 28 
Car tail fins in the 1950s were largely decorative.[34]

Reshaping human feet with swim fins, rather like the tail fin of a fish, add thrust and efficiency to the kicks of a swimmer or underwater diver[35][36] Surfboard fins provide surfers with means to maneuver and control their boards. Contemporary surfboards often have a centre fin and two cambered side fins.[37]

The bodies of reef fishes are often shaped differently from open water fishes. Open water fishes are usually built for speed, streamlined like torpedoes to minimise friction as they move through the water. Reef fish operate in the relatively confined spaces and complex underwater landscapes of coral reefs. For this manoeuvrability is more important than straight line speed, so coral reef fish have developed bodies which optimize their ability to dart and change direction. They outwit predators by dodging into fissures in the reef or playing hide and seek around coral heads.[38]

The pectoral and pelvic fins of many reef fish, such as butterflyfish, damselfish and angelfish, have evolved so they can act as brakes and allow complex maneuvers.[39] Many reef fish, such as butterflyfish, damselfish and angelfish, have evolved bodies which are deep and laterally compressed like a pancake, and will fit into fissures in rocks. Their pelvic and pectoral fins are designed differently, so they act together with the flattened body to optimise maneuverability.[38] Some fishes, such as puffer fish, filefish and trunkfish, rely on pectoral fins for swimming and hardly use tail fins at all.[39]

Other uses
Swim fins add thrust to the kicks of a human swimmer.
Surfboard fins allow surfers to maneuver their boards.
In some Asian countries shark fins are a culinary delicacy.[40]
In recent years, car fins have evolved into highly functional spoilers and wings.[41]
Many reef fish have pectoral and pelvic fins optimised for flattened bodies.[38]
Frog fish use their pectoral and pelvic fins to walk along the ocean bottom.[42]
Flying fish use enlarged pectoral fins to glide above the surface of the water.[43]

Evolution

[edit]
Aquatic animals typically use fins for locomotion
(1) pectoral fins (paired), (2) pelvic fins (paired), (3) dorsal fin, (4) adipose fin, (5) anal fin, and (6) caudal (tail) fin.

Aristotle recognised the distinction between analogous and homologous structures, and made the following prophetic comparison: "Birds in a way resemble fishes. For birds have their wings in the upper part of their bodies and fishes have two fins in the front part of their bodies. Birds have feet on their underpart and most fishes have a second pair of fins in their under-part and near their front fins."

– Aristotle, De incessu animalium [44]

There is an old theory, proposed by anatomist Carl Gegenbaur, which has been often disregarded in science textbooks, "that fins and (later) limbs evolved from the gills of an extinct vertebrate". Gaps in the fossil record had not allowed a definitive conclusion. In 2009, researchers from the University of Chicago found evidence that the "genetic architecture of gills, fins and limbs is the same", and that "the skeleton of any appendage off the body of an animal is probably patterned by the developmental genetic program that we have traced back to formation of gills in sharks".[45][46][47] Recent studies support the idea that gill arches and paired fins are serially homologous and thus that fins may have evolved from gill tissues.[48]

Fish are the ancestors of all mammals, reptiles, birds and amphibians.[49] In particular, terrestrial tetrapods (four-legged animals) evolved from fish and made their first forays onto land 400 million years ago. They used paired pectoral and pelvic fins for locomotion. The pectoral fins developed into forelegs (arms in the case of humans) and the pelvic fins developed into hind legs.[50] Much of the genetic machinery that builds a walking limb in a tetrapod is already present in the swimming fin of a fish.[51][52]

Comparison between A) the swimming fin of a lobe-finned fish and B) the walking leg of a tetrapod. Bones considered to correspond with each other have the same color.
In a parallel but independent evolution, the ancient reptile Ichthyosaurus communis developed fins (or flippers) very similar to fish (or dolphins).

In 2011, researchers at Monash University in Australia used primitive but still living lungfish "to trace the evolution of pelvic fin muscles to find out how the load-bearing hind limbs of the tetrapods evolved."[53][54] Further research at the University of Chicago found bottom-walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods.[55][56]

In a classic example of convergent evolution, the pectoral limbs of pterosaurs, birds and bats further evolved along independent paths into flying wings. Even with flying wings there are many similarities with walking legs, and core aspects of the genetic blueprint of the pectoral fin have been retained.[57][58]

About 200 million years ago the first mammals appeared. A group of these mammals started returning to the sea about 52 million years ago, thus completing a circle. These are the cetaceans (whales, dolphins and porpoises). Recent DNA analysis suggests that cetaceans evolved from within the even-toed ungulates, and that they share a common ancestor with the hippopotamus.[59][60] About 23 million years ago another group of bearlike land mammals started returning to the sea. These were the pinnipeds (seals).[61] What had become walking limbs in cetaceans and seals evolved further, independently in a reverse form of convergent evolution, back to new forms of swimming fins. The forelimbs became flippers and, in pinnipeds, the hind limbs became a tail terminating in two fins (the cetacean fluke, conversely, is an entirely new organ).[62] Fish tails are usually vertical and move from side to side. Cetacean flukes are horizontal and move up and down, because cetacean spines bend the same way as in other mammals.[63][64]

Ichthyosaurs are ancient reptiles that resembled dolphins. They first appeared about 245 million years ago and disappeared about 90 million years ago.

"This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing — the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor."[65]

The biologist Stephen Jay Gould said the ichthyosaur was his favorite example of convergent evolution.[66]

Robotics

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In the 1990s the CIA built a robotic catfish called Charlie to test the feasibility of unmanned underwater vehicles.
External videos
video icon Charlie the catfish – CIA video
video icon AquaPenguin – Festo, YouTube
video icon AquaRay – Festo, YouTube
video icon AquaJelly – Festo, YouTube
video icon AiraCuda – Festo, YouTube

The use of fins for the propulsion of aquatic animals can be remarkably effective. It has been calculated that some fish can achieve a propulsive efficiency greater than 90%.[4] Fish can accelerate and maneuver much more effectively than boats or submarine, and produce less water disturbance and noise. This has led to biomimetic studies of underwater robots which attempt to emulate the locomotion of aquatic animals.[67] An example is the Robot Tuna built by the Institute of Field Robotics, to analyze and mathematically model thunniform motion.[68] In 2005, the Sea Life London Aquarium displayed three robotic fish created by the computer science department at the University of Essex. The fish were designed to be autonomous, swimming around and avoiding obstacles like real fish. Their creator claimed that he was trying to combine "the speed of tuna, acceleration of a pike, and the navigating skills of an eel".[69][70][71]

The AquaPenguin, developed by Festo of Germany, copies the streamlined shape and propulsion by front flippers of penguins.[72][73] Festo also developed AquaRay,[74] AquaJelly[75] and AiraCuda,[76] respectively emulating the locomotion of manta rays, jellyfish and barracuda.

In 2004, Hugh Herr at MIT prototyped a biomechatronic robotic fish with a living actuator by surgically transplanting muscles from frog legs to the robot and then making the robot swim by pulsing the muscle fibers with electricity.[77][78]

Robotic fish offer some research advantages, such as the ability to examine part of a fish design in isolation from the rest, and variance of a single parameter, such as flexibility or direction. Researchers can directly measure forces more easily than in live fish. "Robotic devices also facilitate three-dimensional kinematic studies and correlated hydrodynamic analyses, as the location of the locomotor surface can be known accurately. And, individual components of a natural motion (such as outstroke vs. instroke of a flapping appendage) can be programmed separately, which is certainly difficult to achieve when working with a live animal."[79]

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fin

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from Grokipedia
A fin is a thin, membranous extending from the body of many aquatic animals, particularly , consisting of supported by bony or cartilaginous rays or spines and controlled by underlying muscles; it serves essential roles in locomotion, balance, , and sensory functions. Fins are classified into unpaired and paired types, with unpaired fins including the (for stability and protection), caudal fin (for propulsion), and anal fin (for steering and balance), while paired fins comprise the pectoral and pelvic fins (analogous to limbs in tetrapods, aiding in maneuvering and braking). In , fins exhibit diverse structures adapted to specific environments and behaviors; for instance, the caudal fin's shape varies from forked in fast-swimming to rounded in more maneuverable ones, directly influencing hydrodynamic . Some , like certain salmonids, possess an adipose fin—a small, fleshy dorsal structure without rays—whose exact function remains under study but may aid in sensory perception or stability. Fin development in fish, such as the pectoral fin, mirrors evolutionary patterns seen in limbs, originating from fin folds in embryonic stages and involving complex vascular and skeletal formation. Beyond locomotion, fins play critical roles in , display, and defense; males of many use elongated fins for rituals, while spines in fins of predatory fish like deter attackers. Evolutionary adaptations have led to fin loss or modification in some aquatic vertebrates, such as eels, where reduced fins prioritize undulatory swimming over fin-based propulsion.

Anatomy and Types

Structure of Fins

Fins in aquatic animals, particularly , are primarily composed of a thin or web of that stretches between supportive skeletal elements, enabling flexibility and interaction with water. In ray-finned (), these supports are fin rays known as lepidotrichia, which are dermal structures formed from two opposing hemirays that articulate at overlapping joints, creating segmented, flexible rays capable of independent movement. In contrast, chondrichthyans such as and rays feature fins supported by cartilaginous radials—elongated, segmented elements that radiate from the fin base—along with fibrous ceratotrichia that provide additional rigidity without bony . The key structural components of fins include the , which faces the direction of motion and may incorporate stiff spines for reinforcement or protection in certain ; the trailing edge, where the membrane tapers to minimize drag; and the base, which attaches to the body via muscles, ligaments, and skeletal girdles such as the pectoral or pelvic structures, without direct articulation to the in most cases. Variations in these components, such as anterior spines on the , enhance durability while maintaining flexibility through the . At the microscopic level, fin surfaces are richly vascularized with blood vessels that supply nutrients and oxygen to the tissues, intertwined with that innervate sensory receptors and control musculature for precise movements. In bony , the skin covering includes overlapping scales—typically or ctenoid—that reduce friction and protect underlying structures, while shark fins bear dermal denticles, tooth-like placoid scales embedded in the that provide abrasion resistance and hydrodynamic benefits. For example, the in consists of a rigid, triangular supported by cartilaginous radials and often an anterior spine, with the entire surface clad in dermal denticles for enhanced toughness. In comparison, the of ray-finned fish forms flexible, fan-like lobes through arrays of lepidotrichia that allow undulating motion, covered by smooth scales that facilitate seamless water flow.

Classification of Fins

Fins in vertebrates are broadly classified into and paired types based on their anatomical position and developmental origins. Median fins are unpaired structures located along the body's midline, including the on the back, the anal fin on the ventral surface anterior to the tail, and the caudal fin at the posterior end. These fins typically consist of soft rays supported by fin membranes and contribute to overall body orientation. Paired fins, in contrast, occur symmetrically on either side of the body and include the pectoral fins positioned behind the gills and the pelvic fins located ventrally further posterior. These structures are evolutionarily homologous to the limbs of tetrapods, sharing developmental pathways involving similar genetic controls such as . From an evolutionary perspective, fins are further categorized by their structural composition and phylogenetic distribution within bony fishes (). Ray-finned fishes (), comprising over 30,000 species, feature fins supported by slender, unbranched or branched lepidotrichia (fin rays), enabling flexible and lightweight appendages. In contrast, lobe-finned fishes (), a smaller group including coelacanths and lungfishes, possess fleshy fins with robust internal bones resembling limb precursors, often lacking extensive ray support. Some species exhibit vestigial fins, such as in certain eels (e.g., swamp eels in Synbranchidae), where pectoral and pelvic fins are absent and dorsal/anal fins are reduced to rudimentary ridges. Fin morphology varies significantly across taxa, reflecting adaptations to aquatic lifestyles. In fishes, caudal fins often display homocercal , with equal upper and lower lobes for balanced propulsion in advanced species like . Primitive fishes, such as and early bony fishes, typically have heterocercal caudal fins, where the upper lobe is enlarged and the vertebral column extends into it, aiding lift in ancestral forms. Cetaceans, fully aquatic mammals, retain a for stability, as seen in dolphins and whales, while their paired appendages are modified into flippers without true fin rays. Ichthyosaurs, extinct marine reptiles, evolved paddle-like paired fins from forelimbs and possible dorsal fins for streamlined swimming. In amphibians and reptiles, fins are generally absent, replaced by limbs derived from paired fin homologues during the fin-to-limb transition, though some aquatic forms like sea turtles exhibit flipper modifications.

Primary Functions in Locomotion

Thrust Generation

Fins generate forward propulsion in aquatic animals primarily through oscillatory motions that exploit hydrodynamic interactions with , producing net forces directed rearward on the fluid and forward on the body. The caudal fin is the dominant structure for this in most fishes, oscillating laterally to shed vortices into the wake, forming linked vortex rings or loops that create a reactive jet propelling the animal forward. This vortex shedding mechanism is evident in the wakes of freely swimming fishes, where each tail beat expels coherent vortical structures, enhancing transfer. In undulatory swimmers, pectoral fins contribute to via coordinated flapping or motions that interact with body waves, recapturing wake vortices to boost overall efficiency. Hydrodynamic thrust arises from two key principles: lift-based generation and reactive forces from fluid displacement. In lift-based propulsion, fins function as dynamic airfoils, with oscillatory angles of attack creating pressure differences across the fin surface; faster flow over the curved upper surface lowers pressure per , yielding upward lift that, when inclined, provides rearward . Reactive forces, meanwhile, stem from the of accelerated by the fin's motion, producing a via change as fluid is displaced laterally or posteriorly during beats. These mechanisms combine in most species, with vortex dynamics—such as leading-edge vortices stabilizing flow—amplifying lift and during the oscillatory cycle. Efficiency in thrust generation depends on fin morphology and kinematics, notably the aspect ratio (span squared divided by area, or length-to-width) and stroke phasing. High-aspect-ratio caudal fins, as in fast cruisers, minimize induced drag while maximizing lift for sustained high speeds, achieving efficiencies up to 60-70% in cruising. Low-aspect-ratio fins favor maneuverability but at the cost of cruising efficiency. In flapping propulsion, the power stroke (typically the downstroke or high-angle phase) generates most thrust through strong vortex shedding, while the recovery stroke feathers the fin to reduce drag; at higher speeds, both strokes become active, with upstrokes contributing via reversed angles of attack. Swimming modes illustrate these principles' variations: thunniform locomotion in relies on high-frequency, low-amplitude caudal oscillations for thrust-dominated , enabling speeds over 10 body lengths per second with body stability. Conversely, anguilliform mode in eels uses full-body undulations propagating as waves, with the amplifying through vortex rings but distributing effort for flexibility in low-speed environments. The magnitude of thrust can be quantified using the equation T=12ρv2ACTT = \frac{1}{2} \rho v^2 A C_T where TT is force, ρ\rho is water density, vv is the fin's through water, AA is effective fin area, and CTC_T is the dimensionless encapsulating shape, kinematics, and flow effects (typically 0.2-1.0 for efficient swimmers). This formulation highlights how scales with and geometry, guiding evolutionary adaptations for speed versus efficiency.

Motion Control

Fins facilitate steering in aquatic locomotion primarily through the asymmetric deployment of paired structures, allowing to generate differential hydrodynamic forces for directional changes. Pectoral and pelvic fins, positioned laterally, enable control over yaw (lateral turning), pitch (nose-up or nose-down adjustments), and roll (tilting along the longitudinal axis) by varying their or oscillation amplitude on one side relative to the other. In sunfish ( macrochirus), for instance, pectoral fins actively modulate these rotational to maintain precise orientation during steady swimming or evasion maneuvers. Dorsal and anal fins, located medially, contribute to yaw stability by acting as passive keels that resist unwanted lateral deviations, particularly during high-speed travel. Stability during swimming is achieved through strategic fin placement that dampens oscillatory motions and balances hydrodynamic forces. Fins positioned posterior to the center of gravity (COG) shift the center of pressure (COP)—the point where net hydrodynamic force acts—aft of the COG, promoting inherent static stability and preventing uncontrolled tumbling or rolling. This configuration generates restoring moments that counteract perturbations, such as currents or sudden accelerations, while drag from fin surfaces further damps dynamic oscillations in pitch and yaw. In sharks, for example, the pectoral fins' forward positioning relative to the caudal fin helps trim the body by adjusting the COP to offset the upward lift from the heterocercal tail, ensuring level progression. Fins enhance maneuverability by enabling rapid, high-agility turns distinct from linear . In many coral-reef fishes, pectoral fins facilitate tight turns with radii as small as 0.2–0.5 body lengths through independent flapping or feathering, allowing precise amid complex habitats. Larger cetaceans, such as humpback whales (Megaptera novaeangliae), employ elongate pectoral fins to generate lift during acrobatic maneuvers like breaching, where asymmetric fin extension provides the necessary for body rotation and reorientation out of water. Sensory feedback from fins integrates with for real-time adjustments during motion. Fin rays contain arrays of mechanosensory neurons that detect , , and flow, functioning as proprioceptors to monitor fin position and external loads relative to the body. This proprioceptive input allows to reflexively modulate fin , such as altering to correct for drag imbalances during turns, thereby maintaining equilibrium without relying solely on visual or cues. The effectiveness of fin-based turning is quantitatively linked to the moment arm of the applied force, influencing the minimum achievable . (τ\tau) generated by a fin is given by τ=r×F\tau = r \times F, where rr is the perpendicular distance (lever arm) from the rotation axis (typically near the COG) to the FF produced by the fin. Longer moment arms, as in elongated pectoral fins, amplify for tighter radii, enabling agile species like reef fish to execute turns with radii under 0.3 body lengths, while shorter arms in streamlined swimmers prioritize speed over sharp maneuvers.

Secondary Biological Roles

Temperature Regulation

Fins facilitate in aquatic animals primarily through their high surface-to-volume ratio, which enhances heat exchange with the surrounding , acting as efficient radiators for both heat gain and loss. In species with specialized vascular arrangements, countercurrent heat exchange in fin blood vessels allows to warm cooler returning from the periphery, thereby conserving metabolic heat and minimizing passive loss to colder environments. This mechanism is particularly vital in maintaining thermal gradients, where the fin's vascular structure—briefly referenced in anatomical studies—enables precise control over . For ectothermic aquatic animals, such as most , fins play a key role in absorbing environmental heat to elevate body temperature above ambient levels. During basking behaviors, species like common carp ( carpio) position their fins near the water surface to capture solar radiation, resulting in body temperatures 0.7–2.2°C warmer than surrounding water and supporting faster growth rates. Similarly, basking sharks (Cetorhinus maximus), traditionally viewed as ectotherms, exhibit surface-oriented behaviors that expose fins to warmer waters, aiding initial heat uptake despite their emerging regional endothermic traits. In endothermic species like tunas (Thunnus spp.), adaptations such as intricate vascular patterns enable controlled cooling to counteract overheating from elevated metabolic rates. These can rapidly adjust whole-body thermal conductivity by up to two orders of magnitude, to balance internal production with environmental conditions. Vascular countercurrent systems further prevent excessive heat loss during dives into cooler waters while allowing dissipation when needed. Physiological mechanisms in fin tissues, including to increase blood flow for enhanced cooling and to reduce flow for heat retention, help maintain core temperature gradients across varying activity levels. Environmental water temperature significantly influences these processes, with colder conditions triggering reduced fin circulation rates to conserve heat and warmer waters promoting increased flow to avert . In tunas, such adjustments ensure stable muscle temperatures despite ambient shifts of 10–15°C.

Ornamentation and Sensory Uses

In many fish species, fins serve ornamental roles through vibrant coloration and exaggerated shapes that enhance mating displays. Male guppies (Poecilia reticulata) exhibit polymorphic coloration on their caudal fins, with orange, black, and iridescent patches that are actively displayed during sigmoid courtship behaviors to attract females. These ornaments signal genetic quality and are preferred by females, influencing mate choice. Similarly, in threadfin rainbowfish (Iriatherina werneri), males possess elongated, filamentous dorsal and anal fins that are flared during both courtship and intrasexual competitions, amplifying visual appeal. Fins also facilitate threat postures and aggressive signaling. (family Balistidae), such as the gray triggerfish (Balistes capriscus), can erect and lock the spines of their first into a rigid position when threatened, deterring predators by anchoring themselves in crevices or displaying an imposing silhouette. In poeciliid fishes like sailfin mollies ( latipinna), males extend their enlarged s during confrontations to intimidate rivals, a linked to establishing dominance. Beyond static ornamentation, fins play key roles in dynamic communication. Fin flicking and flaring serve as visual signals for and across species; for instance, male guppies flare their fins in circular swims to court females, while in (Betta splendens), opercular and fin flaring escalates during male-male to assess opponent intent. In characin fishes like the (Hemigrammus erythrozonus), rapid fin flicking acts as an alarm signal to conspecifics, indicating predator detection and prompting evasive responses. Deep-sea species further employ in fins for signaling; viperfish (Chauliodus sloani) possess a glowing lure on their dorsal fin ray, which may facilitate mate attraction or species recognition in low-light environments. Fins contribute to sensory functions through specialized receptors that detect environmental cues. Paired fins in damselfishes (family Pomacentridae) bear extraoral , enabling short-range chemoreception to identify prey or suitable habitats via dissolved chemicals in water. These gustatory structures, integrated with the fish's system, allow precise localization of food sources. In sharks and other elasmobranchs, the —electroreceptive pores concentrated around the head—detect weak bioelectric fields from prey muscle contractions, aiding hunting even in murky waters. Camouflage via fins involves adaptive patterns generated by chromatophores, pigment cells that enable rapid color shifts. Flatfishes like the peacock flounder (Bothus lunatus) use dermal chromatophores to match fin patterns to sandy or rocky substrates, reducing visibility to predators through disruptive coloration. In reef fishes such as wrasses, fin chromatophores produce mottled or banded patterns that blend with coral environments, with neural control allowing instantaneous adjustments to background changes. Peacock-like displays, seen in male peacock gudgeons (Tateurndina ocellicauda), contrast this by temporarily overriding camouflage for courtship, flaring iridescent fins to reveal bold spots and edges.

Evolutionary Development

Origins of Fins

Fins originated in early chordates during the , with median fins evolving as dorsal and ventral structures from a continuous median fin fold around 535 million years ago, providing stability and propulsion in primitive swimming. Fossils from the Middle Cambrian , such as Pikaia gracilens, reveal these early chordates possessed a notochord-supported body with tail structures resembling simple fins, enabling through myomere contractions. This transition from notochord-based support to fin-augmented structures marked a key step in vertebrate swimming efficiency, predating more specialized appendages. Paired fins emerged later in the period, approximately 420 million years ago, among jawless vertebrates known as ostracoderms, particularly in osteostracans, where pectoral fin-like extensions arose from the pectoral girdle for enhanced maneuverability. These structures were homologous to the limb girdles in later vertebrates, consisting of dermal and endoskeletal elements that anchored to the body wall. In placoderms, the first jawed vertebrates during the , fins underwent elaboration with the addition of paired pelvic fins supported by internal girdles, increasing diversity in locomotion and body control. However, paired fins were subsequently lost in certain jawless lineages, such as modern lampreys and , where regulatory genes like Tbx5 fail to extend expression into the necessary for appendage initiation. Developmentally, fin origins involve conserved genetic mechanisms, with regulating the positioning and patterning of fin buds during embryogenesis in . In , posterior Hoxa and Hoxd cluster genes display tri-phasic expression in pectoral fin , initiating bud formation and proximal-distal outgrowth akin to early limb development. The apical ectodermal ridge (AER), a thickened ectodermal fold at the fin bud's distal margin, secretes signaling molecules like Fgf to sustain mesenchymal proliferation and prevent , ensuring proper fin elongation. Comparative embryology highlights the homology between fish fins and tetrapod limbs, particularly in sarcopterygians like lungfish and coelacanths, where shared Hox expression domains and AER-like structures underpin the fin-to-limb transition around 390 million years ago. In these lobe-finned fish, fin endoskeletons exhibit proximodistal patterning similar to limb buds, with genetic modules enabling the evolutionary innovation of digits from fin radials. This developmental framework illustrates how fins provided the scaffold for terrestrial appendage evolution without altering core patterning genes.

Adaptations and Diversity

Fins exhibit remarkable morphological and functional diversity shaped by environmental pressures, enabling species to exploit varied aquatic niches. In fast-swimming such as tunas and mackerels, fins are streamlined and to minimize hydrodynamic drag, facilitating sustained high-speed through open water. Conversely, bottom-dwelling species like sea robins (Prionotus spp.) possess enlarged pectoral fins that aid in substrate manipulation, , and slow maneuvering over benthic environments. This contrast highlights how fin shape correlates with demands, with pelagic forms prioritizing and demersal ones emphasizing stability and utility. A striking example of specialized environmental is the ribbon-like anal fin in gymnotiform knifefishes (e.g., Eigenmannia spp.), which integrates myogenic electric organs derived from modified muscle tissue to generate for and communication in murky freshwater habitats. These electric fin rays, comprising up to 150 undulating segments, produce weak discharges (around 1-10 V) that support electrolocation without relying on traditional vision. Such innovations underscore the evolutionary plasticity of fins beyond mere propulsion, adapting to sensory challenges in low-visibility ecosystems. Beyond , fin-like structures have convergently evolved in other vertebrates, demonstrating broad . In cetaceans like dolphins (Delphinidae), flukes represent modified tail structures analogous to caudal fins, providing thrust via oscillatory movements while the body maintains streamlining for efficient cruising. Similarly, (Spheniscidae) have transformed forelimbs into rigid, flattened flippers derived from avian wings, optimized for rapid underwater through reduced drag and enhanced lift in polar marine environments. These non-homologous appendages illustrate how selective pressures for aquatic locomotion can repurpose diverse anatomical precursors across taxa. Sexual dimorphism further diversifies fin morphology, often driven by reproductive competition. In many species, males develop elaborate, elongated fins—such as the extended dorsal and anal fins in male guppies (Poecilia reticulata)—to attract females or intimidate rivals during displays. fishes exhibit polymorphic fins, where color and shape variations (e.g., in labrids like Thalassoma bifasciatum) signal alternative strategies, with terminal-phase males sporting brighter, larger fins for defense. These traits enhance success but impose energetic costs, balancing display with survival. Pathological alterations reveal fins' vulnerability to external stressors and regenerative potential. Teleost fishes, including (Danio rerio), demonstrate robust fin regeneration, restoring amputated caudal fins to near-original size within 2-3 weeks via formation and signaling pathways like Wnt/β-catenin. This capacity, conserved across actinopterygians, involves coordinated proliferation of epidermal and mesenchymal cells. However, pollution induces deformities; exposure to contaminants like in California's Sacramento-San Joaquin Delta causes spinal and fin malformations in native fish such as (Hypomesus transpacificus), reducing mobility and survival. Heavy metals similarly lead to eroded or absent fins in polluted waters, as observed in Swedish coastal . Evolutionary trends in fin morphology reflect long-term adaptations to extreme conditions. Cave-dwelling populations of the Mexican tetra (Astyanax mexicanus) show pelvic fin reduction or loss, an energy-saving regression in nutrient-poor, dark environments where vision and active swimming are deprioritized. In contrast, (Exocoetidae) exhibit hyper-specialized pectoral fins, hypertrophied into wing-like structures spanning up to 40% of body length, enabling glides of 200-400 meters to evade predators. These enlarged, asymmetrical fins generate lift at speeds of 15-20 m/s, with pelvic fins providing additional stability during aerial phases. Such polarizations—from simplification to elaboration—exemplify fins' role in niche specialization across evolutionary timescales.

Contemporary Applications

Biomimicry in Robotics

Biomimicry in robotics draws inspiration from fish fins to develop propulsion systems for underwater vehicles, emphasizing flexible structures that replicate the undulating and oscillating motions observed in aquatic locomotion. These designs often incorporate soft robotics principles, utilizing compliant materials such as silicone or elastomers to mimic the ray-supported architecture of pectoral or caudal fins, enabling distributed deformation along the fin surface for enhanced hydrodynamic interaction. Oscillating fin propulsors, in particular, emulate the flapping or waving patterns of biological fins, where actuators drive periodic motions to generate thrust through vortex shedding and lift forces. This approach contrasts with rigid propellers by allowing adaptive shaping in response to flow conditions, as demonstrated in soft-rigid hybrid robots like those inspired by pangasius fish, which use servo-driven fin rays for precise control. Such fin-inspired mechanisms find primary application in autonomous underwater vehicles (AUVs) designed for , where they facilitate in complex marine environments such as reefs or deep-sea trenches. Early examples include the RoboTuna, developed at MIT in the 1990s, which used a flexible tail fin to mimic thunniform for efficient cruising speeds up to 1.25 m/s, serving as a foundational model for biomimetic testing. Modern undulating fin drones, such as those employing long-based fin arrays, extend this to multi-degree-of-freedom maneuvering, enabling AUVs to perform tasks like seabed mapping or environmental sampling with reduced acoustic signatures compared to traditional rotors. Fin-based systems offer distinct advantages over conventional propellers, including superior maneuverability in turbulent currents due to their ability to generate vectored through asymmetric , allowing for agile turns and hovering without additional control surfaces. Biomimetic also enhances energy efficiency, with studies showing up to 30-40% lower power consumption for steady-state in soft robotic prototypes, attributed to optimized wake structures that minimize drag. These benefits are particularly evident in undulating propulsors, which provide better stability in variable flows, outperforming propellers in scenarios requiring low-speed precision. Despite these gains, challenges persist in material durability, as flexible polymers degrade under prolonged saltwater exposure, leading to reduced elasticity and accumulation that impairs . Developing robust, corrosion-resistant composites remains critical, alongside advancing control algorithms to synchronize multi-actuator fin motions, where real-time feedback from sensors is needed to counteract hydrodynamic instabilities. Recent developments as of 2025 have integrated to enhance robot designs, such as MIT's use of AI to optimize shapes for autonomous gliders, improving efficiency in tasks like current tracking and .

Other Technological and Cultural Uses

In naval , bilge keels serve as fin-like appendages attached along the hulls of ships near the to counteract rolling motions by generating hydrodynamic lift and drag forces that dampen oscillations. These structures, typically plates extending from one-third to one-half the ship's length, enhance stability in rough seas without significantly impeding forward motion. Hydrofoils, analogous to wings, are engineered lifting surfaces that elevate vessel hulls above the surface at high speeds, reducing drag and enabling efficient travel for ferries and military craft. In , bioinspired designs drawing from flexibility have informed morphing wing technologies, such as 3D-printed prototypes that adapt shapes for improved aerodynamic control in and drones. Fin concepts permeate cultural narratives, notably in mermaid mythology where hybrid human-fish figures feature tail fins symbolizing the enigmatic boundary between land and sea, often embodying themes of allure, danger, and transformation across global from European sirens to Asian sea spirits. In modern sports, swim fins—footwear extending the surface area of human feet for propulsion—were pioneered by inventor Owen Churchill, who developed and patented a practical rubber in 1943 based on observations of Pacific Islanders, revolutionizing underwater activities like diving and training. These devices, first commercialized in the late , amplify swimmer efficiency by mimicking hydrodynamics. Artistic expressions of fins appear in and adornments, with or fin motifs in tattoos representing , strength, or marine heritage, often stylized in tribal or realistic forms to evoke oceanic power. incorporating fin shapes, such as pendants or rings mimicking dorsal or pectoral structures, draws from nautical themes to symbolize resilience and fluidity in . In , fin-like elements manifest as decorative protrusions or structural fins on buildings, inspired by aquatic forms to enhance and provide shading, as seen in modern coastal designs blending form and function. Aquariums and marine parks utilize fin displays in exhibits to educate visitors on aquatic locomotion, highlighting how these appendages enable species survival while fostering public appreciation for marine . Conservation efforts have targeted shark fins through international regulations, with finning—the practice of removing fins and discarding carcasses—addressed by FAO's 1999 International Plan of Action for Sharks, which urged sustainable practices, followed by binding prohibitions such as the ICCAT's 2004/2005 finning ban and measures enforced by over two dozen nations since the mid-2000s to curb by requiring full shark retention or fin-to-body ratios. Emerging applications include fin-shaped heat sinks on photovoltaic panels, where extended surfaces like L-shaped or pin fins facilitate water or to mitigate efficiency losses from overheating, with studies showing temperature reductions of up to 3-5% under natural convection. In , fin-inspired devices for swimmers, such as webbed gloves or smart attachments, enhance propulsion while integrating sensors for performance tracking, blending biomimicry with for training optimization.

References

  1. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/fins
  2. https://www.dnr.sc.gov/fish/anatomy.html
  3. https://manoa.hawaii.edu/exploringourfluidearth/biological/fish/structure-and-function-fish
  4. https://www.floridamuseum.ufl.edu/discover-fish/fish/anatomy/spines-rays-caudal-fins/
  5. https://pubmed.ncbi.nlm.nih.gov/35132436/
  6. https://pubmed.ncbi.nlm.nih.gov/32709626/
  7. https://www.sciencedirect.com/science/article/abs/pii/S1742706117300259
  8. https://pubmed.ncbi.nlm.nih.gov/32965713/
  9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6561483/
  10. https://pmc.ncbi.nlm.nih.gov/articles/PMC8694198/
  11. https://pmc.ncbi.nlm.nih.gov/articles/PMC3552419/
  12. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/osteichthyes
  13. https://flexbooks.ck12.org/cbook/ck-12-advanced-biology/section/16.11/primary/lesson/fish-classification-advanced-bio-adv/
  14. https://www.fishbase.se/summary/SpeciesSummary.php?ID=5319&AT=Eel
  15. https://www.biologydiscussion.com/fisheries/fish/7-main-types-of-system-in-fishes-phylum-chordata/40861
  16. https://pmc.ncbi.nlm.nih.gov/articles/PMC5770300/
  17. https://www.mdpi.com/1424-2818/16/6/349
  18. https://journals.physiology.org/doi/10.1152/nips.01398.2002
  19. https://journals.biologists.com/jeb/article/205/12/1709/8955/Hydrodynamics-of-caudal-fin-locomotion-by-chub
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC10762429/
  21. https://www.ias.ac.in/article/fulltext/reso/014/01/0032-0046
  22. https://www.sciencedirect.com/science/article/abs/pii/S1095643301004615
  23. https://link.aps.org/doi/10.1103/PhysRevFluids.8.073101
  24. https://pmc.ncbi.nlm.nih.gov/articles/PMC11274942/
  25. https://academic.oup.com/icb/article/42/2/243/652624
  26. https://www.pnas.org/doi/10.1073/pnas.2113206118
  27. https://pubs.aip.org/aip/pof/article/30/9/091901/1013581/Study-of-the-thrust-drag-balance-with-a-swimming
  28. https://www.sciencedirect.com/science/article/abs/pii/S0029801821006211
  29. https://bpb-us-e1.wpmucdn.com/sites.harvard.edu/dist/6/58/files/2022/03/Lauder.Drucker.2004.pdf
  30. https://journals.biologists.com/jeb/article/220/23/4351/33683/Control-surfaces-of-aquatic-vertebrates-active-and
  31. https://cdnsciencepub.com/doi/10.1139/z99-086
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC7173857/
  33. https://www.nature.com/articles/ncomms2751
  34. https://www.mdpi.com/2311-5521/5/3/106
  35. https://www.researchgate.net/figure/Fig6-Mean-fin-surface-area-at-the-time-of-maximum-excursion-Fin-area-at-swimming_fig1_7742884
  36. https://journals.biologists.com/jeb/article/205/22/3475/9163/The-relationship-between-heat-flow-and-vasculature
  37. https://libres.uncg.edu/ir/uncw/f/barbierim2005-1.pdf
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC5998101/
  39. https://www.int-res.com/articles/esr2023/51/n051p227.pdf
  40. https://www.nature.com/articles/358410a0
  41. https://www.researchgate.net/publication/255614587_Thermoregulation_in_Yellowfin_Tuna_Thunnus_albacares
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC7661287/
  43. https://www.sciencedirect.com/science/article/pii/S0003347224001271
  44. https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2435.12097
  45. https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/gray-triggerfish/
  46. https://academic.oup.com/evolut/article/73/2/360/6882156
  47. https://www.researchgate.net/publication/12821487_Fin-flicking_behaviour_A_visual_antipredator_alarm_signal_in_a_characin_fish_Hemigrammus_erythrozonus
  48. https://oceanexplorer.noaa.gov/wp-content/uploads/2025/04/bioluminescent-creatures.pdf
  49. https://academic.oup.com/iob/article/4/1/obac035/6661421
  50. https://pmc.ncbi.nlm.nih.gov/articles/PMC5354974/
  51. https://ocean.si.edu/ocean-life/fish/incredible-diversity-fish-how-form-equals-function
  52. https://caribbeancompass.com/more-than-skin-deep-color-and-pattern-in-marine-fishes-part-one/
  53. https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-017-0370-x
  54. https://pmc.ncbi.nlm.nih.gov/articles/PMC3390900/
  55. https://www.sciencedirect.com/science/article/pii/S0960982224006699
  56. https://www.researchgate.net/publication/258494420_Paired_fins_of_jawless_vertebrates_and_their_homologies_across_the_agnathan-gnathostome_transition
  57. https://www.semanticscholar.org/paper/4128a0b6a30d7f9398b765d055b6a63f8cdef60b
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC3645028/
  59. https://pmc.ncbi.nlm.nih.gov/articles/PMC5018757/
  60. https://www.sciencedirect.com/science/article/pii/S0012160608010166
  61. https://journals.biologists.com/dev/article/139/16/2916/45235/Mechanism-of-pectoral-fin-outgrowth-in-zebrafish
  62. https://www.science.org/doi/10.1126/sciadv.abc3510
  63. https://onlinelibrary.wiley.com/doi/abs/10.1002/dvg.23052
  64. https://files.dnr.state.mn.us/education_safety/education/minnaqua/leadersguide/chapter_2/2_6_adapted_for_habitat.pdf
  65. https://seagrant.sunysb.edu/ifishny/pdfs/lessons/inclass/elementary/FormFunction-Background.pdf
  66. https://www.britannica.com/animal/fish/Locomotion
  67. https://evodevojournal.biomedcentral.com/articles/10.1186/s13227-022-00194-5
  68. https://www.fishbase.se/summary/Eigenmannia-virescens.html
  69. https://pmc.ncbi.nlm.nih.gov/articles/PMC4509401/
  70. https://pmc.ncbi.nlm.nih.gov/articles/PMC7507958/
  71. https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.20541
  72. https://pmc.ncbi.nlm.nih.gov/articles/PMC4322438/
  73. https://www.aquarium.co.za/foundation/news/locomotion-in-the-ocean-how-sea-animals-move-part-2
  74. https://academic.oup.com/beheco/article/17/4/670/215942
  75. https://pmc.ncbi.nlm.nih.gov/articles/PMC7126040/
  76. https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-10-391
  77. https://www.pnas.org/doi/10.1073/pnas.1900475116
  78. https://elifesciences.org/articles/77614
  79. https://pubs.acs.org/doi/10.1021/acs.est.9b06419
  80. https://www.sciencedirect.com/science/article/pii/0166445X82900133
  81. https://pmc.ncbi.nlm.nih.gov/articles/PMC3594788/
  82. https://pubmed.ncbi.nlm.nih.gov/20833919/
  83. https://www.int-res.com/articles/ab2013/18/b018p105.pdf
  84. https://www.smithsonianmag.com/smart-news/fossils-show-how-flying-fish-gained-their-ability-glide-180953830/
  85. https://pmc.ncbi.nlm.nih.gov/articles/PMC8778375/
  86. http://vigir.missouri.edu/~gdesouza/Research/Conference_CDs/IEEE_IROS_2009/papers/0657.pdf
  87. https://www.nature.com/articles/s41598-022-26179-x
  88. https://news.mit.edu/1994/robotuna
  89. https://academic.oup.com/icb/article/42/1/118/559938
  90. https://www.mdpi.com/2313-7673/8/3/318
  91. https://www.researchgate.net/publication/272272012_Advantages_of_Natural_Propulsive_Systems
  92. https://www.sciencedirect.com/science/article/pii/S2590123020300918
  93. https://pmc.ncbi.nlm.nih.gov/articles/PMC10807579/
  94. https://advanced.onlinelibrary.wiley.com/doi/10.1002/aisy.202300299
  95. https://scispace.com/pdf/advancements-and-challenges-in-underwater-soft-robotics-2avmjekkg9.pdf
  96. https://news.mit.edu/2025/ai-shapes-autonomous-underwater-gliders-0709
  97. https://asmedigitalcollection.asme.org/OMAE/proceedings-pdf/OMAE2006/47462/575/4561265/575_1.pdf
  98. https://www.marineinsight.com/naval-architecture/importance-of-ships-keel-and-types-of-keel/
  99. https://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html
  100. https://www.techbriefs.com/component/content/article/39887-a-3d-printed-fish-fin-offers-new-ideas-for-morphing-aircraft
  101. https://daily.jstor.org/mermaids-myth-kith-and-kin/
  102. https://www.churchillswimfins.com/pages/history
  103. https://www.divinghistory.org/owen-churchill-fin/
  104. https://www.tattoolife.com/life-and-death-central-motifs-in-barims-tattoo-art/
  105. https://www.prestigeonline.com/id/style/jewellery/cece-jewellery-turning-tattoo-art-into-jewellery/
  106. https://www.jaefr.com/articles/unveiling-the-mysteries-of-fish-fins-a-comprehensive-exploration-106179.html
  107. https://awionline.org/content/international-shark-protection-measures
  108. https://www.fao.org/4/i3036e/i3036e.pdf
  109. https://sharkleague.org/2024/11/01/iccats-finning-ban-twenty-years-on/
  110. https://www.sciencedirect.com/science/article/pii/S2214157X25006562
  111. https://the-gadgeteer.com/2018/12/23/become-aquaman-with-these-wearable-swimming-fins/
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