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Sea cucumber
Temporal range: Middle Ordovician-present
DendrochirotidaSynallactidaApodidaHolothuriidaElasipodidaMolpadidaPersiculidaArthrochirotida
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Echinozoa
Class: Holothuroidea
Blainville, 1834
Orders
Thelenota ananas, a giant sea cucumber from the Indo-Pacific tropics

Sea cucumbers are echinoderms from the class Holothuroidea (/ˌhɒləˌθjʊəˈrɔɪdiə, ˌhoʊlə-/ HOL-ə-thyuu-ROY-dee-ə, HOH-lə-). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786,[1] with the greatest number being in the Asia–Pacific region.[2] Sea cucumbers serve a useful role in the marine ecosystem as detritivores who help recycle nutrients, breaking down detritus and other organic matter, after which microbes can continue the decomposition process.[2]

Sea cucumbers have a leathery skin and an elongated body containing a single, branched gonad; they are named for their overall resemblance to the fruit of the cucumber plant. Like all echinoderms, sea cucumbers have a calcified dermal endoskeleton, which is usually reduced to isolated microscopic ossicles (or sclerietes) joined by connective tissue. In some species these can sometimes be enlarged to flattened plates, forming an armoured cuticle. In some abyssal or pelagic species such as Pelagothuria natatrix (order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.[3]

Many species of sea cucumbers are foraged as food by humans, and some species are cultivated in aquaculture systems. They are considered a delicacy seafood, especially in Asian cuisines, and the harvested product is variously referred to as trepang, namako, bêche-de-mer, or balate.

Overview

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Sea cucumber : a - Tentacles, b - Cloaca, c - Ambulacral feet on the ventral side, d - Papillae on the back

Most sea cucumbers have a soft and cylindrical body, rounded off and occasionally fat in the extremities, and generally without solid appendages. Their shape ranges from almost spherical for "sea apples" (genus Pseudocolochirus) to serpent-like for Apodida or the classic sausage-shape, while others resemble caterpillars. The mouth is surrounded by tentacles, which can be pulled back inside the animal.[4] Holothuroids measure generally between 10 and 30 centimetres (3.9 and 11.8 in) long, with extremes of some millimetres for Rhabdomolgus ruber and up to more than 3 metres (9.8 ft) for Synapta maculata. The largest American species, Holothuria floridana, which abounds just below low-water mark on the Florida reefs, has a volume of well over 500 cubic centimeters (31 cu in),[5] and 25–30 cm (10–12 in) long. Most possess five rows of tube feet (called "podia"), but Apodida lacks these and moves by crawling; the podia can be of smooth aspect or provided with fleshy appendages (like Thelenota ananas). The podia on the dorsal surface generally have no locomotive role, and are transformed into papillae. At one of the extremities opens a rounded mouth, generally surrounded with a crown of tentacles which can be very complex in some species (they are in fact modified podia); the anus is postero-dorsal.

Holothuroids do not look like other echinoderms at first glance, because of their tubular body, without visible skeleton nor hard appendixes. Furthermore, the fivefold symmetry, classical for echinoderms, although preserved structurally, is doubled here by a bilateral symmetry which makes them look like chordates. However, a central symmetry is still visible in some species through five 'radii', which extend from the mouth to the anus (just like for sea urchins), on which the tube feet are attached. There is thus no "oral" or "aboral" face as for sea stars and other echinoderms, but the animal stands on one of its sides, and this face is called trivium (with three rows of tube feet), while the dorsal face is named bivium. A remarkable feature of these animals is the "catch" collagen that forms their body wall.[Notes 1] This can be loosened and tightened at will, and if the animal wants to squeeze through a small gap, it can essentially liquefy its body and pour into the space. To keep itself safe in these crevices and cracks, the sea cucumber will hook up all its collagen fibers to make its body firm again.[6]

The most common way to separate the subclasses is by looking at their oral tentacles. Order Apodida have a slender and elongate body lacking tube feet, with up to 25 simple or pinnate oral tentacles. Aspidochirotida are the most common sea cucumbers encountered, with a strong body and 10 to 30 leaflike or shield-like oral tentacles. Dendrochirotida are filter-feeders, with plump bodies and eight to 30 branched oral tentacles (which can be extremely long and complex).

Anatomy

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Sea cucumbers are typically 10 to 30 cm (4 to 12 in) in length, although the smallest known species are just 3 mm (0.12 in) long, and the largest can reach 3 meters (10 ft). The body ranges from almost spherical to worm-like, and lacks the arms found in many other echinoderms, such as starfish. The anterior end of the animal, containing the mouth, corresponds to the oral pole of other echinoderms (which, in most cases, is the underside), while the posterior end, containing the anus, corresponds to the aboral pole. Thus, compared with other echinoderms, sea cucumbers can be said to be lying on their side.[7]

Conspicuous Sea Cucumber, Coconut Island, Hawaii

Body plan

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The body of a holothuroid is roughly cylindrical. It is radially symmetrical along its longitudinal axis, and has weak bilateral symmetry transversely with a dorsal and a ventral surface. As in other echinozoans, there are five ambulacra separated by five ambulacral grooves, the interambulacra. The ambulacral grooves bear four rows of tube feet but these are diminished in size or absent in some holothuroids, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.[8]

At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth. These are called the primary tentacles and were present in the common ancestor of echinoderms, but have been lost in all the other classes of the phylum,[9] and may be simple, branched or arborescent. They are known as the introvert and posterior to them there is an internal ring of large calcareous ossicles. Attached to this are five bands of muscle running internally longitudinally along the ambulacra. There are also circular muscles, contraction of which cause the animal to elongate and the introvert to extend. Anterior to the ossicles lie further muscles, contraction of which cause the introvert to retract.[8]

The body wall consists of an epidermis and a dermis and contains smaller calcareous ossicles, the types of which are characteristics which help to identify different species. Inside the body wall is the coelom which is divided by three longitudinal mesenteries which surround and support the internal organs.[8]

Digestive system

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A sea cucumber atop gravel, feeding

A pharynx lies behind the mouth and is surrounded by a ring of ten calcareous plates. In most sea cucumbers, this is the only substantial part of the skeleton, and it forms the point of attachment for muscles that can retract the tentacles into the body for safety as for the main muscles of the body wall. Many species possess an oesophagus and stomach, but in some the pharynx opens directly into the intestine. The intestine is typically long and coiled, and loops through the body three times before terminating in a cloacal chamber, or directly as the anus.[7]

Nervous system

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Sea cucumbers have no true brain. A ring of neural tissue surrounds the oral cavity, and sends nerves to the tentacles and the pharynx. The animal is, however, quite capable of functioning and moving about if the nerve ring is surgically removed, demonstrating that it does not have a central role in nervous coordination. In addition, five major nerves run from the nerve ring down the length of the body beneath each of the ambulacral areas.[7]

Most sea cucumbers have no distinct sensory organs, although there are various nerve endings scattered through the skin, giving the animal a sense of touch and a sensitivity to the presence of light. There are, however, a few exceptions: members of the Apodida order are known to possess statocysts, while some species possess small eye-spots near the bases of their tentacles.[7]

Respiratory system

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Sea cucumbers use cloacal respiration via a pair of "respiratory trees" that branch in the cloaca just inside the anus, so that they "breathe" by drawing water in through the anus, extracting dissolved oxygen from water, and then expelling it.[10][11] The "trees" consist of a series of narrow tubules branching from a common duct, and lie on either side of the digestive tract. Gas exchange occurs across the thin walls of the tubules, to and from the fluid of the main body cavity.

Together with the intestine, the "respiratory trees" also act as excretory organs, with nitrogenous waste diffusing across the tubule walls in the form of ammonia and phagocytic coelomocytes depositing particulate waste.[7]

Circulatory systems

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Like all echinoderms, sea cucumbers possess both a water vascular system that provides hydraulic pressure to the tentacles and tube feet, allowing them to move, and a haemal system. The latter is more complex than that in other echinoderms, and consists of well-developed vessels as well as open sinuses.[7]

A central haemal ring surrounds the pharynx next to the ring canal of the water vascular system, and sends off additional vessels along the radial canals beneath the ambulacral areas. In the larger species, additional vessels run above and below the intestine and are connected by over a hundred small muscular ampullae, acting as miniature hearts to pump blood around the haemal system. Additional vessels surround the respiratory trees, although they contact them only indirectly, via the coelomic fluid.[7]

Indeed, the blood itself is essentially identical with the coelomic fluid that bathes the organs directly, and also fills the water vascular system. Phagocytic coelomocytes, somewhat similar in function to the white blood cells of vertebrates, are formed within the haemal vessels, and travel throughout the body cavity as well as both circulatory systems. An additional form of coelomocyte, not found in other echinoderms, has a flattened discoid shape, and contains hemoglobin. As a result, in many (though not all) species, both the blood and the coelomic fluid are red in colour.[7]

Pearsonothuria graeffei showing its three rows of podia on its trivium
Sea cucumber ossicles (here "wheels" and "anchors")

Vanadium has been reported in high concentrations in holothuroid blood,[12] however researchers have been unable to reproduce these results.[13]

Locomotive organs

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Like all echinoderms, sea cucumbers possess pentaradial symmetry, with their bodies divided into five nearly identical parts around a central axis. However, because of their posture, they have secondarily evolved a degree of bilateral symmetry. For example, because one side of the body is typically pressed against the substratum, and the other is not, there is usually some difference between the two surfaces (except for Apodida). Like sea urchins, most sea cucumbers have five strip-like ambulacral areas running along the length of the body from the mouth to the anus. The three on the lower surface have numerous tube feet, often with suckers, that allow the animal to crawl along; they are called trivium. The two on the upper surface have under-developed or vestigial tube feet, and some species lack tube feet altogether; this face is called bivium.[7]

In some species, the ambulacral areas can no longer be distinguished, with tube feet spread over a much wider area of the body. Those of the order Apodida have no tube feet or ambulacral areas at all, and burrow through sediment with muscular contractions of their body similar to that of worms, however five radial lines are generally still obvious along their body.[7]

Even in those sea cucumbers that lack regular tube feet, those that are immediately around the mouth are always present. These are highly modified into retractile tentacles, much larger than the locomotive tube feet. Depending on the species, sea cucumbers have between 10 and 30 such tentacles and these can have a wide variety of shapes depending on the diet of the animal and other conditions.[7]

Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.[14] These can even evolve into long antennae-like structures, especially on the abyssal genus Scotoplanes.

Endoskeleton

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Echinoderms typically possess an internal skeleton composed of plates of calcium carbonate within the dermis. In most sea cucumbers, however, these have become reduced to microscopic ossicles embedded beneath the skin. A few genera, such as Sphaerothuria, retain relatively large plates, giving them a scaly armour.[7]

Life history and behaviour

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Habitat

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The mysterious Pelagothuria natatrix is the only truly pelagic echinoderm known to date.
Benthopelagic sea cucumbers, such as this Enypniastes, are often confused with jellyfish, have webbed swimming structures enabling them to swim up off the surface of the seafloor and journey as much as 1,000 metres (3,300 ft) up the water column
Spanish dancer (Benthodytes sp.), another swimming sea cucumber, hovering at 2,789 metres (9,150 ft) by the Davidson Seamount

Sea cucumbers can be found in great numbers on the deep seafloor, where they often make up the majority of the animal biomass.[15] At depths deeper than 8,900 m (5.5 mi), sea cucumbers comprise 90% of the total mass of the macrofauna.[16] Sea cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothuroids, such as Enypniastes eximia, Peniagone leander and Paelopatides confundens,[17] is made of a tough gelatinous tissue with unique properties that makes the animals able to control their own buoyancy, making it possible for them to either live on the ocean floor or to actively swim [18] or float over it in order to move to new locations,[19] in a manner similar to how the group Torquaratoridae floats through water.

Holothuroids appear to be the echinoderms best adapted to extreme depths, and are still very diversified beyond 5,000 metres (16,000 ft) deep: several species from the family Elpidiidae ("sea pigs") can be found deeper than 9,500 metres (31,200 ft), and the record seems to be some species of the genus Myriotrochus (in particular Myriotrochus bruuni), identified down to 10,687 metres (35,062 ft) deep.[20] In more shallow waters, sea cucumbers can form dense populations. The strawberry sea cucumber (Squamocnus brevidentis) of New Zealand lives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of 1,000 animals/m2 (93 animals/sq ft). For this reason, one such area in Fiordland is called the strawberry fields.[21]

Locomotion

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Some abyssal species in the abyssal order Elasipodida have evolved to a "benthopelagic" behaviour: their body is nearly the same density as the water around them, so they can make long jumps (up to 1,000 metres or 3,300 feet high), before falling slowly back to the ocean floor. Most of them have specific swimming appendages, such as some kind of umbrella (like Enypniastes), or a long lobe on top of the body (Psychropotes). Only one species is known as a true completely pelagic species, that never comes close to the bottom: Pelagothuria natatrix.[22]

Diet

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Holothuroidea are generally scavengers, feeding on debris in the benthic zone of the ocean. Exceptions include some pelagic cucumbers and the species Rynkatorpa pawsoni, which has a commensal relationship with deep-sea anglerfish.[23] The diet of most cucumbers consists of plankton and decaying organic matter found in the sea. Some sea cucumbers position themselves in currents and catch food that flows by with their open tentacles. They also sift through the bottom sediments using their tentacles. Other species can dig into bottom silt or sand until they are completely buried. They then extrude their feeding tentacles, ready to withdraw at any hint of danger.

In the South Pacific, sea cucumbers may be found in densities of 40 individuals/m2 (3.7 individuals/sq ft). These populations can process 19 kilograms per square metre (3.9 lb/sq ft) of sediment per year.[24]

The shape of the tentacles is generally adapted to the diet, and to the size of the particles to be ingested: the filter-feeding species mostly have complex arborescent tentacles, intended to maximize the surface area available for filtering, while the species feeding on the substratum will more often need digitate tentacles to sort out the nutritional material; the detritivore species living on fine sand or mud more often need shorter "peltate" tentacles, shaped like shovels. A single specimen can swallow more than 45 kilograms (99 lb) of sediment a year, and their excellent digestive capacities allow them to reject a finer, purer and homogeneous sediment. Therefore, sea cucumbers play a major role in the biological processing of the sea bed (bioturbation, purge, homogenization of the substratum etc.).

  • The mouth of Euapta godeffroyi, showing pinnate tentacles.
    The mouth of Euapta godeffroyi, showing pinnate tentacles.
  • Mouth of Holothuria sp., showing peltate tentacles.
    Mouth of Holothuria sp., showing peltate tentacles.
  • Mouth of Cucumaria miniata, with dendritic tentacles, for filtering the water.
    Mouth of Cucumaria miniata, with dendritic tentacles, for filtering the water.
  • Faeces of a holothuroid. This participates in sediment homogenization and purification.
    Faeces of a holothuroid. This participates in sediment homogenization and purification.

Communication and sociability

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Reproduction

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"Auricularia" larva (by Ernst Haeckel)

Most sea cucumbers reproduce by releasing sperm and ova into the ocean water. Depending on conditions, one organism can produce thousands of gametes. Sea cucumbers are typically dioecious, with separate male and female individuals, but some species are protandric. The reproductive system consists of a single gonad, consisting of a cluster of tubules emptying into a single duct that opens on the upper surface of the animal, close to the tentacles.[7]

At least 30 species, including the red-chested sea cucumber (Pseudocnella insolens), fertilize their eggs internally and then pick up the fertilized zygote with one of their feeding tentacles. The egg is then inserted into a pouch on the adult's body, where it develops and eventually hatches from the pouch as a juvenile sea cucumber.[25] A few species are known to brood their young inside the body cavity, giving birth through a small rupture in the body wall close to the anus.[7]

Development

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In all other species, the egg develops into a free-swimming larva, typically after around three days of development. The first stage of larval development is known as an auricularia, and is only around 1 mm (39 mils) in length. This larva swims by means of a long band of cilia wrapped around its body, and somewhat resembles the bipinnaria larva of starfish. As the larva grows it transforms into the doliolaria, with a barrel-shaped body and three to five separate rings of cilia. The pentacularia is the third larval stage of sea cucumber, where the tentacles appear. The tentacles are usually the first adult features to appear, before the regular tube feet.[7]

Symbiosis and commensalism

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Emperor shrimp Periclimenes imperator on a Bohadschia ocellata sea cucumber

Numerous small animals can live in symbiosis or commensalism with sea cucumbers, as well as some parasites.

Some cleaner shrimps can live on the tegument of holothuroids, in particular several species of the genus Periclimenes (genus which is specialized in echinoderms), in particular Periclimenes imperator.[26] A variety of fish, most commonly pearl fish, have evolved a commensalistic symbiotic relationship with sea cucumbers in which the pearl fish will live in sea cucumber's cloaca using it for protection from predation, a source of food (the nutrients passing in and out of the anus from the water), and to develop into their adult stage of life. Many polychaete worms (family Polynoidae[27]) and crabs (like Lissocarcinus orbicularis) have also specialized to use the mouth or the cloacal respiratory trees for protection by living inside the sea cucumber.[28] Nevertheless, holothuroids species of the genus Actinopyga have anal teeth that prevent visitors from penetrating their anus.[29]

Sea cucumbers can also shelter bivalves as endocommensals, such as Entovalva sp.[30]

Predators and defensive systems

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Tonna perdix, a selective predator of tropical sea cucumbers
A sea cucumber in Mahé, Seychelles ejects sticky filaments from the anus in self-defense.

Sea cucumbers are often ignored by most of the marine predators because of the toxins they contain (in particular, holothurin) and because of their often spectacular defensive systems. However, they remain a prey for some highly specialized predators which are not affected by their toxins, such as the big mollusks Tonna galea and Tonna perdix, which paralyzes them using powerful poison before swallowing them completely.[31] Some other less specialized and opportunist predators can also prey on sea cucumbers sometimes when they cannot find any better food, such as certain species of fish (triggerfish, pufferfish) and crustaceans (crabs, lobsters, hermit crabs).

Some species of coral-reef sea cucumbers within the order Aspidochirotida can defend themselves by expelling their sticky cuvierian tubules (enlargements of the respiratory tree that float freely in the coelom) to entangle potential predators. When startled, these cucumbers may expel some of them through a tear in the wall of the cloaca in an autotomic process known as evisceration. Replacement tubules grow back in one and a half to five weeks, depending on the species.[2][32] The release of these tubules can also be accompanied by the discharge of a toxic chemical known as holothurin, which has similar properties to soap. This chemical can kill animals in the vicinity and is one more method by which these sedentary animals can defend themselves.[6]

Estivation

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If the water temperature becomes too high, some species of sea cucumber from temperate seas can aestivate. While they are in this state of dormancy, they stop feeding, their gut atrophies, their metabolism slows down and they lose weight. The body returns to its normal state when conditions improve.[2]

Phylogeny and classification

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See also: List of prehistoric sea cucumbers and List of echinodermata orders
Apodida like this Euapta godeffroyi are snake-shaped, without podia, and have pinnate tentacles.
Holothuriida like this Holothuria cinerascens are sausage-shaped, with peltate tentacles.
Dendrochirotida like this Cercodemas anceps are curled-bodied and have arborescent tentacles.
Elasipodida like this "sea pig" Scotoplanes have a translucent body with specific appendages; they live in the abyss.
Synallactida like this Stichopus herrmanni still lack a definition.

Holothuroidea (sea cucumbers) are one of five extant classes that make up the phylum Echinodermata. This is one of the most distinctive and diverse phyla, ranging from starfish to urchins to sea cucumbers and many other organisms. The echinoderms are mainly distinguished from other phyla by their body plan and organization. The earliest sea cucumbers are known from the middle Ordovician, over 450 million years ago.[33] The apodida is the sister group to the other orders of sea cucumbers.[34]

All echinoderms share three main characteristics. When mature, echinoderms have a pentamerous radial symmetry. While this can easily be seen in a sea star or brittle star, in the sea cucumber it is less distinct and seen in their five primary tentacles. The pentamerous radial symmetry can also be seen in their five ambulacral canals.[35] The ambulacral canals are used in their water vascular system which is another characteristic that binds this phylum together.

The water vascular system develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet". Echinoderms tube feet (including sea cucumbers) can be seen aligned along the side of their axes.

While echinoderms are invertebrates, meaning they do not have a spine, they do all have an endoskeleton that is secreted by the mesenchyme. This endoskeleton is composed of plates called ossicles. They are always internal but may only be covered by a thin epidermal layer like in sea urchin's spines. In the sea cucumber, the ossicles are only found in the dermis, making them a very supple organism. For most echinoderms, their ossicles are found in units making up a three dimensional structure. However, in sea cucumbers, the ossicles are found in a two-dimensional network.[36]

All echinoderms also possess anatomical feature(s) called mutable collagenous tissues, or MCTs.[37] Such tissues can rapidly change their passive mechanical properties from soft to stiff under the control of the nervous system and coordinated with muscle activity. Different echinoderm classes use MCTs in different ways. The asteroids, sea stars, can detach limbs for self-defense and then regenerate them. The Crinoidea, sea fans, can go from stiff to limp depending on the current for optimal filter feeding. The Echinoidea, sand dollars, use MCTs to grow and replace their rows of teeth when they need new ones. The Holothuroidea, sea cucumbers, use MCTs to eviscerate their gut as a self-defense response. MCTs can be used in many ways but are all similar at the cellular level and in mechanics of function. A common trend in the uses of MCTs is that they are generally used for self-defense mechanisms and in regeneration.[37]

Holothuroid classification is complex and their paleontological phylogeny relies on a limited number of well-preserved specimens. The modern taxonomy is based first of all on the presence or the shape of certain soft parts (podia, lungs, tentacles, peripharingal crown) to determine the main orders, and secondarily on the microscopic examination of ossicles to determine the genus and the species. Contemporary genetic methods have been helpful in clarifying their classification.

Taxonomic classification according to World Register of Marine Species:

Scientific history and naming

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The word Holothurion (ὁloqoύrion or "holothoύrion") was first coined by the Greek philosopher Aristotle in his Historia animalium (Book I, Part 1), although it remains unsure whether he is referring to the same animal as us.

The earliest known mention of the Greek term ὁλοθούριον (holotoúrion) [38] · [39] is found in the poet Epicharmus around 450 BC.[40] Plato, Aristotle and Theophrastus all used the generic name πλεύμον θαλάσσιος (pleúmon thalássios) to refer to a soft, flabby marine zoophyte – often translated as "sea lung"; the term may designate sea cucumbers, but possibly also ascidians or even jellyfish.[41]

One of the oldest scientific texts concerning sea cucumbers dates back to Aristotle, in his Parts of Animals[42] (around 343 BC): he names an animal "holothurion" without describing it, but classifies it among the animals lacking sensation (along with sponges and "sea lungs", apparently corresponding to tunicates); this name would later be retained and used to refer to sea cucumbers, though there is no definitive proof that this was the animal the Philosopher meant.[40]

Today, the word "Holothurian" is often used, although it is considered wrong as it would refer mostly to the genus Holothuria rather than to the whole class of Holothurioidea, which should rather be called in English "Holothuroids".[43]

In the East, medical or zoological treatises mention sea cucumbers as early as the 8th century, notably the Kojiki in China (712), then the Wamyō ruijushō in Japan (934), initiating a long tradition of excellent representations of these animals in Chinese and Japanese treatises.[40]

Western scientists began to take renewed interest in echinoderms during the Renaissance, and Pierre Belon in 1553 was the first to propose a link between them and starfish and sea urchins.[44] The first unambiguous use of this term to name a sea cucumber, accompanied by an illustration, is found in the Libri de Piscibus Marinis by Guillaume Rondelet, published in 1554 (although he describes two species, the second being an ascidian, and he wrongly separates the "vit de mer"[45]).[40] He noted that these beings "are of a middle nature between plants and animals".[45]

Real progress came during the Age of Enlightenment: in 1751, an article titled "Holothurie" was written for the Encyclopédie, based on the commentaries of Aristotle and Rondelet, but their taxonomic position (and even their description) remained unclear:

HOLOTHURIE, s.f. holothurium, (Nat. Hist. Zool.) marine animal. Mr. Linnæus placed it among the zoophytes, which are naked and have limbs. Rondelet mentioned two species of holothuries which he illustrated. The first species has a hard shell, it is oblong; one end is blunt and finished with a shell pierced with several holes. The second species has a body covered with spines; it ends at one end with a sort of round head pierced with a round, wrinkled hole that opens and closes, and which is the animal's mouth; the other end is narrow and elongated like a tail. On each side is an extension which is a leg, or rather a fin, since the animal uses it to move. One of the extensions is narrower than the other, scalloped all around, and pointed.[46]

In 1758, sea cucumbers appeared in the Systema Naturae of Carl von Linné, but the term still did not refer specifically to echinoderms,[47] and included diverse creatures such as the physalia.[48] It was only in 1767 that Linnaeus revised the Holothuria entry.[47]

Nathanael Gottfried Leske created the phylum echinoderms in 1778 (systematized by Jean-Guillaume Bruguière in 1791[47]), thereby formally incorporating this clade into scientific classifications. Henri-Marie Ducrotay de Blainville provided the scientific description of the subclass Holothuroidea in 1834, identifying it specifically with sea cucumbers (then still grouped under a single genus).

During the 19th century, many species were discovered (notably by Edmond Perrier), and were rapidly divided into orders and families, particularly by Grube, Théel, and Haeckel.

In the 20th century, increasingly intensive commercial fishing driven by Asian markets led to the rapid collapse of numerous stocks.[49] This situation began to alarm the industry and governments from the 1970s onward, prompting scientific studies on population status, which helped revive interest in sea cucumber research. In 1990, the SPC Beche-de-mer Information Bulletin was launched, the first scientific journal devoted exclusively to holothuroids.[50] Today, sea cucumbers are studied by numerous specialists from around the world, including Chantal Conand, Gustav Paulay, Sven Uthicke, Nyawira Muthiga, Maria Byrne, Steven Purcell, François Michonneau, and Yves Samyn.[51]

Relation to humans

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Food

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Main article: Sea cucumber as food
Dried sea cucumbers in a Japanese pharmacy

Sea cucumbers are consumed in various Asian countries such as China, Indonesia, Japan, South Korea,[52] and Malaysia.[53] They are commonly eaten as winter delicacies in the Noto Peninsula of Japan, with local harvest reportedly dating back to the 8th century.[54]

Sea cucumbers have also been harvested for local consumption in some Pacific Island countries like Palau, Samoa, Tonga, Fiji and the Federated States of Micronesia.[55]

To supply the markets of Southern China, Makassar trepangers traded with the Indigenous Australians of Arnhem Land from at least the 18th century and probably earlier. This is the first recorded example of trade between the inhabitants of the Australian continent and their Asian neighbours.[56]

There are many commercially important species of sea cucumber that are harvested and dried for export to the Asian market, often to be used in Chinese cuisine as hoisam.[57] Some of the more commonly found species in markets include:[57][58]

Medicine

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According to the American Cancer Society, although it has been used in traditional Asian folk medicine for a variety of ailments, "there is little reliable scientific evidence to support claims that sea cucumber is effective in treating cancer, arthritis, and other diseases" but research is examining "whether some compounds made by sea cucumbers may be helpful against cancer".[59]

Various pharmaceutical companies emphasize gamat, the Malay traditional medicinal usage of this animal.[60] Extracts are prepared and made into oil, cream or cosmetics. Some products are intended to be taken internally.

A review article found that chondroitin sulfate and related compounds found in sea cucumbers can help in treating joint-pain, and that dried sea cucumber is "medicinally effective in suppressing arthralgia".[61]

Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair.[62] Sea cucumbers are under investigation for use in treating ailments including colorectal cancer.[63] Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring.[64] One study found that a lectin from Cucumaria echinata impaired the development of the malaria parasite when produced by transgenic mosquitoes.[65]

  • "Teripang" in a market in Asia.
    "Teripang" in a market in Asia.
  • Sea cucumber in sauce in China.
    Sea cucumber in sauce in China.
  • Haisom cah jamur, Chinese Indonesian sea cucumber with mushroom.
    Haisom cah jamur, Chinese Indonesian sea cucumber with mushroom.
  • Deep fried sea cucumbers.
    Deep fried sea cucumbers.
  • Kripik teripang, Indonesian sea cucumber cracker.
    Kripik teripang, Indonesian sea cucumber cracker.

Procurement

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Sea cucumbers are harvested from the environment, both legally and illegally, and are increasingly farmed via aquaculture. The harvested animals are normally dried for resale.[66] In 2016, prices on Alibaba ranged up to $1,000 per kilogram ($450/lb).[67]

Commercial harvest

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In recent years, the sea cucumber industry in Alaska has increased due to increased demand for the skins and muscles to China.[68] Wild sea cucumbers are caught by divers. Wild Alaskan sea cucumbers have higher nutritional value and are larger than farmed Chinese sea cucumbers. Larger size and higher nutritional value has allowed the Alaskan fisheries to continue to compete for market share.[68]

One of Australia's oldest fisheries is the collection of sea cucumber, harvested by divers from throughout the Coral Sea in far North Queensland, Torres Straits and Western Australia. In the late 1800s, as many as 400 divers operated from Cook Town, Queensland.

Overfishing of sea cucumbers in the Great Barrier Reef is threatening their population.[69] Their popularity as luxury seafood in East Asian countries poses a serious threat.[70]

Black market

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As of 2013, a thriving black market was driven by demand in China where 450 grams (1 lb) at its peak might have sold for the equivalent of US$300[66] and a single sea cucumber for about US$160.[71] A crackdown by governments both in and out of China reduced both prices and consumption, particularly among government officials who had been known to eat (and were able to afford purchasing) the most expensive and rare species.[71] In the Caribbean Sea off the shores of the Yucatán Peninsula near fishing ports such as Dzilam de Bravo, illegal harvesting had devastated the population and resulted in conflict as rival gangs struggled to control the harvest.[66]

Aquaculture

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Main article: Aquaculture of sea cucumber

Overexploitation of sea cucumber stocks in many parts of the world provided motivation for the development of sea cucumber aquaculture in the early 1980s. The Chinese and Japanese were the first to develop successful hatchery technology on Apostichopus japonicus, prized for its high meat content and success in commercial hatcheries.[72] Using techniques pioneered by the Chinese and Japanese, a second species, Holothuria scabra, was cultured for the first time in India in 1988.[73] In recent years Australia, Indonesia, New Caledonia, Maldives, Solomon Islands and Vietnam have successfully cultured H. scabra using the same technology, and now culture other species.[72]

Conservation

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In India, the commercial harvest and transportation of sea cucumbers has been strictly banned under Schedule I of the Wild Life (Protection) Act, 1972 (WLPA) since 2001. In 2020, the Indian government created the world's first sea cucumber conservation area, the Dr. K.K. Mohammed Koya Sea Cucumber Conservation Reserve, to protect the sea cucumber species.[74][75]

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Holothurians plate by Ernst Haeckel from his Kunstformen der Natur (1904)

Sea cucumbers have inspired thousands of haiku in Japan, where they are called namako (海鼠), written with characters that can be translated as "sea mice" (an example of gikun). In English translations of these haiku, they are usually called "sea slugs". According to the Oxford English Dictionary, the English term "sea slug" was originally applied to holothuroids during the 18th century. The term is now applied to several groups of sea snails, marine gastropod mollusks that have no shell or only a very reduced shell, including the nudibranchs. Almost 1,000 Japanese holothuroid haiku translated into English appear in the book Rise, Ye Sea Slugs! by Robin D. Gill.[76]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sea cucumber

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from Grokipedia
Sea cucumbers comprise the class Holothuroidea within the phylum Echinodermata, consisting of elongated, flexible-bodied that lack the rigid endoskeletons typical of other echinoderms like and urchins, instead featuring microscopic embedded in leathery skin. Over 1,700 extant exist, distributed across all oceans in benthic habitats ranging from shallow intertidal zones to deep-sea floors exceeding 8,000 meters. These animals primarily function as detritivores, ingesting seafloor to extract , thereby performing bioturbation that oxygenates substrates, recycles nutrients, and mitigates organic buildup to support and resilience, including against sediment overload and disease. Their feeding tentacles, modified arranged in a crown around the mouth, facilitate suspension or deposit feeding depending on and environment. Sea cucumbers hold substantial commercial value, particularly in where dried preparations (bêche-de-mer) are consumed for purported nutritional and medicinal benefits, driving intensive fisheries that have depleted stocks in numerous regions and prompted calls for to avert ecological collapse. Defining traits include defensive evisceration of viscera, which they can regenerate, and diverse morphologies from worm-like forms to robust, warty bodies adapted to varied pressures and predation.

Introduction

General Description

Sea cucumbers are belonging to the class Holothuroidea in the phylum Echinodermata, distinguished by their soft, elongated, cylindrical bodies that resemble cucumbers, typically covered in a leathery integument rather than the rigid ossicles or spines characteristic of other echinoderm classes such as or sea urchins. These bottom-dwelling animals inhabit seabeds from shallow coastal waters to deep ocean trenches, exhibiting a flexible body plan adapted for slow crawling or burrowing via arranged in rows along the ventral surface. A notable defensive in many holothuroids involves the expulsion of —branching, adhesive filaments discharged from the to ensnare predators, providing an effective, albeit sacrificial, barrier that exploits the entangling properties of mucus and coelomocytes for mechanical deterrence. Complementing this, sea cucumbers possess pronounced regenerative abilities, capable of restoring lost or autotomized structures, including the aforementioned tubules, through processes involving cellular and proliferation over weeks. In benthic ecosystems, sea cucumbers function predominantly as detritivores and deposit feeders, processing large volumes of to extract organic detritus, which drives bioturbation, enhances organic matter mineralization, and facilitates the efflux of nutrients like to support overlying algal and microbial . This reworking aerates the substrate and recycles essential elements such as and , underscoring their causal role in maintaining dynamics and resilience on the floor.

Global Distribution and Diversity

Sea cucumbers inhabit marine environments worldwide, ranging from intertidal zones in coastal areas to the deepest trenches at depths up to approximately 9,000 . In hadal zones beyond 8,900 , they dominate the macrofaunal , comprising up to 90% of the total mass. Over 1,100 extant have been described, with estimates suggesting up to 1,800 when accounting for recent discoveries, primarily distributed across benthic habitats in tropical, temperate, and polar seas. The greatest occurs in the Indo-West Pacific, particularly systems of , where biodiversity hotspots include , the , and , as evidenced by fishery surveys and taxonomic inventories. These organisms demonstrate physiological adaptability to fluctuating environmental conditions, tolerating salinities typically between 27 and 35 parts per thousand through osmotic regulation mechanisms observed in species like Stichopus japonicus. Temperature tolerances vary by species, with tropical forms thriving in warmer waters and temperate or polar species enduring cooler regimes, as supported by experimental studies on growth and survival responses.

Taxonomy and Phylogeny

Evolutionary Origins and Fossil Record

Sea cucumbers, classified within the class Holothuroidea, represent an early-diverging lineage within the subphylum Echinozoa of , with analyses estimating their divergence from other echinoderm clades around 540 million years ago during the early period. This timing aligns with the , when predation pressures intensified, favoring morphological innovations such as the reduction of pentaradial symmetry in favor of a bilateral, elongated . The shift to a vermiform shape enhanced burrowing efficiency in soft sediments, enabling infaunal lifestyles that minimized exposure to surface predators through streamlined propulsion via longitudinal muscles and reduced cross-sectional area for sediment displacement. The fossil record of holothuroids is sparse, primarily consisting of isolated from the rather than complete body fossils, due to their predominantly soft-bodied construction and low mineralization. Putative early holothurians appear in deposits, such as the (~505 million years ago), where Charles Walcott identified soft-bodied forms resembling modern holothuroids based on tubular structures and tentative ossicle-like elements, though subsequent re-evaluations have questioned their precise affinity, suggesting they may represent stem-group echinoderms or unrelated soft-bodied taxa. More definitive body fossils emerge in the , including articulated specimens from Middle (~460 million years ago) strata in , described as the earliest unambiguous holothurians with features like elongated bodies and simple . Fossil-calibrated phylogenies and molecular data indicate that crown-group holothuroids diversified during the , with several lineages surviving the end-Permian mass extinction (~252 million years ago), after which ossicle-based records become more abundant. The Late yields evidence of specialized groups like synallactids, implying a pre- evolutionary history extending into the for stem forms, consistent with the adaptive advantages of sediment-dwelling under causal pressures from biotic interactions in expanding marine benthic ecosystems. The oldest confirmed body fossil, Porosothyone from the late of (~420 million years ago), exhibits primitive traits like simple tentacles and reduced , underscoring gradual refinement of the holothuroid bauplan.

Classification and Major Orders

The class Holothuroidea encompasses approximately 1,800 extant species of echinoderms, classified into seven orders based on integrative analyses of morphological traits and molecular data from mitochondrial and nuclear genes. This framework, established through phylogenomic studies, overturned the traditional six-order system by demonstrating the of Aspidochirotida and elevating subordinate clades to ordinal rank. and multigene phylogenies since 2010 have further refined family-level boundaries, particularly within Dendrochirotida, by resolving cryptic species and adjusting generic placements based on ossicle morphology and genetic divergence. The orders reflect adaptive diversification in body architecture and ossicle arrays, with Apodida comprising vermiform species lacking tube feet and featuring reduced or absent ossicles, representing about 10% of holothuroid diversity. Dendrochirotida includes forms with branched oral tentacles and arborescent body plans, encompassing over 500 species across 15 families, where recent phylogenomics have consolidated monophyletic groupings despite prior morphological ambiguities. Elasipodida and Molpadida exhibit specialized deep-sea adaptations in tube foot arrangements and body elongation, while Persiculida and Synallactida are smaller clades distinguished by unique calcareous ring structures and gonad positions. Holothuriida, formerly the core of Aspidochirotida, dominates with roughly 50% of , characterized by peltate tentacles, respiratory trees, and deposit-feeding specializations supported by robust ossicle tables; this order includes commercially significant families like Holothuriidae and Stichopodidae. These revisions underscore causal links between genetic lineages and morphological innovations, such as the evolution of sediment-processing mechanisms in Holothuriida versus suspension-capture in certain dendrochirotids, without implying normative superiority in ecological roles. Family counts vary, with Holothuriida alone holding over 10 , while Apodida has fewer but phylogenetically basal forms.
OrderApproximate Species ShareKey Morphological Traits
Apodida~10%Vermiform body, absent/reduced and
Dendrochirotida~30%Branched tentacles, dendriform habitus
Elasipodida~5%Modified for substrate interaction
Holothuriida~50%Peltate tentacles, respiratory trees, table
Molpadida~3%Elongated, burrowing forms
Persiculida<1%Specific gonad and ring features
Synallactida~1%Derived from aspidochirotid stock, unique

Anatomy and Physiology

Body Plan and Endoskeleton

Sea cucumbers possess an elongated, sausage-shaped body that contrasts sharply with the rigid, plated structures of other echinoderms such as and sea urchins. The body wall is composed of a thick, leathery dermis rich in , providing tensile strength while allowing significant flexibility for burrowing and evasion. Unlike the continuous calcareous test of many echinoderm relatives, the endoskeleton of sea cucumbers is highly reduced, consisting of isolated microscopic ossicles—tiny, calcified spicules embedded singly or in small clusters within the dermis. These ossicles, often rod-shaped, buttons, or tables varying by species and body region, prevent complete liquefaction of the integument under tension but do not confer rigidity. This dispersed skeletal architecture supports a hydrostatic system, where coelomic fluid acts as the incompressible core, opposed by circular and longitudinal muscle bands in the body wall for controlled contraction and elongation—enabling body lengths to shorten to one-fifth or extend beyond normal during locomotion or stress. Morphological variations in body form occur across orders; for instance, members of Apodida exhibit slender, vermiform shapes lacking tube feet, with some deep-sea genera like Synaptula developing highly branched, dendriform extensions that enhance surface area for attachment to substrates such as sponges. Anteriorly, the mouth is encircled by 8 to 30 retractable tentacles, which vary from simple digitate forms in deposit-feeders to peltate or pinnate structures in suspension-feeders, facilitating particle capture without altering the core body plan's flexibility. This adaptable endoskeleton underpins exceptional regenerative capacity; experimental evisceration studies on species like Holothuria demonstrate that torn mesentery edges thicken into blastemal tissue, reforming a functional intestine within 3 to 4 weeks under ambient seawater conditions, with ossicle redeposition restoring dermal integrity concurrently. Such regrowth relies on the modular spicule array, allowing rapid remodeling without structural collapse, as verified in controlled aquarium trials where full anterior viscera restoration occurred by day 21 post-injury.

Digestive and Respiratory Systems


The digestive system of sea cucumbers forms a continuous tubular tract from the mouth to the cloaca, facilitating deposit feeding on organic-rich sediments. The mouth is encircled by 8 to 30 tentacles—modified tube feet that are typically peltate or digitate—which collect and direct particulate matter, including detritus and microorganisms, into a short esophagus for initial ingestion. The foregut, comprising the anterior intestine, processes incoming material amid a microbiota dominated by Proteobacteria, while the midgut (medial intestine) supports microbial fermentation and nutrient extraction through symbionts like Bacteroidetes. The hindgut, or posterior intestine, compacts residues and expels feces via the cloaca, yielding processed material cleaner than ingested sediment due to selective microbial breakdown. Respiration occurs via a pair of dendritic respiratory trees branching from the cloaca, which actively pump seawater through their thin-walled branches for diffusive oxygen uptake, compensating for low ambient levels in benthic habitats. These structures, often filling much of the posterior body cavity, exhibit plasticity in function, with oxygen consumption rates increasing significantly during metabolic demands like reproduction—up to twofold in Apostichopus japonicus—and correlating positively with body mass (r=0.913, p<0.001). Water influx supports not only gas exchange but also excretion, enabling survival in oxygen-poor sediments where diffusion alone would suffice minimally. Evisceration serves as an antipredator defense, involving rapid expulsion of the digestive tract and respiratory trees—either anteriorly via the mouth or posteriorly—often laced with viscous, distasteful fluids to deter attackers. Laboratory inductions using 0.45 M KCl injections in species like Eupentacta quinquesemita trigger autotomy at mesentery junctions within 15 minutes, followed by regeneration: anterior portions reform via mesenchymal-epithelial transitions forming tubular rudiments, fusing with posterior regrowth from the cloaca remnant in 2–3 weeks, as confirmed by histological tracking of 86 specimens. This capacity underscores anatomical adaptations prioritizing escape over immediate organ integrity.

Nervous, Circulatory, and Locomotive Systems

Sea cucumbers possess a decentralized nervous system lacking a centralized brain, consisting primarily of a circumoral nerve ring encircling the mouth and five radial nerve cords extending posteriorly along the body axes. These nerves are hollow tubular structures divided into ectoneural (superficial, sensory-motor) and hyponeural (deeper, motor) components, with the ectoneural part featuring longitudinal strands and the hyponeural part thickened into cords. This diffuse organization supports reflexive coordination, such as the rapid retraction of tube feet or body wall contraction in response to mechanical stimuli, without requiring complex central processing suited to their often sedentary or slow-moving habits. The circulatory system is open and relies on coelomic fluid as the primary transport medium for nutrients, gases, and waste, directly bathing internal organs rather than being confined to vessels. Unlike most echinoderms, sea cucumbers contain hemoglobin within coelomocytes suspended in this fluid, enhancing oxygen-carrying capacity in low-oxygen sediments. Fluid circulation occurs via peristaltic contractions of the body wall and respiratory trees, augmented by a rudimentary hemal system of sinuses and vessels lacking a distinct heart; some species, like Stichopus moebii, exhibit ciliated epithelial linings in blood vessels to aid flow. Coelomocytes also contribute to immune functions, migrating through the fluid to sites of injury or infection. Locomotion in adult sea cucumbers depends on the water vascular system, which hydraulically operates thousands of tube feet (podia) arranged in three longitudinal rows for adhesion, crawling, and burrowing into sediments at slow rates typically under 10 cm per minute. Tube feet extend and contract via ampullae reservoirs drawing seawater through the madreporite, enabling stepwise propulsion coordinated by radial nerves; species in flowing waters adjust podia deployment to counter currents, reducing velocity with increasing flow speed. In larvae, such as the auricularia stage, movement shifts from microtubule-based ciliary bands for planktonic swimming to preparatory tube foot development during metamorphosis, reflecting ontogenetic adaptation from dispersive to benthic lifestyles.

Ecology and Life History

Habitats and Environmental Adaptations

Sea cucumbers (class Holothuroidea) occupy diverse benthic habitats across global oceans, ranging from intertidal zones to abyssal plains beyond 8,000 meters depth, with preferences for soft sediments like sandy mud and silt, as well as hard substrates including rocky reefs, gravel, and shell debris. Species distribution data from trawl surveys indicate niche partitioning, where low-value species predominate in shallow waters (1–10 m) on finer sediments, while commercial holothuroids favor deeper hard substrates (20–100 m). Intertidal and subtidal forms, such as those in rocky pools or seagrass beds, tolerate emersion through physiological adjustments including enhanced antioxidant defenses to mitigate oxidative stress from desiccation. In deeper waters, holothuroids adapt to gradients of pressure and low temperatures, with abyssal species like Chiridota inhabiting mud-covered seafloors where hydrostatic pressure exceeds 800 atmospheres; their coelomic fluid provides buoyant support analogous to hydraulic systems in shallower kin. Trawl-based abundance estimates from surveys in regions like the Southern California Bight reveal associations with specific invertebrate assemblages on gravelly bottoms at 100–200 m, contrasting with epifaunal clusters on reefs at shallower depths. Deep-sea forms exhibit reduced metabolic rates suited to near-freezing conditions (around 2–4°C), enabling persistence in oxygen-minimum zones. Physiological experiments quantify tolerances, with many species enduring salinities of 22–36 ppt (optimal 27–31.5 ppt for growth) and temperatures spanning 5–35°C depending on biogeographic origin—temperate Apostichopus japonicus thriving at 10–15°C and 28–34 ppt, while tropical shows narrower thermal limits. Salinity stress beyond 40 ppt induces evisceration in sensitive taxa like , underscoring substrate-mediated buffering in estuarine habitats. These parameters, derived from laboratory assays and field distributions, highlight species-specific resilience without implying uniform vulnerability across clades.

Locomotion, Feeding, and Diet

Sea cucumbers achieve locomotion primarily through the coordinated action of tube feet arranged in three longitudinal rows along the ventral surface, which operate via hydraulic pressure from the water vascular system to grip the substrate and propel the body forward. Many species supplement this with peristaltic body undulations or waves, particularly during burrowing into soft sediments, where the anterior end is anchored while the posterior contracts to advance. Movement speeds vary significantly among species and are influenced by factors such as body size, substrate type, temperature, and flow velocity; for instance, Holothuria arguinensis exhibits an average daily displacement of approximately 10 meters, while smaller or reef-dwelling forms like the warty sea cucumber cover only about 15 cm per day. Burrowing species, such as , can descend into sediment to depths of several centimeters, using tube feet to excavate and stabilize their position. Feeding in most sea cucumbers occurs via a specialized oral apparatus consisting of 8 to 30 peltate or pinnate tentacles, which are extended to the seafloor to collect surface sediments laden with organic detritus, microalgae, and bacteria. These tentacles sweep material into the mouth in a rhythmic motion, with particles passed to the pharynx and esophagus for initial sorting; inorganic fractions are often rejected as pseudofeces. Deposit-feeding dominates, with individuals processing substantial volumes of sediment—equivalent to 75% or more of their body weight daily during active periods—to extract nutrients, as observed in species like those in the genus . Field measurements indicate ingestion rates of 10-50 grams of sediment per day for typical individuals under 100 grams body mass, contributing to bioturbation and oxygenation of benthic layers. The diet consists predominantly of refractory organic matter from decaying plant and animal debris, enriched by microbial films on sediment grains, though some suspension-feeding forms, such as Cucumaria miniata, capture plankton via extended tentacles. Gut microbial symbionts play a crucial role in digestion, with communities enriched for anaerobic bacteria that break down complex carbohydrates and xenobiotics, facilitating decomposition and releasing bioavailable nutrients like nitrogen and phosphorus for recycling into the ecosystem. This symbiosis enhances digestive efficiency, as evidenced by elevated metabolic genes for organic matter processing in sea cucumber feces compared to ambient sediments.

Reproduction, Development, and Growth

Most species of sea cucumbers are gonochoristic, with separate sexes and a single gonad per individual, though differentiation between males and females typically requires microscopic examination of gametes due to subtle external differences. Sexual reproduction predominates via broadcast spawning, in which adults synchronously release gametes into the water column for external fertilization, often triggered by environmental cues such as lunar cycles, temperature rises, or phytoplankton blooms. A minority of species display simultaneous hermaphroditism or protandric sex change, enabling self-fertilization or sequential mating, but these are exceptions rather than the norm across the class Holothuroidea. Fertilized ova typically develop into planktotrophic auricularia larvae, ciliated planktonic forms that actively feed on microalgae such as diatoms and flagellates to fuel growth through the larval stages. The auricularia phase transitions to non-feeding doliolaria and pentactula larvae, culminating in settlement onto suitable benthic substrates—often algae, rocks, or sediments—followed by metamorphosis into pentaradial juveniles, a process spanning 20 to 60 days post-fertilization depending on species, water temperature (optimal at 24–28°C for many tropical forms), and larval nutrition. For example, in Holothuria mammata, juveniles emerge around 21 days, while in Holothuria forskali, full metamorphosis requires over 40 days. Juvenile growth occurs benthically, with individuals transitioning to deposit or suspension feeding as they develop the adult body plan, including tube feet and respiratory trees; rates vary by habitat and food supply, but sexual maturity is generally attained in 1 to 5 years. Temperate species like the giant red sea cucumber (Parastichopus californicus) may require 4 years, whereas faster-growing tropical species such as can mature in 1–2 years under aquaculture conditions with abundant organic sediments. Viviparity or brooding, observed in roughly 30 primarily within orders Apodida and Dendrochirotida, deviates from this pattern by retaining embryos internally or externally until they hatch as fully formed juveniles, bypassing the dispersive larval stage. This strategy, rarer than broadcast spawning (prevalent in >95% of ), empirically correlates with low densities in patchy or deep-sea habitats, as internal development maximizes fertilization probability and offspring survival by shielding them from planktonic predation and dispersal losses, though it limits compared to pelagic larvae.

Behavior, Symbiosis, Predation, and Defenses

Sea cucumbers exhibit limited , typically maintaining solitary lifestyles outside of reproductive periods, though some form temporary aggregations during spawning events synchronized by chemical cues released from males. In Holothuria arguinensis, water conditioned by spawning males attracts both sexes and induces spawning responses, demonstrating olfactory-mediated conspecific aggregation. Similarly, triterpenoid saponins with structures serve as pheromones promoting aggregation in certain holothuroids, distinct from those in non-aggregating congeners. These cues facilitate mass spawning but do not indicate advanced kin discrimination, with empirical studies focusing primarily on sex- and -specific signaling rather than familial recognition. Symbiotic associations in sea cucumbers often involve commensal relationships that provide shelter to associates without apparent detriment to the host. (Carapidae) enter the of species like Holothuria and Stichopus, residing internally during the day to evade predators, emerging nocturnally to feed; this interaction benefits the fish via protection while sea cucumbers show no significant physiological cost in documented cases. Certain and also utilize sea cucumbers as mobile habitats or cleaning stations, clinging externally or associating with respiratory structures for and . These mutualisms enhance associate survival in predator-rich reefs but remain facultative, with hosts tolerating symbionts through behavioral indifference rather than active recruitment. Predators of sea cucumbers include , octopuses, sea stars, and predatory gastropods such as triton snails (), which target exposed individuals via drilling or engulfment. In response, many species deploy mechanical and chemical defenses; holothuroids lacking rigid structures rely on of viscera or ejection of specialized from the , which rapidly elongate into sticky, adhesive strands that entangle attackers. These tubules contain holothurin, a triterpenoid that deters predation through and repellency, biosynthesized via mevalonate pathways and stored in high concentrations. Additional defenses include body wall and metabolic evasion, with some species reducing activity to minimize detection. Under environmental stress such as or hypoxia in intertidal zones, aestivating species like Apostichopus japonicus enter a state characterized by profound metabolic depression, reducing oxygen consumption by up to 71% over weeks to months. This adaptation involves transcriptional downregulation of energy-intensive pathways, shifts, and of non-essential tissues, enabling survival in anoxic sediments for periods exceeding 100 days without feeding. Recovery upon rehydration restores metabolic rates, underscoring the strategy's reversibility and reliance on biochemical reprogramming rather than structural modifications.

Human Interactions

Culinary Uses and Nutritional Value

Sea cucumbers are harvested and processed primarily for consumption in dried form, known as bêche-de-mer, which serves as a delicacy in East Asian cuisines, especially Chinese, where the rehydrated product is incorporated into soups, stews, and braised dishes for its gelatinous texture. Traditional preparation involves eviscerating the fresh animal, boiling it to remove impurities and achieve contraction, salting if needed, and repeated sun-drying cycles until the body wall hardens into a lightweight, storable product that can expand significantly upon rehydration in water or broth prior to cooking. In Japanese cuisine, dried forms are sometimes boiled in green tea or used in other preparations, reflecting regional adaptations of this ancient practice dating back over a millennium. High-value species such as (sandfish) command premium prices in international markets due to their thick body walls and desirable texture post-processing, often fetching up to $2000 per kilogram dry weight. Global trade in bêche-de-mer reached a market value of approximately $510 million in 2019, driven largely by demand from , though values have fluctuated with supply constraints and species availability. Nutritionally, dried sea cucumbers exhibit high protein content ranging from 41% to 63% by dry weight, derived mainly from the collagen-rich body wall, alongside low levels typically under 1%. Mineral composition includes notable amounts of sodium as a primary component, with trace elements varying by and processing method. Mucopolysaccharides constitute a portion of the fraction, contributing to the product's structural qualities upon cooking.
Nutrient (dry weight basis)Approximate Range (%)Source Notes
Protein41–63Predominantly collagen-based; varies by species like Parastichopus californicus.
<1Minimal lipid content across tissues.
Ash (minerals)9–12Includes sodium and other inorganics.
Carbohydrates5–9Partly mucopolysaccharides.

Medicinal Applications and Bioactive Compounds

Sea cucumbers contain sulfated polysaccharides, particularly fucosylated chondroitin sulfates (FCS), which exhibit anticoagulant and antithrombotic activities by inhibiting thrombin and factor Xa in the coagulation cascade, surpassing the potency of heparin in some in vitro assays. These compounds also demonstrate anti-inflammatory effects through modulation of cytokine production and reduction of inflammatory markers in cellular models. FCS structures vary by species, with branches of fucose sulfated at specific positions contributing to their bioactivity, as isolated from species like Holothuria and Cucumaria. Recent research highlights FCS from sea cucumbers, such as Holothuria floridana, as potent inhibitors of sulfatase-2 (Sulf-2), an enzyme that promotes heparan sulfate desulfation and facilitates cancer cell metastasis by enhancing extracellular matrix remodeling and tumor invasion. In 2025 studies, HfFucCS bound tightly to Sulf-2's active site, blocking its activity without observed cytotoxicity, suggesting potential for adjunct cancer therapies targeting metastasis in solid tumors like hepatocellular carcinoma. Complementary extracts rich in sulfated polysaccharides have shown in vitro suppression of hepatocellular carcinoma cell proliferation via apoptosis induction and cell cycle arrest. Extracts from sea cucumber body walls and viscera provide hepatoprotective effects in animal models, reducing alcohol-induced oxidative stress by lowering malondialdehyde levels and elevating antioxidant enzymes like superoxide dismutase and glutathione peroxidase in mouse livers. Ether-phospholipids isolated from species such as Stichopus japonicus mitigated lipid dysregulation and hepatic inflammation, though human trials remain absent. Antidiabetic potential arises from peptides and polysaccharides that enhance glucose uptake and modulate insulin signaling; for instance, hydrolysates from Holothuria nobilis activated the PI3K/Akt pathway in streptozotocin-induced diabetic rats, lowering fasting blood glucose and improving insulin sensitivity. Ethanolic extracts promoted glucose consumption in HepG2 hepatocytes via upregulated expression, indicating peripheral insulin-mimetic effects. Despite promising preclinical data, therapeutic translation is limited by poor oral bioavailability of polysaccharides due to gastrointestinal degradation and low absorption, necessitating nanoparticle formulations or structural modifications for efficacy. Few randomized controlled trials exist, with most evidence from in vitro and rodent studies; traditional claims from systems like lack rigorous validation and often extrapolate beyond empirical findings. Further clinical investigation is required to confirm safety, dosing, and efficacy in humans.

Other Commercial Products

Sea cucumber byproducts, particularly the body wall and viscera discarded during food processing, have been explored for extraction of collagen used in cosmetics. Type I collagen derived from species such as Stichopus japonicus and Apostichopus japonicus yields approximately 1.2–10% of the wet body wall weight, offering a marine alternative to mammalian or fish collagens with high bioavailability for skin applications. This collagen promotes fibroblast proliferation and wound healing in vitro, supporting its incorporation into anti-aging creams and serums that enhance skin elasticity and reduce fine lines, as evidenced by formulations from Australian brands utilizing extracts for topical repair. Such value addition incentivizes processors to utilize processing waste, potentially increasing revenue streams and reducing disposal costs in fisheries where edible portions account for only 20–30% of harvested biomass. Certain live sea cucumber species, including Holothuria and Pseudocolochirus genera, enter minor commercial markets as ornamental invertebrates for marine aquaria, valued for their detritus-sifting behavior that aids tank maintenance. Global trade volumes remain small, comprising less than 1% of total sea cucumber exploitation, with specimens like tiger-tail cucumbers (Holothuria thomasi) retailed at $20–50 per individual for reef setups requiring stable water parameters to prevent mass mortality events. This niche demand drives selective harvesting or aquaculture of hardy species, providing economic diversification without overlapping primary fisheries, though high failure rates in captivity—due to sensitivity to poor oxygenation—limit scalability. Inedible byproducts like viscera and exoskeletal remnants have potential for fertilizer production, leveraging high nitrogen content from protein hydrolysates to enrich soil amendments, akin to general marine waste valorization yielding 5–15% nutrient recovery. Pilot efforts in regions with intensive processing, such as Asia-Pacific fisheries, demonstrate that converting these wastes into organic fertilizers reduces landfill burdens by up to 40% per ton processed, with market incentives tied to byproduct sales offsetting overexploitation risks through higher per-unit value rather than volume increases. Empirical data from small-scale trials indicate saponin-rich extracts may also serve niche detergent-like applications due to their surfactant properties, though commercial scaling remains underdeveloped pending toxicity assessments.

Harvesting, Aquaculture, and Trade

Wild Harvesting Methods and Challenges

Wild sea cucumbers are harvested primarily through labor-intensive methods such as hand collection by free-diving, snorkeling, or SCUBA diving in shallow coastal waters, where divers manually pick individuals and collect them in mesh bags or baskets for surfacing. In regions like Alaska and parts of the Pacific, regulations often restrict harvesting to hand-picking to minimize environmental impact, with divers using surface-supplied air or SCUBA for depths up to 20-30 meters. For deeper waters or larger-scale operations, mechanized techniques including bottom trawling, beam trawling, or dredging are employed, particularly in fisheries off California or in the Indian Ocean, where nets or dredges are dragged across the seafloor to capture sedentary or slow-moving holothuroids. Global wild capture production for sea cucumbers peaked in the 2010s, with FAO-reported figures reaching approximately 59,300 tonnes by 2019, though earlier estimates for wet weight harvests in major producers like and the suggested totals exceeding 100,000 tonnes annually during high-demand periods driven by Asian markets. These methods yield variable efficiencies, with catch per unit effort (CPUE) often ranging from 2 to 25 kg wet weight per diver-hour in artisanal fisheries, constrained by the animals' patchy distribution and sessile habits. Harvesting faces significant challenges from overexploitation, which has reduced population densities in many fisheries to levels impairing reproduction and recruitment, as sea cucumbers require minimum thresholds for effective spawning—often dropping below 0.1 individuals per square meter post-fishing. Market preferences for larger specimens (>20-30 cm) lead to size-selective harvesting, leaving smaller juveniles that may face higher predation or slower growth, exacerbating stock depletion without improving yields. Mechanized and , while enabling access to deeper stocks, generate substantial of non-target benthic species and cause degradation by scouring seafloors, burying organisms, and reducing structural complexity in coral reefs and beds, with recovery times spanning years to decades. Additionally, diving operations pose safety risks to fishers, including from prolonged SCUBA use, and yield inefficiencies persist due to post-overfishing scarcity, often requiring longer search times for .

Illegal Trade and Black Markets

Several high-value sea cucumber species, including , Thelenota anax, and Thelenota rubralineata (collectively known as teatfish), were listed in Appendix II effective January 2021 following adoption at the 18th in 2019, aiming to regulate through permits and non-detriment findings due to evidence of population declines from overharvesting. Similarly, Isostichopus fuscus has been included in Appendix III by since 2004 to monitor exports from that region. Despite these measures, illegal trade continues to drive depletion, with black market supply chains routing product from overexploited Pacific fisheries (e.g., , Galápagos, and Pacific Island nations) and the toward major Asian consumers like and , often bypassing quotas via underreporting, species substitution, or laundering through intermediary countries. Seizures in the 2020s underscore the scale of evasion, including 1,930 kg intercepted in Western Australia in October 2025 alongside other contraband fisheries products, and a British Columbia case in 2025 involving the illegal sale of over 39 metric tons (87,000 pounds) generating more than $1 million in revenue. In Mexico and the US, documented incidents from 2011 to 2021 yielded seizures equivalent to 100,611 kg of dried sea cucumbers valued at $29.55 million, with ongoing reports of hundreds of kilograms confiscated annually in ports and border operations. These activities are propelled by premium prices for dried bêche-de-mer, which range from $100 to over $1,000 per kg for rare species, reflecting intense demand for their use in traditional Asian cuisine and medicine. Enforcement gaps persist due to inadequate in fragmented global s, where illegal catches are mixed with legal quotas and exported without verifiable , complicating detection at import points. Limited molecular identification tools and inconsistent reporting further enable misdeclaration of or origins, as evidenced by persistent volumes post-CITES listings without corresponding reductions in illicit flows. Enhancing transparency through mandatory species-level tracking and digital verification could mitigate these issues more effectively than expanding prohibitions, which have historically shifted rather than curtailed demand-driven exploitation.

Aquaculture Innovations and Sustainability

Aquaculture of sea cucumbers, particularly Apostichopus japonicus in , has advanced through pond-based systems and sea ranching, where juveniles are released into controlled coastal areas for grow-out to market size. These methods leverage natural and supplemental feeding to achieve high densities, with pond culture dominating production due to its scalability in regions like Province. Sea ranching enhances stock enhancement while minimizing infrastructure costs compared to intensive ponds. China's output exceeds 200,000 metric tons annually, primarily from A. japonicus, representing the bulk of global farmed sea cucumbers and driven by demand for dried products. Recent innovations include programs yielding varieties like "Dongke No.1," which exhibit faster growth rates and improved stress resistance through targeted genetic selection for traits such as and tolerance. in ponds further optimizes water quality by promoting heterotrophic to recycle nutrients, reducing feed costs and effluent discharge. Integrated multi-trophic aquaculture (IMTA) systems pair sea cucumbers with fed species like or , where deposit-feeding holothuroids process organic wastes, lowering and loads by up to 30% in trial setups. This enhances overall farm efficiency, with sea cucumbers providing benefits documented in FAO technical workshops, while diversified outputs improve economic resilience for operators. Model-based assessments indicate that scaled has alleviated wild harvest pressures by substituting 20-30% of market supply in high-demand regions, supporting stock recovery without relying on unsubstantiated conservation claims.

Conservation Status

Global stock assessments of sea cucumber fisheries reveal widespread declines driven by overharvesting, with reductions documented in 81% of evaluated fisheries across regions. These decreases are causally linked to escalating effort, particularly in response to international demand for dried products, as evidenced by serial depletion patterns where high-value are sequentially targeted until local collapses occur. The IUCN Species Survival Commission has classified at least 16 sea cucumber species on its Red List as threatened primarily due to , with additional species approaching endangered status amid intensified exploitation for luxury markets. In many tropical fisheries, capture production has fallen by more than 70% since the , correlating directly with expanded harvest volumes that exceed natural recruitment rates. Catch per unit effort (CPUE) metrics further quantify this ; for example, in the Galápagos Marine Reserve, CPUE for Isostichopus fuscus halved from 102.6 individuals per diver-hour in 1999 to 54.5 in 2005, reflecting biomass depletion unresponsive to subsequent closures. Regional patterns underscore fishing intensity as the dominant factor, with tropical —often multispecies and historically small-scale—experiencing more severe collapses than temperate ones, where fisheries are typically newer, species-specific, and subject to comparatively lower pressure or slower exploitation trajectories. In areas like the , overfishing has reduced species richness from 13 to 7 and overall abundance by 82.6% over 16 years, exemplifying localized extirpation risks tied to unchecked diver-based collection. Temperate populations, such as those of Parastichopus californicus, show relative resilience in less intensively fished zones, though emerging markets pose analogous threats.

Threats Beyond Fishing and Management Responses

Habitat degradation, including sedimentation from coastal development and dredging, poses a secondary threat to sea cucumber populations by smothering benthic substrates and disrupting deposit-feeding behaviors essential for nutrient recycling. Such disturbances contribute modestly to declines—estimated at 10-20% of impacts in localized reef systems per empirical assessments—far subordinate to overexploitation, as confirmed by global meta-analyses attributing primary causality to harvesting pressures. Climate-driven factors exacerbate vulnerabilities, with ocean warming and acidification reducing larval survival rates by impairing ciliary development and metamorphosis; laboratory studies on species like Apostichopus japonicus demonstrate up to 50% lower settlement success under elevated temperatures (24-28°C) and pCO₂ levels projected for 2100. These effects compound density-dependent recruitment failures but remain marginal relative to fishing, per syntheses emphasizing biological traits like slow growth that amplify harvest sensitivity over environmental stressors. Management responses have varied in efficacy, with temporary moratoria proving more successful than static quotas in permitting recovery; for instance, post-1990s bans in Australian fisheries stabilized stocks through enforced closures, allowing biomass rebound via natural replenishment. In contrast, quota-based systems frequently falter due to inaccurate baseline data and weak , leading to serial depletions in 60-70% of monitored fisheries as per 2024-2025 governance audits. Recent evaluations reveal systemic gaps, including inadequate monitoring in 70% of global operations and overreliance on top-down regulations that ignore local incentives, resulting in persistent illegal harvests despite nominal protections. Data-driven harvest strategies, incorporating real-time stock assessments and adaptive total allowable catches, offer superior outcomes by aligning exploitation with empirical carrying capacities, as demonstrated in data-poor models for multi-species fisheries. Incentives for sea ranching—such as subsidies for restocking wild habitats with hatchery-reared juveniles—further mitigate pressures by shifting economic reliance toward aquaculture, which has restored local yields in Pacific trials without exacerbating wild overharvest. These approaches prioritize causal mechanisms like reproductive bottlenecks over blanket prohibitions, fostering resilience through market-aligned conservation rather than enforcement-heavy mandates prone to circumvention.

Scientific Research and History

Historical Discovery and Naming

Indigenous peoples of the Pacific region, including and , harvested sea cucumbers for food, tools, and trade well before European contact, with oral histories documenting regular visits by Southeast Asian (Makassan) traders to collect trepang (dried sea cucumbers) as early as the late 17th or early . These practices involved , , and the animals for export to Asian markets, particularly , where they were valued for culinary and medicinal purposes, demonstrating sophisticated of their biology and habitat despite lacking formal taxonomic systems. The scientific naming of sea cucumbers originated in Western natural history with Carl Linnaeus, who in 1758 established the genus Holothuria in the tenth edition of Systema Naturae, drawing on earlier descriptions of Mediterranean and Atlantic specimens that highlighted their soft, cylindrical bodies and tentaculate mouths. This classification initially placed them tentatively among vermes or mollusks due to their worm-like appearance and lack of obvious hard parts, but dissections revealing a radial canal system akin to starfish began to suggest echinoderm relations. Eighteenth-century exploratory voyages, such as those led by Captain from 1768 to 1779, expanded European awareness of sea cucumber diversity through collections of Pacific specimens by naturalists like and , who documented numerous holothuroids during stops in , , and other islands. These expeditions revealed tropical species far more varied than temperate forms previously known, prompting refinements in description amid initial uncertainties about their affinities. By the , improved enabled detailed study of the microscopic dispersed in the —tiny plates, buttons, or rods unique to each species—which provided definitive traits for distinguishing genera and solidified their placement within Echinodermata via shared endoskeletal features with other classes.

Recent Advances and Emerging Research

Recent transcriptomic studies have advanced understanding of sea cucumber regeneration, identifying genes such as Myc, SoxB1, and Klf13 as key regulators in intestinal and body wall repair processes in species like Holothuria glaberrima and Apostichopus japonicus. Single-cell RNA sequencing conducted in 2025 on regenerating intestines of H. glaberrima mapped cellular interactions involving apoptosis, proliferation, and differentiation, revealing conserved pathways with vertebrate analogs that underscore sea cucumbers' utility as evo-devo models despite their invertebrate morphology. These findings build on 2023 analyses of developmental gene regulation, classifying adhesion, cell cycle, and signaling genes into functional groups that parallel vertebrate organogenesis, though empirical validation in wild populations remains limited. In , 2025 research identified single nucleotide polymorphisms (SNPs) linked to resistance in A. japonicus via transcriptome profiling, enabling programs to counter pathogens like splendidus and skin ulceration syndrome. Complementary studies demonstrated dietary interventions, such as ferrous sulfate supplementation, enhancing innate immunity and survival rates against bacterial challenges, with maps supporting targeted improvements in resilience. However, approaches, including butyrate-supplemented feeds, showed promise in modulating for broader tolerance, highlighting the need for integrated genomic-field trials to address overreliance on controlled simulations. Biomechanical investigations from 2023 onward modeled water retention dynamics in Atlantic sea cucumbers (Cucumaria frondosa), quantifying seasonal body water-holding capacity as a of total weight to inform processing yields and stress responses. These models integrate environmental factors like , revealing adaptive mechanisms for hydration that parallel , yet underscore gaps in predictive where field-derived data lags behind lab-centric predictions. Neuropeptidomic profiling in 2025 of radial nerve cords in Holothuria scabra and Stichopus cf. horrens further elucidated neural contributions to and regeneration, providing unbiased endogenous inventories for non-model . Hong Kong market analytics from 2023-2025 indicate sustained high values for dried premium sea cucumbers, with averages exceeding US$500 per kg for select species amid fluctuating export prices (e.g., $3.80-$9.40/kg projections), driving research toward sustainable over short-term yield optimizations. Persistent genomic incompleteness, particularly in wild ecotypes, necessitates prioritizing empirical datasets to refine causal models of beyond simulation-heavy approaches.

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