Sea cucumber

Sea cucumbersareechinodermsfrom theclassHolothuroidea(/ˌhɒləˌθjʊəˈrɔɪdiə,ˌhoʊlə-/HOL-ə-thyuu-ROY-dee-ə,HOH-lə-). They are marine animals with a leathery skin and an elongated body containing a single, branchedgonad.  They are found on the sea floor worldwide. The number of knownholothurian(/ˌhɒləˈθjʊəriən,ˌhoʊlə-/HOL-ə-THURE-ee-ən,HOH-lə-)species worldwide is about 1,786,with the greatest number being in theAsia–Pacificregion.Many of these are gathered forhuman consumption, and some species are cultivated inaquaculturesystems. The harvested product is variously referred to astrepang,namako,bêche-de-mer, orbalate. Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and otherorganic matter, after which bacteria can continue the decomposition process.
Like allechinoderms, sea cucumbers have anendoskeletonjust below the skin, calcified structures that are usually reduced to isolated microscopicossicles(or sclerietes) joined byconnective tissue. In some species these can sometimes be enlarged to flattened plates, forming an armour. Inpelagicspecies such asPelagothuria natatrix(orderElasipodida, familyPelagothuriidae), the skeleton is absent and there is nocalcareousring.
Sea cucumbers are named for their resemblance to the fruit of thecucumberplant.
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" (genusPseudocolochirus) to serpent-like forApodidaor the classic sausage-shape, while others resemble caterpillars. The mouth is surrounded by tentacles, which can be pulled back inside the animal.Holothurians measure generally between 10 and 30 centimetres (3.9 and 11.8 in) long, with extremes of some millimetres forRhabdomolgus ruberand up to more than 3 metres (9.8 ft) forSynapta maculata. The largestAmericanspecies,Holothuria floridana, which abounds just below low-water mark on theFloridareefs, has a volume of well over 500 cubic centimeters (31 cu in),and 25–30 cm (10–12 in) long. Most possess five rows of tube feet (called "podia"), butApodidalacks these and moves by crawling; the podia can be of smooth aspect or provided with fleshy appendages (likeThelenota 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.
Holothurians 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 likechordates. 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 calledtrivium(with three rows of tube feet), while the dorsal face is namedbivium.
A remarkable feature of these animals is the "catch"collagenthat forms their body wall.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.
The most common way to separate the subclasses is by looking at their oral tentacles. OrderApodidahave a slender and elongate body lacking tube feet, with up to 25 simple orpinnateoral tentacles.Aspidochirotidaare the most common sea cucumbers encountered, with a strong body and 10 to 30 leaflike or shield-like oral tentacles.Dendrochirotidaare filter-feeders, with plump bodies and eight to 30 branched oral tentacles (which can be extremely long and complex).
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, andthe largestcan reach 3 meters (10 ft). The body ranges from almost spherical to worm-like, and lacks the arms found in many other echinoderms, such asstarfish. 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.
The body of a holothurian 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 otherechinozoans, there are fiveambulacraseparated 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 holothurians, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.
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,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.
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 longitudinalmesenterieswhich surround and support the internal organs.
Apharynxlies behind the mouth and is surrounded by a ring of tencalcareousplates. 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 anoesophagusandstomach, but in some the pharynx opens directly into theintestine. The intestine is typically long and coiled, and loops through the body three times before terminating in acloacalchamber, or directly as theanus.
Sea cucumbers have no truebrain. A ring of neural tissue surrounds the oral cavity, and sends nerves to the tentacles and thepharynx. 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.
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 possessstatocysts, while some species possess small eye-spots near the bases of their tentacles.
Sea cucumbers extract oxygen from water in a pair of "respiratory trees" that branch in thecloacajust inside theanus, so that they "breathe" by drawing water in through the anus and then expelling it.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 ofammoniaandphagocyticcoelomocytesdepositing particulate waste.
Like all echinoderms, sea cucumbers possess both awater vascular systemthat provides hydraulic pressure to the tentacles and tube feet, allowing them to move, and ahaemal system. The latter is more complex than that in other echinoderms, and consists of well-developed vessels as well as opensinuses.
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 thecoelomic fluid.
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 thewhite blood cellsofvertebrates, 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 containshemoglobin. As a result, in many (though not all) species, both the blood and the coelomic fluid are red in colour.
Vanadiumhas been reported in high concentrations in holothurian blood,however researchers have been unable to reproduce these results.
Like all echinoderms, sea cucumbers possesspentaradial 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 forApodida). Likesea 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 numeroustube feet, often with suckers, that allow the animal to crawl along; they are calledtrivium. The two on the upper surface have under-developed or vestigial tube feet, and some species lack tube feet altogether; this face is calledbivium.
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 orderApodidahave 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.
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 retractiletentacles, 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.
Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.These can even evolve into long antennae-like structures, especially on the abyssal genusScotoplanes.
Echinoderms typically possess an internal skeleton composed of plates ofcalcium carbonate. In most sea cucumbers, however, these have become reduced to microscopic ossicles embedded beneath the skin. A few genera, such asSphaerothuria, retain relatively large plates, giving them a scaly armour.
Sea cucumbers can be found in great numbers on the deep seafloor, where they often make up the majority of the animal biomass.At depths deeper than 8,900 m (5.5 mi), sea cucumbers comprise 90% of the total mass of the macrofauna.Sea cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothurians, such asEnypniastes eximia,Peniagone leanderandPaelopatides confundens,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 swimor float over it in order to move to new locations,in a manner similar to how the groupTorquaratoridaefloats through water.
Holothurians 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 familyElpidiidae("sea pigs") can be found deeper than 9,500 metres (31,200 ft), and the record seems to be some species of the genusMyriotrochus(in particularMyriotrochus bruuni), identified down to 10,687 metres (35,062 ft) deep.In more shallow waters, sea cucumbers can form dense populations. The strawberry sea cucumber (Squamocnus brevidentis) ofNew Zealandlives on rocky walls around the southern coast of the South Island where populations sometimes reach densities of 1,000 animals/m(93 animals/sq ft). For this reason, one such area inFiordlandis called the strawberry fields.
Some abyssal species in the abyssal orderElasipodidahave 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 (likeEnypniastes), or a long lobe on top of the body (Psychropotes). Only one species is known as a true completelypelagicspecies, that never comes close to the bottom:Pelagothuria natatrix.
Holothuroidea are generallyscavengers, feeding on debris in thebenthic zoneof the ocean. Exceptions include somepelagiccucumbers and the speciesRynkatorpa pawsoni, which has acommensalrelationship with deep-seaanglerfish.Thedietof most cucumbers consists ofplanktonand decaying organic matter found in the sea. Some sea cucumbers position themselves incurrentsand catch food that flows by with their open tentacles. They also sift through the bottomsedimentsusing 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/m(3.7 individuals/sq ft). These populations can process 19 kilograms per square metre (3.9 lb/sq ft) of sediment per year.
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.).
Most sea cucumbers reproduce by releasingspermandovainto the ocean water. Depending on conditions, one organism can produce thousands ofgametes. Sea cucumbers are typicallydioecious, with separate male and female individuals, but some species areprotandric. The reproductive system consists of a singlegonad, consisting of a cluster of tubules emptying into a single duct that opens on the upper surface of the animal, close to the tentacles.
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.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.
In all other species, the egg develops into a free-swimminglarva, typically after around three days of development. The first stage of larval development is known as anauricularia, and is only around 1 mm (39mils) in length. This larva swims by means of a long band ofciliawrapped around its body, and somewhat resembles thebipinnarialarva of starfish. As the larva grows it transforms into thedoliolaria, with a barrel-shaped body and three to five separate rings of cilia. Thepentaculariais 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.
Numerous small animals can live insymbiosisorcommensalismwith sea cucumbers, as well as some parasites.
Some cleaner shrimps can live on the tegument of holothurians, in particular several species of the genusPericlimenes(genus which is specialized in echinoderms), in particularPericlimenes imperator.A variety of fish, most commonlypearl fish, have evolved acommensalisticsymbiotic 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. Manypolychaeteworms (familyPolynoidae) andcrabs(likeLissocarcinus orbicularis) have also specialized to use the mouth or the cloacal respiratory trees for protection by living inside the sea cucumber.Nevertheless, holothurians species of the genusActinopygahave anal teeth that prevent visitors from penetrating their anus.
Sea cucumbers can also shelter bivalves as endocommensals, such asEntovalvasp.
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 mollusksTonna galeaandTonna perdix, which paralyzes them using powerful poison before swallowing them completely.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 orderAspidochirotidacan defend themselves by expelling their stickycuvierian tubules(enlargements of the respiratory tree that float freely in thecoelom) to entangle potential predators. When startled, these cucumbers may expel some of them through a tear in the wall of thecloacain anautotomicprocess known asevisceration. Replacement tubules grow back in one and a half to five weeks, depending on the species.The release of these tubules can also be accompanied by the discharge of a toxic chemical known asholothurin, 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.
If the water temperature becomes too high, some species of sea cucumber from temperate seas canaestivate. 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.
Holothuroidea (sea cucumbers) are one of fiveextantclasses 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 middleOrdovician, over 450 million years ago.The apodida is the sister group to the other orders of sea cucumbers.
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.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 themesenchyme. 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.
All echinoderms also possess anatomical feature(s) calledmutable collagenous tissues, or MCTs.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.
Holothurian 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 toWorld Register of Marine Species:
To supply the markets ofSouthern China,Makassartrepangerstraded with theIndigenous AustraliansofArnhem Landfrom at least the 18th century and probably earlier. This is the first recorded example oftrade between the inhabitants of the Australian continent and their Asian neighbours.
There are many commercially important species of sea cucumber that are harvested and dried for export for use inChinese cuisineashoisam.Some of the more commonly found species in markets include:
According to theAmerican 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".
Various pharmaceutical companies emphasizegamat, theMalaytraditional medicinal usage of this animal.Extracts are prepared and made into oil, cream or cosmetics. Some products are intended to be taken internally.
A review article found thatchondroitin sulfateand related compounds found in sea cucumbers can help in treating joint-pain, and that dried sea cucumber is "medicinally effective in suppressingarthralgia".
Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair.Sea cucumbers are under investigation for use in treating ailments includingcolorectal cancer.Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring.One study found that alectinfromCucumaria echinataimpaired the development of themalaria parasitewhen produced by transgenicmosquitoes.
Sea cucumbers are harvested from the environment, both legally and illegally, and are increasingly farmed viaaquaculture. The harvested animals are normally dried for resale.In 2016, prices onAlibabaranged up to $1,000 per kilogram ($450/lb).
In recent years, the sea cucumber industry in Alaska has increased due to increased demand for the skins and muscles to China.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.
One of Australia's oldest fisheries is the collection of sea cucumber, harvested by divers from throughout theCoral Seain farNorth Queensland,Torres StraitsandWestern Australia. In the late 1800s, as many as 400 divers operated fromCook Town, Queensland.
Overfishingof sea cucumbers in the Great Barrier Reef is threatening their population.Their popularity as luxury seafood inEast Asiancountries poses a serious threat.
As of 2013, a thrivingblack marketwas driven by demand in China where 450 grams (1 lb) at its peak might have sold for the equivalent ofUS$300and a single sea cucumber for aboutUS$160.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.In theCaribbean Seaoff the shores of theYucatán Peninsulanear 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.
Overexploitationof sea cucumber stocks in many parts of the world provided motivation for the development of sea cucumberaquaculturein the early 1980s. The Chinese and Japanese were the first to develop successful hatchery technology onApostichopus japonicus, prized for its high meat content and success in commercial hatcheries.Using techniques pioneered by the Chinese and Japanese, a second species,Holothuria scabra, was cultured for the first time in India in 1988.In recent years Australia, Indonesia, New Caledonia, Maldives, Solomon Islands and Vietnam have successfully culturedH. scabrausing the same technology, and now culture other species.
In India, the commercial harvest and transportation of sea cucumbers has been strictly banned under Schedule I of theWild Life (Protection) Act, 1972(WLPA) since 2001. In 2020, the Indian government created the world's first sea cucumber conservation area, theDr. K.K. Mohammed Koya Sea Cucumber Conservation Reserve, to protect the sea cucumber species.
Sea cucumbers have inspired thousands ofhaikuinJapan, where they are callednamako(海鼠), written with characters that can be translated as "sea mice" (an example ofgikun). In English translations of these haiku, they are usually called "sea slugs". According to theOxford English Dictionary, the English term "sea slug" was originally applied to holothurians during the 18th century. The term is now applied to several groups ofsea snails,marinegastropodmollusksthat have no shell or only a very reduced shell, including thenudibranchs. Almost 1,000 Japanese holothurian haiku translated into English appear in the bookRise, Ye Sea Slugs!byRobin D. Gill.
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