Sea cucumber

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 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 holothurians, especially on the dorsal surface. The two dorsal ambulacra make up the bivium while the three ventral ones are known as the trivium.[10]

At the anterior end, the mouth is surrounded by a ring of tentacles which are usually retractable into the mouth. These are modified tube feet 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.[10]

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.[10]

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.[9]

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.[9]

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.[9]

Sea cucumbers extract oxygen from water in 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 and then expelling it.[11][12] 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.[9]

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.[9]

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.[9]

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.[9]

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 holothurian blood,[13] however researchers have been unable to reproduce these results.[14]

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.[9]

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.[9]

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 ten and thirty such tentacles and these can have a wide variety of shapes depending on the diet of the animal and other conditions.[9]

Many sea cucumbers have papillae, conical fleshy projections of the body wall with sensory tube feet at their apices.[15] These can even evolve into long antennae-like structures, especially on the abyssal genus Scotoplanes.

Echinoderms typically possess an internal skeleton composed of plates of calcium carbonate. 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.[9]

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 m (3,300 ft) up the water column

Spanish dancer (Benthodytes sp.), another swimming sea cucumber, hovering at 2789 meters 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.[16] At depths deeper than 8.9 km (5.5 mi), sea cucumbers comprise 90% of the total mass of the macrofauna.[17] Sea cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothurians, such as Enypniastes eximia, Peniagone leander and Paelopatides confundens,[18] 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 [19] or float over it in order to move to new locations,[20] in a manner similar to how the group Torquaratoridae floats through water.

Holothurians appear to be the echinoderms best adapted to extreme depths, and are still very diversified beyond 5,000 m deep: several species from the family Elpidiidae ("sea pigs") can be found deeper than 9,500 m, and the record seems to be some species of the genus Myriotrochus (in particular Myriotrochus bruuni), identified down to 10,687 meters deep.[21] 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 per square meter (93 /sq ft). For this reason, one such area in Fiordland is called the strawberry fields.[22]

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 m (3,300 ft) 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.[23]

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.[24] 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 per square meter (33 /sq yd). These populations can process 19 kilograms of sediment per square meter (34 lb /sq yd) per year.[25]

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 kg 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.).

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

"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.[9]

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.[26] 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.[9]

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.[9]

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 holothurians, in particular several species of the genus Periclimenes (genus which is specialized in echinoderms), in particular Periclimenes imperator.[27] 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[28]) 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.[29] Nevertheless, holothurians species of the genus Actinopyga have anal teeth that prevent visitors from penetrating their anus.[30]

Sea cucumbers can also shelter bivalvia as endocommensals, such as Entovalva sp.[31]

• 
  Lissocarcinus orbicularis, a symbiotic crab.
• 
  Periclimenes imperator, a symbiotic shrimp.
• 
  Polynoid worms on a king sea cucumber.

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.[32] 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.[4][33] 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.[8]

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.[4]

Dried sea cucumbers in a Japanese pharmacy

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.[39]

There are many commercially important species of sea cucumber that are harvested and dried for export for use in Chinese cuisine as hoisam.[40] Some of the more commonly found species in markets include:[40][41]

• Holothuria nobilis
• Thelenota ananas
• Actinopyga echinites
• Actinopyga palauensis
• Holothuria scabra
• Holothuria fuscogilva
• Actinopyga mauritiana
• Stichius japonicus
• Apostichopus californicus
• Acaudina molpadioides
• Isostichopus fuscus

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".[42]

Various pharmaceutical companies emphasize gamat, the traditional medicinal usage of this animal.[43] 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".[44]

Another study suggested that sea cucumbers contain all the fatty acids necessary to play a potentially active role in tissue repair.[45] Sea cucumbers are under investigation for use in treating ailments including colorectal cancer.[46] Surgical probes made of nanocomposite material based on the sea cucumber have been shown to reduce brain scarring.[47] One study found that a lectin from Cucumaria echinata impaired the development of the malaria parasite when produced by transgenic mosquitoes.[48]

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

In recent years, the sea cucumber industry in Alaska has increased due to increased demand for the skins and muscles to China.[51] 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.[51]

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.[52] Their popularity as luxury seafood in East Asian countries poses a serious threat.[53]

As of 2013 a thriving black market was driven by demand in China where 1 lb (0.5 kg) at its peak might have sold for the equivalent of US$300[49] and a single sea cucumber for about US$160.[54] 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.[54] 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.[49]

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.[55] Using techniques pioneered by the Chinese and Japanese, a second species, Holothuria scabra, was cultured for the first time in India in 1988.[56] 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.[55]