When expelling organs, sea cucumbers usually release one or both respiratory trees, the gut and the gonads. This behavior may occur seasonally or in response to predation.
Holothurians have the most developed hemal system of echinoderms, having well developed vessels and several single chambered hearts along the intestinal system. The hemal system functions for gas and food transport.
The non-centralized nervous system of echinoderms allows them to sense their environment from all sides. Holothurians have a nerve ring near the base of the tentacles. The podia are touch-sensitive. Adult pheromones may attract larvae, which tend to settle near conspecific adults.
Communication Channels: chemical
Other Communication Modes: pheromones
Perception Channels: tactile ; chemical
Some populations of sea cucumbers have been overfished, which has an effect on the ecosystem. Overfishing has in some places reduced their role in breaking down organics on the ocean floor. Areas without the sea cucumbers have become unihabitable for other organisms.
Commercially exploitable species are mainly in the order Aspidochirotida. Large amounts of dried sea cucumbers are traded in Galapagos Islands to Asian markets, mainly Japan, Hong Kong, Taiwan, and Singapore. Stocks have become depleted in these countries, so they have been looking for other sources.
Sea cucumbers in Baja California, eastern Russia, and the Galapagos Archipelago have been the focus of recent attention. In Baja California Isostichopus fuscus has been overharvested. In 1994, the National Institute of Ecology in Mexico declared that I. fuscus was in danger of extinction. In eastern Russia, increasing demand on Cucumaria japonica has led to concern for this species, which is harvested for both food and cosmetic products. Because of commercial exploitation in the Galapagos, Ecuador passed the Galapagos Marine Management Plan in 1999 to regulate conservation of sea cucumbers.
The Australian government is trying to seed juveniles of sandfish, Holothuroidea scabra which were reduced by overfishing.
As an echinoderm, members of the Holothuroidea are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults.
The larvae develop in sea water. After three days the larval stage is called an auricularia and is similar to the bipinnaria larvae of asteroids. The auricularia has a ciliated locomotor band, then further develops into a larval stage called a doliolaria, where the ciliated band is broken up into three to five ciliated "girdles". Many species of holothurians have another non-feeding, barrel shaped larval stage called a vitellaria. Likely a specialized condition, it develops gradually, retaining many of the larval features. As it is metamorphosing it is sometimes called a pentactula larva.
After larval metamorphosis, the young sea cucumbers ultimately settle on the substrate and become adults.
Development - Life Cycle: metamorphosis
Holothruroidea, or sea cucumbers, have around 1100 described extant species.
Dried sea cucumbers are an important food source and flavoring source in Asia. Before drying, the sea cucumbers are boiled and the bodies contract and thicken and organs are expelled. Sometimes sea cucumbers are considered an aphrodisiac.
Macerated sea cucumbers that release the toxin holothurin with the Cuvierian tubules have been used by South Pacific Islanders to catch tide pool fish.
Positive Impacts: food ; source of medicine or drug
Holothurians have an important role as large scale detritus feeders. They cycle up to 90% benthic biomass in ocean.
Ecosystem Impact: biodegradation
As suspension or deposit feeders holothurians trap particles and plankton on mucus-covered tentacles. The tentacles are pushed into the mouth to ingest food. Secretory cells from papillae of the tentacles and gland cells of the foregut secrete mucus.
In sedentary forms, holothurians hold out extended tentacles to trap particles and plankton. Motile species crawl across the substrate and use tentacles to capture sediment and organic detritus. Sediment feeders are highly selective deposit feeders, generally consuming highly organic sediments. Members of the subclass Apodacea ingest sediments as they burrow through the substrate.
Branched buccal tentacles surround the mouth. From the mouth, the esophogus leads to the foregut and then intestine, where digestion and absorbtion occur.
Primary Diet: planktivore ; detritivore
Holothurians are found in oceans all over the world.
Biogeographic Regions: indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea
Sea cucumbers are common in shallow water areas to deep ocean floors. While most are benthic, a few are pelagic.
Habitat Regions: saltwater or marine
Aquatic Biomes: coastal
Other Habitat Features: intertidal or littoral
Most species live from five to ten years.
Although they vary in color, most holothurians are black, brown, or olive green. Ranging from three cm to one m long, the largest sea cucumbers may have a diameter of 24 cm.
Holothurians generally look long and worm-like, but retain the pentaradial symmetry characteristic of the Echinodermata. Some may be spherical in body shape. The mouth and anus are located on opposite poles, and five rows of tube feet run from the mouth to the anus along the cylindrical body. Ten to 30 branching tentacles surround the mouth. The tentacles are actually part of the water vascular system.
The water vascular system, found in all echinoderms, accommodates the elongated body of the holothurians. Coelomic fluid, rather than sea water, circulates through the water vascular system. The ring canal around the gut has 1-50 polian vessicles, which may function for hydraulic regulation. Each radial canal has rows of ampullae. Podia, which are the external portion of the tube feet, may, be suckered, reduced, or lost. Podia are more randomly scattered along the body than in other echinoderms. The esophagus, foregut and radial canal of the water vascular system are supported by calcareous plates.
Letters are used to describe parts of echinoderms. The ambulacrum opposite the madreorite is section A. Moving clockwise, other parts are coded B through E. Sections C and D are termed the bivium while all the others are collectively termed the trivium. Holothurians mainly orient themselves to have the trivium on the substrate (ventral side) and the bivium facing up (dorsal side).
In the Holothuroidea, the madreporite is unattached to the coelom and is internal, lying beneath the pharynx in the CD-interambulacral position. A short stone canal follows the madreporite.
While support in most echinoderms is from the skeletal structure, in sea cucumbers, thick sheets of body wall muscles provide support. Microscopic ossicles (or sclerietes) are on the dermal layer and are used in taxonomic identification.
Respiratory trees, which branch out near the rectum of the animal are used for gas exchange as water is pumped through the anus. The respiratory trees are part of the organs that are expelled occasionally by the sea cucumber.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; radial symmetry
Holothurians in general are most vulnerable in their larval stage. Some holothurians discharge sticky tubules, known as Cuvierian tubules, at a potential predator. The tubules are sticky clusters found at the base of the respiratory tree. Predators include sea stars, fish, gastropods, and crustaceans as well as humans. Holothurians also expell their organs, which are later regenerated. This is a seasonal event, but is also thought to be an anti-predator defense.
Known Predators:
Anti-predator Adaptations: cryptic
Holothurians have a single gonad, and most are dioecious. Although most spawn and are fertilized externally, there are approximately thirty brooding species. Some capture eggs with tentacles, placing the eggs at the sole or dorsal body surface for incubation. A few have internal fertilization and development, where hatched young are released.
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); simultaneous hermaphrodite; sexual ; fertilization (External , Internal ); ovoviviparous ; oviparous
While most species release eggs and have no perental investment after spawning, some species brood eggs. A few species also brood the eggs internally until they hatch.
Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Provisioning: Female, Protecting: Female)
Sea Cucumbers (Class Holothuroidea)
A person need wade out only knee-deep around many of the Florida keys to encounter, lying con- spicuously exposed on the muddy bottom, large sausage-shaped creatures 1 foot or more in length and better than 2 inches in diameter. Against the pale gray surface on which they lie, the contrast may be striking: dark brown with light spots, or brick red with raised lumps of black or dark brown. They are sea cucumbers, with a name given them (Cucumis marinus) in the first century a.d. by Pliny, the Roman elder and encyclopedist.
Upon closer examination, none of the usual clues is evident to show which is the head end of the animal. As it lies there quietly, both ends of the cucumber appear to be doing something. At one end, an opening appears, sometimes as much as 1 inch in diameter. If the water is shallow, a current may be noted pouring out of the opening. Or the sea cucumber may be taking in water just as rapidly. The movements of the opening and the slow enlargements and contractions of the whole body suggest a sort of underwater whistling. Actually, they are breathing movements and, in this unusual animal, occur at its rear end.
As though further to astonish the beachcomber on tropical and subtropical shores, a sea cucumber may be found from which a fish's head projects. The fish is very much alive, and the cucumber's breathing movements simply take in water or expel it around the fish. If nudged, the fish may swim out, exposing a slender tapering body as much as six inches long.
Usually a minor drama follows at once. The blenny-like fish turns back immediately to the side of the sea cucumber and moves about along the surface, evidently searching for the respiratory opening again. Often the sea cucumber closes the aperture tightly, as though to keep the fish from returning to its refuge. But eventually the need for oxygen becomes too great. The cucumber opens again, the fish slips in either tail first or head first (and turns around immediately).
The cavity into which the fish goes is the cloaca of the sea cucumber, a chamber serving not only respiration but also as a common exit for wastes from the digestive tract and sex cells from the reproductive system. Sea cucumbers are unique in having a pair of generously branched "respiratory trees" extending blindly from the cloaca far forward in the body cavity. Through their walls oxygen and water pass, keeping the other internal organs aerated and maintaining the plumpness of the cucumber's body.
At the opposite end of the animal a set of tentacles moves slowly, obtaining food. In most sea cucumbers, including the large kinds found near shore in tropical and subtropical waters, these soft organs around the mouth shovel the surface mud into the digestive tract, letting the animal get the nourishment from a great assortment of microscopic life, especially diatoms. The gritty residue is expelled from the cloaca, and sometimes accumulates into conspicuous heaps. The late Professor W. J. Crozier estimated from measurements of the cones of debris that the sea cucumbers on each acre of bottom in one region off Bermuda would pass between 100 and 200 pounds of sand through their bodies annually.
Substantial amounts of the nourishment obtained by a sea cucumber are stored in its body wall. There the food reserve usually gains protection from a slimy, leathery skin in which are embedded little limy secretions of remarkable variety. Some are microscopic plates perforated by many holes. Others are knobby rods, or anchor-shaped, or resembling a concrete bird bath or a wheel with spokes but no rim. Each species has its own distinctive limy granules. Only a few kinds lack them altogether.
Many of the larger sea cucumbers that live close to shore supposedly discourage attack by fish and crabs through the presence of a poison (holothurin) in their skins. If extracts of it are injected into mice, they die quickly. The presence of certain sea cucumbers in an aquarium tank may be enough to poison any fish present. With some of the large subtropical and tropical cucumbers, the effect sometimes persists in a tank for weeks after the echinoderm has been removed and the water changed repeatedly.
Large cucumbers belonging to the genera Holothuriaand Actinopyga have ready a truly astonishing defense against animals that molest them. Associated with the region where their respiratory trees open into the cloaca they have short tubules of red, pink, or white color. If the echinoderm is disturbed seriously or repeatedly, it slowly turns its body until the cloacal opening faces the molester, then performs a general contraction and proceeds to send out the slender tubules in great numbers. The blind ends of the tubules may be enlarged; almost always they are very sticky. And as they emerge from the cloacal opening, they become darting, adhesive threads that, in a minute or less, can so enmesh a crab or lobster that it is immobilized. The cucumber frees itself from the tubules and moves slowly away as though nothing had happened.
With provocation these and many related sea cucumbers will perform a far more amazing trick. With a single powerful contraction they turn themselves partly inside out–throwing out the respiratory trees, the reproductive organs, and sometimes some of the intestine as well. All of these emerge suddenly through the cloacal opening as a tangled mass over and around a crab or fish. From these too the cucumber separates itself, as one of the most spectacular instances of self-mutilation and evisceration in the animal kingdom. Until new organs are regenerated, the sea cucumber continues its breathing movements, drawing sea water directly into its body cavity. In six weeks or so, the animal recovers completely and is ready to repeat the performance if irritated sufficiently.
In many parts of the South Pacific and along Oriental coasts, people deliberately annoy these large sea cucumbers and gather up the extruded organs (particularly the ovaries of a female) as meat for the soup pot or delicacies to be eaten raw. More widespread is the custom of preparing holothurians as "trepang" or "'beche-de-mer." Usually the animal is eviscerated, its body wall boiled, then dried or smoked. In the Indo-Pacific region the product is very popular as an ingredient for soups or as gelatinous tidbits. Great quantities of trepang are sold commercially to the Chinese.
Trepang from the Mediterranean is almost two-thirds protein, whereas that from the Indo-Pacific averages between one-third and one-half protein. Apparently the protein constituents are completely di- gestible, and the method of preparation removes all toxic materials.
Since the sea cucumbers in which the little pearl fish Carapus seems an unwelcome guest are exactly the ones producing fish poison and sticky threads and eviscerating themselves when irritated, a person can only marvel that the pearl fish is able to use the cucumber's cloaca as a refuge. Actually, Carapus gets enough space for its body by sliding its tapered tail well up into one of the cucumber's respiratory trees. Yet the fish seems never to trigger the common responses and is completely immune to the poison.
The potency of the poison to fish in general is well known among natives on many South Sea islands. On Guam, for example, people cut the common black sea cucumber in two and wring the contents of its body cavity into tidal pools to drive the fish to the surface. In the Marshall Islands, similar sea cucumbers are pounded and the mangled remains dropped into pools at low tide, stupefying the fish enough that they can be caught easily. Yet the poison is not feared by the natives. It is harmless to human skin, and fish caught through its use are often eaten raw with no ill effects.
About 500 different kinds of sea cucumbers have been found, living almost exclusively on or in the bottom sediments. Most of them are dull colored, and only a few have contrasting spots or stripes. Yet they pursue their lethargic way of life on minute food in so many different levels of the sea that a surprising variety of form and body build is represented.
Something in common can be seen between a child solemnly licking its fingers to clean them of jam, and a big sea cucumber in its normal method of feeding with ten or more profusely branched tentacles, each like a shrubby tree. The cucumber spreads its tentacles over the sea bottom and rubs them around, gathering food particles in the mucus coating. Then, one at a time, the animal thrusts a loaded tentacle into its mouth, closes fleshy lips around it, and pulls out the tentacle all clean and ready for reloading.
Sea cucumbers acting in this way can be found in cooler waters between low-tide mark and 1200 feet below the surface. Cucumaria frondosa, found in tide pools along rocky coasts on both sides of the North Atlantic, is one that presents a particularly magnificent set of bushy tentacles when fully expanded. Along the body of a Cucumaria, five lengthwise tracts of short tube-feet show the five-parted symmetry so obvious in most echinoderms.
In Thyone, the whole body is studded with tube- feet and curved into a broad U. Ordinarily these ani- mals bury themselves in the bottom with only the cloacal opening and the bushy tentacles exposed. If a Thyone is dug out and then placed on the sea floor, it usually needs three to four hours to work itself into the hidden position again.
Some other sea cucumbers with bushy tentacles have a scale covering. Usually these animals rest on a solelike area of the lower surface, and give the general appearance of an armored slug with tentacles instead of gills. They creep from place to place, and can climb the vertical walls of a glass aquarium at fair speed. Psolus has tube-feet only around and under the creeping sole, whereas Psolidium extends degenerate tube-feet that lack sucker tips through holes in the body scales.
Psolus antarcticuscarries as many as 22 young along with it, holding to smooth areas of the creeping sole. Cucumaria parva has been seen holding plant material against its body, helping keep young in place. Other species of these two genera have pockets in the body wall, usually around the anterior end, in which the eggs develop.
Large tropical and subtropical sea cucumbers usually have twenty tentacles, but each of these feeding organs has an expanded tip and cannot be withdrawn into the body as is done by cucumbers with bushy tentacles. Holothuria is one genus of particularly inert and sausage-like sea cucumbers, with no obvious flattened surface to indicate a ventral side. Actinopyga has a creeping sole, as has Stichopus. Both of these live in exposed positions on mudflats, reaching record lengths of 40 inches and a diameter of 8 inches. Actinopyga differs from Stichopus in that the anus opens into the cloaca through an armament of five limy teeth. Stichopus lacks these teeth, but has the ability to raise its body in waves of movement, like a giant caterpillar walking, and shift the animal far more rapidly than use of its tube-feet would permit.
Close relatives of these cucumbers live in the great depths of the ocean. There Bathyplotes appears to drift well above the bottom for most of its life, supported by a float extending around the rim of its creeping sole. Mesothuria intestinalis, a grayish white animal often tinged with pink or violet, is sometimes found also near the surface. It covers its body with debris, as though to hide from enemies, and has been found to begin adult life as a male, later transforming into an egg-laying female.
Molpadonias are sea cucumbers with a conspicuous tail, often found buried in the bottom mud with only the tail tip and cloacal opening exposed. These animals lack tube-feet, or have them only around the anus, perhaps used there in keeping the cloaca free of sediments. The feeding tentacles are fleshy, sometimes with a few finger-like extensions at the ends.
According to Japanese scientists, molpadonias feed particularly rapidly. An individual may move from 125 to 150 pounds of bottom sediments through its 7-inch body annually in extracting nourishment. One of this type of cucumber is Caudina arenata, found from Rhode Island to the Gulf of St. Lawrence between 100 feet below the surface and low-tide mark. Its tail tip can be found exposed from the sandy mud, and used to capture the buried cylindrical animal. The body may be 1 inch in diameter and 7 in length, in hues ranging from deep purple to flesh-color.
Some sea cucumbers are wormlike, lacking tube- feet and respiratory trees. Usually the body wall is very thin, often translucent, and the animal itself is more active than most other holothurians. Several kinds with this shape of body burrow in the mud and can bury themselves in five to six minutes. Others, while only partly grown, swim to the surface at night by a curious twitching movement of the body, suggesting a scissors kick.
Synaptula is one of the commoner wormlike sea cucumbers. It can be found clambering among sea- weeds and through coral reefs. When fully extended a Synaptula may reach a length of 3 feet, yet be no more than 1/2 of an inch in diameter. Some members of this genus have openings through the wall of the intestine in the female, through which sperms from sea water reach the eggs in the body cavity. Thus fertilization is internal, and the embryos develop for some time in the body cavity before being cast out into the sea. The young of Chiridota rotifera, a wormlike sea cucumber of shallow water in the West Indies, reach the same body form as the parent before they emerge, and this sea cucumber is truly viviparous.
Listen to theSea Cucumber podcast and take the accompanying Google Earth Tour on the EOL Learning and Education website. You can also meet the featured scientists and find relevant educational materials.
Sea cucumbers are echinoderms from the class Holothuroidea (/ˌhɒləˌθjʊəˈrɔɪdi.ə, ˌhoʊ-/). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian (/ˌhɒləˈθjʊəri.ən, ˌhoʊ-/)[1][2] species worldwide is about 1,717,[3] with the greatest number being in the Asia-Pacific region.[4] Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as trepang, namako, bêche-de-mer, or balate. Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter, after which bacteria can continue the decomposition process.[4]
Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are 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 armour. In pelagic species such as Pelagothuria natatrix (order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.[5]
Sea cucumbers are named for their resemblance to the fruit of the cucumber plant.
Most sea cucumbers, as their name suggests, have a soft and cylindrical body, more or less lengthened, 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.[6] Holothurians measure generally between 10 and 30 centimetres long, with extremes of some millimetres for Rhabdomolgus ruber and up to more than 3 metres 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),[7] 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.
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 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.[8]
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 10–30 leaflike or shield-like oral tentacles. Dendrochirotida are filter-feeders, with plump bodies and 8–30 branched oral tentacles (which can be extremely long and complex).
Details of the mouth with its tentacles.
Synaptula lamperti lives on sponges (here in Indonesia).
Synapta maculata, the longest known sea cucumber (Apodida).
The king sea cucumber (Thelenota anax, family Stichopodidae) is one of the heaviest known holothurians.
Chiridota heheva, abyssal species.
Cucumaria miniata, a filter-feeding sea cucumber.
Pseudocolochirus ("sea apple").
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.[9]
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 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]
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]
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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/m2 (93 animals/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/m2 (3.7 individuals/sq ft). These populations can process 19 kilograms per square metre (3.9 lb/sq ft) of sediment 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.).
The mouth of an Euapta godeffroyi, showing pinnate tentacles.
Mouth of a Holothuria sp., showing peltate tentacles.
Mouth of a Cucumaria miniata, with dendritic tentacles, for filtering the water.
Faeces of an holothurian. This participates in sediment homogenization and purification.
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]
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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.
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]
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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. While the organisms in this phylum may not all look the same from the outside, their make-up is another story. The earlliest sea cucumbers are known from the middle Ordovician, over 450 million years ago.[34] The apodida is the sister group to the other orders of sea cucumbers.[35]
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.[36] 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.[37]
All echinoderms also possess anatomical feature(s) called mutable collagenous tissues, or MCTs.[38] 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.[38]
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 to World Register of Marine Species:
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]
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 Malay 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]
"Teripang" in a market in Asia.
Sea cucumber in sauce in China.
Haisom cah jamur, Chinese Indonesian sea cucumber with mushroom.
Deep fried sea cucumbers.
Kripik teripang, Indonesian sea cucumber cracker.
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.[49] In 2016, prices on Alibaba ranged up to $1,000/kg.[50]
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]
In 2020, the Indian government created the world's first sea cucumber conservation area, the Dr KK Mohammed Koya Sea Cucumber Conservation Reserve, to protect the sea cucumber species. In India, the commercial harvesting and transportation of sea cucumbers is banned.[57][58]
Informational notes
Citations
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{{cite journal}}: Cite journal requires |journal= (help) Thelenota ananas, a giant sea cucumber from the Indo-Pacific tropics Sea cucumbers are echinoderms from the class Holothuroidea (/ˌhɒləˌθjʊəˈrɔɪdi.ə, ˌhoʊ-/). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothurian (/ˌhɒləˈθjʊəri.ən, ˌhoʊ-/) species worldwide is about 1,717, with the greatest number being in the Asia-Pacific region. Many of these are gathered for human consumption and some species are cultivated in aquaculture systems. The harvested product is variously referred to as trepang, namako, bêche-de-mer, or balate. Sea cucumbers serve a useful role in the marine ecosystem as they help recycle nutrients, breaking down detritus and other organic matter, after which bacteria can continue the decomposition process.
Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are 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 armour. In pelagic species such as Pelagothuria natatrix (order Elasipodida, family Pelagothuriidae), the skeleton is absent and there is no calcareous ring.
Sea cucumbers are named for their resemblance to the fruit of the cucumber plant.
