sea cucumber

(Sea cucumbers)

Phylum: Echinodermata

Class: Holothuroidea

Number of families: 25

Thumbnail description
Worm-like echinoderms with a mouth surrounded by feeding tentacles and often a reduced skeleton of microscopic ossicles

Evolution and systematics

As for most soft-bodied animals, the fossil history of holothuroids, or sea cucumbers, is threadbare. Only 19 species have been described from body fossils, although one form, an Achistrum species from the Middle Pennsylvanian Mazon Creek Formation in North America, is known from study of several thousand, often quite well-preserved, specimens. Most ancient species are known from study of isolated fossils of their ossicles. These microscopic skeletal elements, found in the body walls and internal organs of most taxa, are an important feature in defining extinct and living species. However, because ossicle form varies even within a single animal, most fossil ossicles are classified as paraspecies on the basis of unique morphological features. Largely on the basis of this record of ossicles as well as the few known body fossils, approximately 12 of the 25 living families of holothuroids have been found to have ancient representatives.

The earliest undisputed fossils of holothuroids are of isolated ossicles from the Middle Silurian Period circa 425 million years ago (mya). Plate ossicles attributed to holothuroids are known from the Ordovician Period 450 mya, but their identity as holothuroid is uncertain because they resemble the plates of other echinoderms. Holothuroids appear to have evolved perhaps 480 mya from a poorly known group of extinct burrowing echinoderms called ophiocystioids, which resembled spineless sea urchins with a reduced number of large, plated tube feet. The oldest described body fossil is of Palaeocucumaria hunsrueckiana from the Lower Devonian Period 395 mya. This species is unique among known holothuroids in having plated tentacles, a feature that suggests in part a link to the ophiocystioids. Holothuroids continued to diversify during the Paleozoic Era, when members assigned to the orders Apodida, Elasipodida, Dendrochirotida, and Dacytlochirotida first appeared.

Holothuroids were decimated 250 mya by the end-Permian mass extinction event, as were nearly all other marine organisms. During this time, other classes of echinoderms either became extinct or were reduced to representatives from one or two genera. Holothuroids, however, survived as several divergent groups, perhaps aided by a deep-water, burrowing, or detritus-feeding lifestyle. By the Middle Jurassic Period, approximately 180 mya, holothuroids had diversified considerably, and several new important groups arose, including the living orders Aspidochirotida and Molpadiida, as well as the family Synaptidae within Apodida. In addition, several still-living taxa, now known only from deep water, disappeared from the fossil record. This disappearance may indicate that these organisms invaded the deep sea around that time. By the Jurassic, the family Achistridae, one of the most successful holothuroid groups from the late Paleozoic, had again become a dominant component of the fossil fauna. It met its demise in the Lower Cretaceous 140 mya. Holothuroid fossils younger than 65 million years old are surprisingly scarce, possibly because of a lack of collecting effort. Still, several groups at the family and subfamily levels appear to have arisen during this time, including the family Molpadiidae and two subfamilies within Synaptidae—the Leptosynaptinae and Synaptinae.

The class name Holothuroidea is derived from the term holothourion used by Aristotle to describe an animal that was only "slightly different from the sponges," "without feeling," and "unattached," but "plant-like." This puzzling description resulted in use of the latinized term Holothuria, and its variants from at least the early sixteenth century until the late eighteenth century for siphonophore jellyfish, sea squirts (Tunicata), and priapulid worms, in addition to what we now call holothuroids. Linnaeus originally referred most holothuroids to Fistularia, a name preoccupied by a fish and subsequently abandoned. However, by the nineteenth century, Holothuria referred almost exclusively to certain holothuroids. Nevertheless, the historic uncertainty in ascription engendered several alternative names for the entire class that gained some currency. Arguments over the provenance of the term continued into the twentieth century until Holothuria was formally assigned to the eponymous class of echinoderms in 1924.

Living holothuroids are divided into six orders. Ordinal assignment is based largely on the form of the calcareous ring and tentacles as well as the presence of certain organs, such as respiratory trees and the muscles that retract the oral region. Each order is described according to its taxonomic diversity and major diagnostic features:

APODIDA. Footless sea cucumbers. The order contains approximately 269 species in 32 genera and three families. Tentacles are digitate, pinnate, or, in some small species, simple. Respiratory trees are absent. Tube feet are completely absent. The calcareous ring is low and bandlike, without posterior projections. The body wall is very thin and often transparent. These sea cucumber are found in both shallow and deep water.

ELASIPODIDA. Deep-sea sea cucumbers. The order contains approximately 141 species in 24 genera and five families. Tentacles are shield shaped and used in shoveling sediment. Respiratory trees are present. The calcareous ring is without posterior projections. With the exception of one family, Deimatidae, the body wall is soft to gelatinous. All forms live in deep water.

ASPIDOCHIROTIDA. Shield-tentacle sea cucumbers. There are approximately 340 species in 35 genera and three families in this order. Tentacles are shield shaped, that is, flattened and pad-like. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant. Most forms live in shallow water, although one family is restricted to the deep sea.

MOLPADIIDA. Rat-tailed sea cucumbers. Approximately 95 species compose 11 genera and four families in this order. Tentacles are digitate to simple. Respiratory trees are present. The calcareous ring may have short posterior projections. The body wall is generally soft and pliant. Most forms live in relatively shallow water, although one family is restricted to the deep sea.

DENDROCHIROTIDA. Suspension-feeding sea cucumbers. The order contains approximately 550 species in 90 genera and seven families. Tentacles are highly branched. Respiratory trees are present. Some members have a calcareous ring composed of numerous small pieces or have long posterior extensions. These animals have muscles for retracting the oral introvert. In a few species, the body is hardened from enlarged plate-like ossicles and is U shaped. These sea cucumbers live either attached to hard bottoms or burrow in soft sediment. Most species live in shallow water.

DACTYLOCHIROTIDA. U-shaped sea cucumbers. The order contains approximately 35 species in seven genera and three families. Tentacles are simple or have a few small digits. Respiratory trees are present. The calcareous ring is without posterior projections. These sea cucumbers have muscles for retracting the oral introvert. All members have a rigid body encased in enlarged flattened ossicles. The body usually is U shaped. All members live burrowed in soft sediment. Most live in deep water.

Compared with that of the other four living classes of echinoderms, the phylogeny of holothuroids was poorly known. These animals lack the integrated skeleton that provides an extensive fossil record and do not have the numerous morphological characteristics of other groups of echinoderms. The first speculations about evolutionary relations appeared in a tree figured in 1868 by the German zoologist Carl Semper. Several of Semper's suggestions have been corroborated with formal comparative analyses of morphological features and DNA sequences. The morphological work shows that apodans (members of Apodida) branched off quite early from the other holothuroids, which are united by the presence of hemal vessels and tube feet on the body wall. Among the latter forms, the elasipodans diverged next. Holothuroids in the remaining sister group are united by the presence of respiratory trees. This group radiated in the Upper Triassic to Lower Jurassic into the aspidochirotes and a group united by several characteristics, including posterior projections on the calcareous ring. The latter group diverged into the molpadiians and another lineage, comprising the dendrochirotes and dactylochirotes, the members of which have a retractible oral region called an introvert. Evidence from DNA sequences is largely congruent with the morphological data. However, several important points of disagreement remain: The arrangement of family-level branches within the dendrochirotes is poorly supported, and several families with hardened skeletons may turn out to be distantly related or subsumed within soft-bodied groups. The family-level arrangement within Elasipodida is still uncertain. In addition, the aspidochirote family Synallactidae may comprise two or more independently evolved lineages. Finally, the phylogenetic affinities of the enigmatic families Eupyrgidae and Gephyrothuriidae are unknown.

Physical characteristics

Most sea cucumbers are soft bodied and worm- or slug-like. Some tropical species have thick, muscular body walls, whereas many deep-sea forms are gelatinous and transparent. Most species are perhaps 20 in (15 cm) long, although some apodans are as small as a few millimeters or, in one species, Synapta maculata, more than 118 in (3 m) long. Another species in the aspidochirote Stichopodidae, Thelenota anax, may weigh more than 11 lb (5 kg). Although the description "echinoderm worms" is apt, some holothuroids deviate notably from a vermiform appearance. Several burrowing forms have a foreshortened dorsum, giving them an inflated U-shaped appearance. The limit of this trend is seen in the flask-shaped members of Rhopalodinidae, the mouth and anus of which are adjacent atop a long narrow stalk. Varied development of the papillae—modified tube feet—also contributes to a diversity of form. The numerous enlarged papillae in some Stichopodidae and Synallactidae give them a prickly appearance. The elasipodans Deimatidae and Elpidiidae may have elongate ventrolaterally positioned papillae that serve as "legs," locomotor structures for raising them above, and for negotiating, soft deep-sea sediments. Most holothuroids are dark colored or, in burrowing forms, pale gray to white. In contrast, many shallow-water tropical taxa are brightly colored, being green, red, orange, or yellow. Deep-sea species often are transparent or have a violet to pinkish cast.

The skeleton of holothuroids is considerably modified from that of most other echinoderms. In 90 percent of living species, the skeleton of the body wall is reduced to microscopic ossicles approximately 0.0004–0.04 in (10 µm–1 mm) long. The great variety in shape makes ossicles of considerable taxonomic importance. Ossicles are classified as rods, rosettes, crosses, buttons, tables, and wheels and anchors, among other shapes. In some dendrochirotes, such as the family Psolidae, ossicles may be secondarily enlarged and plate-like so that the animal is rigid. Another important skeletal feature, one definitive for Holothuroidea and used in higher-level taxonomy, is an internal calcareous ring that encircles the pharynx or throat. This ring serves as an attachment surface for muscles operating the oral tentacles and the anterior ends of other muscles that contract the body longitudinally.

As in other echinoderms, the holothurian water vascular system powering the tube feet consists of an anterior ring canal around the pharynx from which arise long canals running posteriorly. Despite their similarity to the radial canals of other echinoderms, the latter structures arise embryologically in a quite different manner. For this reason these canals in holothurians have been recently renamed longitudinal canals. In holothuroids, the larval structures that would in other echinoderms extend away from the mouth and form the radial canals instead become the five primary oral tentacles. This circlet of oral tentacles, from five to more than 20 in number, is another definitive feature of Holothuroidea. The tentacles may be simple, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). In most echinoderms, the water vascular system exchanges water with the environment through a sieve plate, or madrepore, that opens externally. In most holothuroids, however, with the notable exception of elasipodans and some molpadiians, the madrepore is internal and opens into the body cavity or coelom.

Sea cucumbers, with the exception of members of Elasipodida and Apodida, have respiratory trees used in gas exchange. These structures are paired, heavily branched tubes inside the body cavity that attach to the rectum. These structures allow a type of breathing called cloacal breathing also present in an unrelated group, the echiuran worms. In many species from the mostly tropical family Holothuriidae, numerous cuvierian tubules insert at the base of the respiratory trees. These tubules apparently serve as defensive structures in most species that have them. Members of the aspidochirote Holothuriidae and Stichopodidae and the molpadiian Molpadiidae and Caudinidae have a rete mirable, a well-developed dorsal plexus of hemal vessels over the left respiratory tree, which facilitates gas exchange. Ciliated funnels, cups, or vibratile urns are small, numerous organs arranged along the insertion of the intestinal mesenteries into the body walls of the apodans Chiridotidae and Synaptidae. Cup interiors are ciliated and appear to function in removing foreign particles from the coelomic fluid. Statocysts are small organs of presumed importance in balance and are arranged along the anterior radial nerves of apodan families, elasipodan Elpidiidae, and some molpadiians. Some species in the apodan family Synaptidae have "eyes" called ocelli, or optic cups. These structures are small patches of pigmented cells that enclose photosensitive cells at the base of the tentacles.

Distribution

Occurs worldwide from the equator to polar regions at all ocean depths. Latitudinal variation in taxonomic composition is pronounced, even at the level of orders. The shallow-water tropics to warm temperate regions are the most diverse and are dominated by members of the aspidochirote families Holothuriidae and Stichopodidae. The diversity of these families peaks on coral reefs, where 20 species per 2.5 acres (1 hectare) is not uncommon. Dendrochirotes live here as well but become a dominant part of the holothuroid fauna only in shallow cool-temperate to polar seas. Whereas the apodan Synaptidae, subfamily Synaptinae, are primarily found in the tropics, the diversity of Apodida generally increases away from the equator. Molpadiidae also are primarily found at higher latitudes or in deeper water. In the tropics, the Caribbean fauna is distinct from that of the Indo-Pacific region. The waters around southern Africa and New Zealand harbor numerous unusual endemic forms.

Marked taxonomic variation in depth also occurs. The aspidochirote Holothuriidae, in addition to being an essentially tropical family, is primarily a shallow-water group. Most holothuroid habitat, however, is in the deep sea. Many families of holothuroids have at least some deep-sea members. Most Dactylochirotida live at depth. All species of the aspidochirote Synallactidae and Gephyrothuriidae, apodan Myriotrochidae, and the order Elasipodida are found in the deep sea. Among the Elasipodida, species in Laetmogonidae live primarily at bathyl depths (3,000–6,000 ft [915–1,830 m]), whereas those in Psychropotidae and the Peniagone species in Elpidiidae characterize abyssal depths (6,000–18,000 ft [1,830–5,490 m]). The region below approximately 18,000 ft (5,490 m) comprises only approximately 1% of the area of the ocean floor, and a noticeable decrease in species diversity occurs. These depths consist of geologically less stable and inclined substrata in oceanic trenches that extend to 36,000 ft (10,970 m). Nevertheless, in these regions holothuroids dominate the benthic fauna in terms of weight of living organisms. Although nearly all holothuroids are restricted to particular depth ranges, a few species are remarkably indiscriminate. For example, Elpidia glacialis live in waters as shallow as 230 ft (70 m) in northern Europe to as deep as 33,000 ft (10,058 m).

Habitat

Holothuroids are found throughout the marine realm. They may be briefly exposed at low tide or occur in large aggregations on the deepest ocean bottom. Other species are limited to wave-hammered reef crests and rocky shorelines. Many species, particularly those in Aspidochirotida and Elasipodida, are epibenthic, living atop either hard or soft substrata. Others in Dactylochirotida, Apodida, and Molpadiida primarily burrow in sediment. There also are several swimming species, which may venture miles above the sea floor, making Holothuroidea the only class of echinoderms with pelagic members.

Behavior

With few exceptions, holothuroids are very slow-moving animals. Many aspidochirotes rear up and extend their anterior ends into the water column when spawning. Other species writhe violently or inflate when they encounter a predator. Some, mostly deep-sea taxa have adaptations for swimming, such as a flattened body or fringes of webbed papillae that can be undulated rhythmically. Epibenthic taxa wander in an apparently random manner as they feed. Many tropical species are nocturnal, living in crevices or under the sand during the day. Others, usually large species, live permanently exposed in shallow water. This lifestyle may be aided by the presence of toxins in the body wall that deter predation by fishes. The juvenile of one aspidochirote species, Pearsonothuria graeffei, appears to mimic the bright coloration of a toxic species of nudibranch gastropod. One species of tiny apodan lives attached to deep-sea fishes. Many species in the aspidochirote Holothuriidae have cuvierian tubules for use in defense. These structures are expelled through the anus, whereupon they expand dramatically in length and become sticky, entangling or deterring would-be predators, such as crabs and gastropods. Disturbance of some holothuroids can cause them to eviscerate. Dendrochirotes eviscerate anteriorly by detaching their tentacle crown. Conversely, many aspidochirotes eviscerate through the anus. The eviscerated animals usually live and regrow the expelled organs.

Feeding ecology and diet

Holothuroids are either deposit feeders or suspension feeders. Approximately 33 percent of species are suspension feeders, nearly all of them within Dendrochirotida. This group has richly branched tentacles that are lightly coated in mucus and extend into currents to capture algae, planktonic animals, or organic matter. Food is captured passively on mucus-coated sites on the tentacles or mechanically. The tentacles are brought into the mouth one at a time and are wiped clean by contracting muscles encircling the pharynx. While they are being withdrawn from the mouth, the tentacles may be reprovisioned with mucus by small glands in the pharynx. The pelagic elasipodan Pelagothuria captures settling floc with a circumoral funnel of webbed papillae. Most holothuroids, however, feed on bacteria, algae, or detritus in surficial deposits. The variety of ways in which holothuroids feed is reflected in the diversity of tentacle form. Epibenthic aspidochirotes and elasipodans shovel in, or mop up, sediment with peltate tentacles. Synaptid apodans lash the surface with plume-like tentacles. Other apodans, as well as dactylochirotes and molpadiians, are burrowers and have digitate tentacles that probably aid ingestion of the surrounding sediment. One enigmatic molpadiian, Ceraplectana, has short tentacles in sclerous, claw-like sheaths of unknown function.

Numerous animals, including sharks, rays, other large fishes, crabs, gastropods, sea stars, and marine mammals such as walrus, occasionally feed on adult holothuroids, However, holothuroids are a regular part of the diets of only a few fishes and sea stars. Only gastropods in the genus Tonna appear to specialize on holothuroids. These large snails engulf holothuroids with an extensible proboscis or, if the holothuroid is too large, rasp out circular sections of the body wall of prey. Harpacticoid copepods are voracious predators of holothuroid larvae in culture and may therefore also be important predators in the wild.

While sea cucumbers have few specialist predators, they host numerous types of commensals and parasites. These include species living on the body surface, tentacles, and in the anal opening such as portunid and pinnotherid crabs, palaemonid shrimp, polynoid polycheate worms, and flattened polyctene comb jellies (Ctenophora). Eulimid snails (Gastropoda) burrow into the body wall. Turbellarian flatworms and eel-like carapodid pearlfishes may live within the body cavity. Pearlfishes feed externally, but seek refuge in the sea cucumber during the day by entering through the anus and a tear in the rectal wall. Juvenile pearlfishes may feed on the sea cucumber gonad.

Reproductive biology

Holothuroids are unique among echinoderms in having a single, anteriorly positioned gonad and gonoduct leading to a dorsal gonopore. Spawning usually is annual, occurring in the spring or summer. Some species may have a second, usually smaller, autumnal spawning event. Species can have separate sexes or be hermaphroditic. At least one deep-sea species appears to be pair forming. Among broadcast spawners, eggs and sperm are released into the water column, where fertilization and development of the larvae take place. Among brooding species, however, females gather the eggs with their tentacles as they emerge and retain them ventrally or in special pouches. A few species brood their larvae within the body cavity. Brooding is most common among littoral, cold-water species, whereas tropical taxa are almost entirely broadcast spawners, as are, apparently, many deep-sea forms. As with other echinoderms, development is largely either indirect or direct. Indirectly developing species pass through a distinct planktonic and feeding larval phase, the auricularia, before metamorphosing into a barrel-shaped doliolaria and settling as a miniature adult called a pentacula. During metamorphosis, the bilaterally arranged organ systems are reorganized into the pentamerous adult body plan. In direct development, radical metamorphosis to adult morphologic features does not occur but development proceeds directly from a nonfeeding vitellaria larva. The larva may be planktonic or not and is provisioned with lipid stores. Some tropical species may also reproduce asexually as adults by transverse binary fission.

Conservation status

No species is listed by the IUCN or under the CITES convention. Nevertheless, drastic declines in local populations have been caused by commercial overharvesting. Dried and processed holothuroids, called beche-de-mer or trepang, are sold as a gourmet food item in Asian markets, where they form the basis of a multimillion dollar industry. Demand for beche-de-mer is increasing, and overfishing is a threat in many areas. The most valuable species are slow-growing, long-lived tropical forms in shallow water, which are easily harvested. Buyers often move into unregulated areas, where lack of management programs allows unsustainable exploitation. Several areas, such as the Galapagos Islands, Fiji, Sulawesi, Solomon Islands, and Cook Islands, have been overfished, and recovery is slow. Anecdotal evidence suggests that the current lack of commercially valuable species around some islands is due to overharvesting there before World War II. Regulation of harvesting in other areas, such as northern Australia and western North America, has led to long-term, stable fisheries.

Significance to humans

Holothuroids are a food item in several Asian and Pacific Island countries. The widespread use of holothuroids as food and medicine in Asia extends to at least the late sixteenth century, when detailed Chinese and European accounts of commerce first began mentioning trade in beche-de-mer. This long-term, domestic familiarity with holothuroids in the region is reflected in a small role for the animal in northern Asian culture as an object of poetry and popular cartoons. Several thousand individuals of colorful tropical species are harvested annually as part of the worldwide marine aquarium trade. Holothuroids are of minor medical significance because the potent dermal toxins of some species cause severe contact dermatitis in some people. These same toxins are of commercial interest because of their pharmacological properties. Compounds extracted from holothuroids exhibit antimicrobial, anticoagulating, tumor-inhibiting, and antiinflammatory activity. Other compounds are potent respiratory toxins in vertebrates. This feature is used by fishers in the Pacific Islands, who use abraded or chopped holothuroids to poison fishes and force octopuses from their lairs. The sticky cuvierian tubules also are spread over coral cuts to stem bleeding.

Species accounts

Resources

Féral, J.-P., and G. Cherbonnier. "Les holothuries." In Guide des étoiles de mer, oursins e autres échinodermes du lagon de Nouvelle-Calédonie, edited by Guille, A., P. Laboute, and J.L. Menou. Paris: ORSTOM, 1986.

Hendler, G., J. E. Miller, D. L. Pawson, and P. M. Kier. Sea Stars, Sea Urchins, and Allies: Echinoderms of Florida and the Caribbean. Washington, DC: Smithsonian Institution Press, 1995.

Lambert, P. Sea Cucumbers of British Columbia, Southeast Alaska and Puget Sound. Vancouver: University of British Columbia Press, 1997.

Picton, B. E., and R. H. Johnson. A Field Guide to the Shallow-Water Echinoderms of the British Isles. London: Immel, 1993.

Smiley, S. "Holothuroidea." In Microscopic Anatomy of Invertebrates, Vol 14, Echinodermata, edited by F. W. Harrison and F.-S. Chia. New York: Wiley-Liss, 1994.

Smiley, S., F. S. McEuen, S. Chaffee, and S. Krishnan. "Echinodermata: Holothuroidea." In Reproduction of Marine Invertebrates, Vol 6, edited by A. C. Giese, J. S. Pearse, and V. B. Pearse. Pacific Grove, CA: Boxwood Press, 1991.

Gilliland, P. M. "The Skeletal Morphology, Systematics and Evolutionary History of Holothurians." Special Papers in Palaeontology 47 (1993): 1–147.

Kerr, A. M., and J. Kim. "Phylogeny of Holothuroidea (Echinodermata) Inferred from Morphology." Zoological Journal of the Linnean Society 133 (2001): 63–81.

Miller, J. E., and D. L. Pawson. "Swimming Sea Cucumbers (Echinodermata: Holothuroidea): A Survey, with Analysis of Swimming Behavior in Four Bathyal Species." Smithsonian Contributions to the Marine Sciences 35 (1990): 1–18.

Kerr, A. M. "Holothuroidea: Sea Cucumbers." Tree of Life. 1 Dec. 2000 [14 July 2003].

[Article by: Alexander M. Kerr, PhD]

A class of Echinozoa characterized by a cylindrical body and smooth leathery skin, and known as sea cucumbers. There are no arms, but a ring of five or more tentacles may surround the mouth, which is usually at one end of the body. There are no pedicellariae. Tube feet may be present or lacking. There are no ambulacral grooves. See also Echinozoa.

Holothurians resemble worms because the pentamerous symmetry is largely concealed by a secondary bilateral symmetry, and the general absence of external spines distinguishes them from the other extant echinoderms (see illustration).

Cucumaria, a representative holothurian.

The 1100 living species have been grouped in 170 genera arranged in six orders: the Dendrochirotida, Dactylochirotida, Aspidochirotida, Elasipodida, Molpadida, and Apodida. Colors vary widely; the most brilliant colors are found among the Synaptidae. Yellow, red, violet, and fawn tints occur, but many species are somber shades or black.

Holothurians occur in all seas, from low-tide level down to the greatest depths explored. At depths below 5.5 mi (8.8 km) holothurians comprise 90% of the total mass of living matter, the rest being mainly starfishes. Two pelagic genera are known.

A marine animal whose name comes from its cucumberlike shape. It has short tentacles at one end. It's also known as sea slug. Though it is seldom found fresh in the United States, it's sold dried (usually marketed as trepang, iriko or bêche-de-mer) in Asian markets. It must be soaked in water for at least 24 hours, during which time it doubles in size and takes on a gelatinous quality. Its texture is rather rubbery and it's therefore most often used in soups.

For more information on sea cucumber, visit Britannica.com.

Holothuroidea
A sea cucumber
Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Echinozoa
Class:	Holothuroidea de Blainville
Orders
Subclass Apodacea Apodida Molpadiida Subclass Aspidochirotacea Aspidochirotida Elasipodida Subclass Dendrochirotacea Dactylochirotida Dendrochirotida

For the food product made from Holothuroidea, see Sea cucumber (food).

Holothuroidea are a class of marine animals (phylum Echinodermata) with an elongated body and leathery skin, which is found on the sea floor worldwide. Many holothurian species and genera, informally known as sea cucumbers, are targeted for human consumption. The harvested product is also referred to as sea cucumber, or as trepang, bêche-de-mer, balate, or sea slug. The body contains a single, branched gonad. 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. These can sometimes be enlarged to flattened plates, forming an armour. In pelagic species such as Pelagothuria natatrix (Order Elasipodida, family Pelagothuriidae), the skeleton and a calcareous ring are absent^[1][2].

Contents 1 Overview 2 Holothurians as food and medicine 3 Commercial harvest 4 In art and literature 5 Captivity 6 Gallery 7 See also 8 Notes

Overview

Conspicuous Sea Cucumber, Coconut Island, Hawaii

A sea cucumber feeding while on gravel

Sea cucumber in Mahé, Seychelles ejects sticky filaments from the anus in self-defence.

Holothuroidea are generally scavengers, feeding on debris in the benthic zone of the ocean. Exceptions include pelagic cucumbers and the species Rynkatropa pawsoni, which has a commensal relationship with deep-sea anglerfish.^[3] 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. Sea cucumbers communicate with each other through sending hormone signals through the water which others pick up. A remarkable feature of these animals is the catch collagen that 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 hooks up all its collagen fibres to make its body firm again.^[4]

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.^[5] 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 any animal in the vicinity and is one more way in which these sedentary animals can defend themselves. ^[6]

They can be found in great numbers on the deep sea floor, where they often make up the majority of the animal biomass.^[7] At depths deeper than 5.5 mi (8.8 km), sea cucumbers comprise 90% of the total mass of the macrofauna^[8]. Sea Cucumbers form large herds that move across the bathygraphic features of the ocean, hunting food. The body of some deep water holothurians 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 float over it to move to new locations with a minimum of energy.^[9], for instance Enypniastes eximia, Peniagone leander and Paelopatides confundens^[10].

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 metre. For this reason, one such area in Fiordland is simply called the strawberry fields.^[11]

Sea cucumbers extract oxygen from water in a pair of 'respiratory trees' that branch off the cloaca just inside the anus, so that they 'breathe' by drawing water in through the anus and then expelling it.^[12][13] 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.

Emperor shrimp Periclimenes imperator on a Bohadschia argus sea cucumber

Many polychaete worms and crabs have also specialized to use the cloacal respiratory trees for protection by living inside the sea cucumber.^[14]

Ten percent of the blood cell pigment of the sea cucumber is vanadium. Just as the horseshoe crab has blue blood rather than red blood (colored by iron in hemoglobin) because of copper in the hemocyanin pigment, the blood of the sea cucumber is yellow because of the vanadium in the vanabin pigment^[15]. Nonetheless, there is no evidence that vanabins carry oxygen, in contrast to hemoglobin and hemocyanin.

Most sea cucumbers reproduce by releasing sperm and ova into the ocean water. Depending on conditions, one organism can produce thousands of gametes. An unusual sea cucumber found off the South African coast, the red-chested sea cucumber (Pseudocnella insolens), fertilises its eggs internally and then picks up the fertilised egg with one of its 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^[16].

The largest American species is Holothuria floridana, which abounds just below low-water mark on the Florida reefs.

Visitors to Guam often encounter the local variation, called balate, which litters the sea floor all around the island, including in water as shallow as 3 feet. These jet black sea cucumbers are normally 10-12 inches long, 1.5-2.0 inches in diameter and are often curled up, partially covered with sand from the sea floor.

The most common way to separate the subclasses is by looking at their oral tentacles. Subclass Dendrochirotacea has 8-30 oral tentacles, subclass Aspidochirotacea has 10-30 leaflike or shieldlike oral tentacles, while subclass Apodacea may have up to 25 simple or pinnate oral tentacles and is also characterized by reduced or absent tube feet, as in the order Apodida.^{[citation needed]}

Holothurians as food and medicine

Main article: Sea cucumber (food)

Dried sea cucumbers in a Japanese pharmacy

To supply the markets of Southern China, Macassan trepangers traded with the Indigenous Australians of Arnhem Land. This Macassan contact with Australia is the first recorded example of trade between the inhabitants of the Australian continent and their Asian neighbours.^{[17][citation needed]}

There are many of commercially important species of sea cucumber that are harvested and dried for export for use in Chinese cuisine as Hoi sam. Some of the more commonly found species in markets include^[18]

• Holothuria scabra
• Holothuria fuscogilva
• Actinopyga mauritiana
• Stichius japonicus
• Parastichopus californicus
• Thelenota ananas
• Acaudina molpadioides

Some varieties of sea cucumber (known as gamat in Malaysia or teripang in Indonesia) are said to have excellent healing properties. There are pharmaceutical companies being built based on this gamat product. Extracts are prepared and made into oil, cream, or cosmetics. Some products are intended to be taken internally. The effectiveness of sea cucumber extract in tissue repair has been the subject of serious study.^[19] It is believed that the sea cucumber contains all the fatty acids necessary to play an active role in tissue repair.^[20]

On December 21, 2007, a study published in PLoS Pathogens found that a lectin from Cucumaria echinata impaired the development of the malaria parasite when produced by transgenic mosquitoes.^[21]

Commercial harvest

In recent years the sea cucumber industry in Alaska has gained strength due to increased export of the skins and muscles to China.^[22]

In China, many commercial sea cucumbers are farmed in artificial ponds. These ponds can be as large as 1,000 acres, and satisfy much of the local demand.^[22] Wild sea cucumbers are caught by divers and these 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 shares, despite the increase in local, Chinese sea cucumber farming.^[22]

In art and literature

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

Sea cucumbers have inspired musical composition: in the first of his Embryons desséchés for piano solo, Erik Satie presents the "(Desiccated embryo) of a Holothurian" and inserts a description of the animal in the score:

The Holothurian crawls across boulders and rocky surfaces.

This sea-animal purrs like a cat; also, it produces disgusting silky threads.

Light appears to have an incommodating effect on it.

Nonetheless it is the sea cucumber's closest relative (the echinoidea or sea urchin) that gets the most attention from scientists, both as an embryo and as a fossil.

Sea cucumbers have also inspired thousands of haiku in Japan, where they are called namako (海鼠), written with characters that can be translated "sea mice". In English translations of these haiku, they are usually called "sea slugs"; according to the Oxford English Dictionary, the term "sea slug" originally referred to holothurians (in the 18th century), though biologists now use the name only for the nudibranch molluscs, marine relatives of land slugs. Almost 1,000 Japanese holothurian haiku translated into English appear in the book Rise, Ye Sea Slugs! by Robin D. Gill (Paraverse Press, 2003, ISBN 0-9742618-0-7).

Captivity

This section may contain original research or unverified claims. Please improve the article by adding references. See the talk page for details. (August 2008)

Sea cucumbers are very common in marine reef aquaria, particularly in reef tanks, where they are prized for their unusual appearance and behavior. Care of sea cucumbers is not complex, but these unusual creatures have unusual requirements. In the hobby, the term sea cucumber refers to only the detrivore sea cucumbers, that is, those that subsist by consuming the detritus that accumulates on the substrate (such as sand or aragonite).

In particular, these creatures have the remarkable ability to live for months, often up to half a year, without feeding. It is very common for these creatures to be introduced into a system that can't support them, and for the owners to have no idea that they are slowly starving to death. When this happens, the sea cucumber will slowly shrink as it digests its own body mass to survive.

In order to be sure a cucumber is feeding one must watch it at work. It will use the feeding tentacles around its mouth to pick up and swallow sand from the bottom of the aquarium. Particles too big will be of no use to the cucumber, so it is important to watch it to make sure it's feeding, and that it's regularly producing castings of excreted substrate.

In addition to the unusual feeding requirements of sea cucumbers, they release highly toxic compounds when injured. In particular, the filter feeding sea cucumbers, known as "Sea Apples" in the aquarium trade, are exceedingly lethal to the other tank inhabitants should they be injured. All powerheads and pumps should be covered as the cucumbers can squeeze into spaces much smaller than their body. Should a sea apple become injured it must be immediately removed from the aquarium, a major water change needs to be performed, and fresh activated carbon will need to be added if there is to be any hope of saving the other inhabitants.^{[citation needed]}

Gallery

Sea cucumber Euapta godeffroyi feeding at night.

Sea cucumber in Fiji

See also

Wikimedia Commons has media related to: Holothuroidea

Wikispecies has information related to: Holothuroidea

• Trepanging
• Gamat

Notes

1. ^ Pelagic sea cucumber: Information from Answers.com
2. ^ Cambrian holothurians ? – The early fossil record and evolution of Holothuroidea
3. ^ Brusca, R.C., Brusca, G.J.; Invertebrates. Sinauer Associates, Massachusetts, 1990.
4. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
5. ^ Flammang, Patrick; Ribesse, Jerome & Jangoux, Michel (2002-12-01). "Biomechanics of adhesion in sea cucumber cuvierian tubules (echinodermata, holothuroidea)". Integrative and Comparative Biology. http://findarticles.com/p/articles/mi_qa4054/is_200212/ai_n9160035/pg_1. Retrieved 2007-10-03. 
6. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
7. ^ Miller, Nat. "Sea Cucumbers". http://jrscience.wcp.muohio.edu/fieldcourses05/PapersMarineEcologyArticles/SeaCucumbers.html. Retrieved 2007-10-03. 
8. ^ Sci-Tech Encyclopedia: Holothuroidea
9. ^ Carney, Bob (2007-06-18). "The Kingdom of the Echinoderm". http://www.oceanexplorer.noaa.gov/explorations/07mexico/logs/june18/june18.html. Retrieved 2007-10-03. 
10. ^ Deep-sea demersal fish zone
11. ^ Alcock, Nick (2007). "Aquatic Biodiversity & Biosecurity: Shedding new light on the humble sea cucumber". http://www.niwascience.co.nz/ncabb/abb/ma/pi/2003-03/cucumber. Retrieved 2007-10-03. 
12. ^ "Holothurians or sea cucumbers". http://www.fegi.ru/prim/sea/golot.htm. Retrieved 2007-10-03. 
13. ^ Ingram, Jocie (2006-06-16). "Knowing Nature... Cool as a Sea Cucumber". http://www.comoxvalleynaturalist.bc.ca/knowing_nature/2006/sea_cucumbers.html. Retrieved 2007-10-03. 
14. ^ Toonen, Rob, Ph.D. (2003). "Aquarium Invertebrates". http://www.advancedaquarist.com/issues/mar2003/invert.htm. Retrieved 2007-10-03. 
15. ^ Natkin, Michael (2007). "Blood Color". Science Facts. Soak (Source Of All Knowledge). http://www.soak.com/topic/sciencefacts/article/tshow/98556/blood+color. Retrieved 2007-11-16. 
16. ^ Branch GM, Griffiths CL, Branch ML and Beckley LE (2005) Two Oceans ISBN 0-86486-672-0
17. ^ Tippett, A. R. 
18. ^ RAMOFAFIA C., BYRNE M., BATTAGLENE S. C (2003), "Development of three commercial sea cucumbers, Holothuria scabra, H. fuscogilva and Actinopyga mauritiana: larval structure and growth", Marine and freshwater research 54 (5): 657-667, ISSN 1323-1650 
19. ^ Study of healing properties (PDF format)
20. ^ Effects on tissue repair
21. ^ Yoshida S, Shimada Y, Kondoh D, et al. (2007). "Hemolytic C-type lectin CEL-III from sea cucumber expressed in transgenic mosquitoes impairs malaria parasite development". PLoS Pathog. 3 (12): e192. doi:10.1371/journal.ppat.0030192. PMID 18159942. http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0030192. 
22. ^ ^{a b c} Ess, Charlie. "Wild product’s versatility could push price beyond $2 for Alaska dive fleet". National Fisherman. http://www.nationalfisherman.com/2008.asp?ItemID=1800&pcid=373&cid=375&archive=yes. Retrieved 2008-08-01. 

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