The Holothuroidea, or sea cucumbers, are an abundant and diverse group of worm-like and usually soft-bodied echinoderms. They are found in nearly every marine environment, but are most diverse on tropical shallow-water coral reefs. They range from the intertidal, where they may be exposed briefly at low tide, to the floor of the deepest oceanic trenches. The oldest undoubted fossils of sea cucumbers are of isolated spicules from the Silurian (ca. 400 million years ago; Gilliland, 1993). Considerable diversification has occurred since then with about 1400 living species in a variety of forms. Some of these are about 20 cm in length, though adults of some diminutive species may not exceed a centimeter, while one large species can reach lengths of 5 m (Synapta maculata). Several species can swim and there are even forms that live their entire lives as plankton, floating with the ocean currents.
Economically, sea cucumbers are important in two main ways. First, some species produce toxins that are of interest to pharmaceutical firms seeking to learn their medical value. Some compounds isolated to date exhibit antimicrobial activity or act as anti-inflammatory agents and anticoagulants. Second, as a gourmet food item in the orient, they form the basis of a multimillion-dollar industry that processes the body wall for sale as beche-de-mer or trepang. However, the high value of some species, the ease with which such shallow-water forms can be collected and their top-heavy age structures all contribute to over-exploitation and collapse of the fisheries in some regions. Fishermen in the Pacific islands use the toxins, some of which act as respiratory inhibitors, to entice fish and octopus from crevices so that they may be more easily speared. Furthermore, the sticky Cuvierian tubules (see description below) are placed over bleeding wounds as a bandage.
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 Holothuria and 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 antarcticus carries 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.
Holothruroidea, or sea cucumbers, have around 1100 described extant species.
- Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
- Beirne, L., K. Fitzmier, M. Miller. 2001. "Holothuroidea" (On-line). Biological Diversity 2001. Accessed January 28, 2005 at http://www.earlham.edu/~beirnlu/seacucumber.htm.
As for most soft-bodied animals, holothuroids have a poor fossilrecord. Published accounts exist of body fossils for about 19 species,though at least that many body-fossil species lay undescribed on museumshelves. Most ancient holothuroids are known from fossils of isolatedossicles. This complicates the taxonomy somewhat since ossicles can differeven within an individual depending on age, habitat and geography. Howthen does one identify a single species? As a result, most fossilholothuroids have been described as paraspecies based on unique ossicletypes. Entire or isolated elements of the calcareous ring are also known,though less work has been done on these potentially informative structures. The rarity of holothuroid fossils in part may be due to a lack ofcollecting effort, since the microscopic ossicles require specialcollecting methods, and there are few specialists working on the group. Inaddition, isolated ring elements may sometimes be confused with the robustplates of other echinoderms.
Figure 4. Isolated pieces of the calcareous rings of fossil holothurians.
Left: Interradial pieces; Center: Radial pieces; both from apodid holothurians from the Upper Liassic of Germany, approx. 180 Ma;
Right: Pieces from fossil molpadiid holothurians from the Hauterivian of Germany, approx. 130 Ma.
Photographs copyright © 2000 Mike Reich.
Holothuroids probably evolved by at least the Lower Silurian, most likelyfrom a little-known group of extinct Palaeozoic echinoderms calledophiocistioids. However, the oldest reported body fossil of a holothuroidis from the Lower Devonian, while the oldest undoubted ossicle is from theUpper Silurian. Plate ossicles are known from the Ordovician, but theiridentity as holothuroid is uncertain because they resemble the plates ofother echinoderms. Still, plate ossicles ascribable to holothuroids arewell known and, when combined with the phylogenetic evidence, suggest thatseveral groups of ancient plated forms existed that are only distantlyrelated to living plated dendrochirotes and dactylochirotes.Alternatively, these living forms have retained their ancient armour andHolothuroidea has had a long and repeated history of losing a platedmorphology.
A comprehensive account of holothurian palaeontology is found in Gilliland(1993), while an up-to-date bibliography and other palaeontologicalinformation is available from Mike Reich's Fossil Holothuroidea Page.
The Orders of Holothuroidea
The ancestors of the Apodida, Elasipodida and the lineage leading to the remaining orders diverged in the middle to late Paleozoic Era between about 350 to 250 million years ago. The Aspidochirotida, Molpadiida, Dendrochirotida and Dactylochirotida began diverging somewhat later in the Triassic and Jurassic of the early Mesozoic Era about 200 million years ago. Assignment to different orders is largely based on the form of the calcareous ring and tentacles, as well as the presence of certain organs, such as the respiratory trees or the muscles that retract the oral region.
Descriptions of each order given below are modified from Pawson (1982) and Smiley (1994):
- Contains about 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 without posterior projections. The body wall is very thin and often transparent. Found in both shallow and deep water.
- Contains about 141 species in 24 genera and five families. Tentacles are shield-shaped and used in shovelling sediment. Respiratory trees are present. The calcareous ring is without posterior projections. With the exception of Deimatidae, the body wall is soft to gelatinous. All forms live in deep water.
- There are about 340 species in 35 genera and three families. Tentacles are shield-shaped. 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, though one family is restricted to the deep sea.
- There are about 95 species in 11 genera and four families. Tentacles are simple. 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, though one family is restricted to the deep sea.
- Contains about 550 species in 90 genera and seven families. Tentacles are highly branched and extended to filter material from the water column. Respiratory trees are present. Some members with a calcareous ring composed of numerous small pieces or having long posterior extensions. Possess muscles for retracting the oral introvert. The body wall may be hardened from enlarged plate-like ossicles. Live either attached to hard bottoms or burrow in soft sediment. Most species live in shallow water.
- Contains about 35 species in seven genera and three families. Tentacles are simple or with a few small digits. Respiratory trees are present. The calcareous ring is without posterior projections. Possess muscles for retracting the oral introvert. All members have a rigid body encased in enlarged flattened ossicles. The body is usually "U" shaped. All members live burrowed in soft sediment. Most live in deep water.
The most important feature distinguishing the sea cucumbers is a calcareous ring that encircles the pharynx or throat. This ring serves as an attachment point for muscles operating the oral tentacles and for the anterior ends of other muscles that contract the body longitudinally. Sea cucumbers are also distinct as echinoderms in having a circlet of oral tentacles. These may be simple, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). A third key feature, found in 90% of living species, is the reduction of the skeleton to microscopic ossicles (Figure 1). In some species, the ossicles may be enlarged and plate-like.
Figure 1. Calcareous skeletal ossicles from the body wall (in situ) of two recent holothurians.
Left: Wheels and hook-shaped rods of Trochodota allani (Apodida: Chiridotidae).
Right: Spinose wheels with perforated hub and simple rods of Siniotrochus phoxus (Apodida: Myriotrochidae).
Photographs copyright © 2000 Mike Reich.
As in other echinoderms, the holothurian water vascular system consists of an anterior ring canal from which arise long canals running posteriorly (not shown in Figure 2). Despite their similarity to the radial canals of other echinoderms, these latter structures arise embryologically in a quite different manner. For this reason these canals in holothurians have been recently renamed longitudinal canals (Mooi and David 1997). In holothurians, the larval structures that would form the radial canals in other echinoderms instead become the five primary tentacles. Also, holothurians with the exception of members in Elasipodida have a madrepore that opens into the coelom (body cavity). In contrast, elasipodans and nearly all other echinoderms have a madrepore that opens externally.
Figure 2: Main internal anatomical features of a cucumariid sea cucumber (Dendrochirotida).
Drawing by Ivy Livingstone. Copyright © 1995 BIODIDAC.
Some sea cucumbers possess organs not found in other invertebrates. In some Aspidochirotida, the respiratory trees display Cuvierian tubules. In most species, these are apparently defensive structures. They can be expelled through the anus, whereupon they dramatically expand in length and become sticky, entangling or deterring would-be predators, such as crabs and gastropods. Many forms, with the exception of members of Elasipodida and Apodida, possess respiratory trees used in gas exchange. These are paired, heavily branched tubes attached to the intestine near the anus. This type of breathing ("cloacal breathing") is also present in an unrelated group, the echiuran worms.
Hyman (1955) provides a useful account of holothuroid gross anatomy, Smiley (1994) covers microscopic aspects, while Smiley et al. (1991) reviews reproduction and larval development.
Holothurians are found in oceans all over the world.
Biogeographic Regions: indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea
- Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
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).
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
- University of Paisley, 1998. "Class Holothuroidea" (On-line). Accessed January 28, 2005 at http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/echi6.htm.
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
- Waggoner, D., B. Spear. 1994. "The Holothuroidea" (On-line). Accessed January 28, 2005 at http://www.ucmp.berkeley.edu/echinodermata/holothuroidea.html.
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 have an important role as large scale detritus feeders. They cycle up to 90% benthic biomass in ocean.
Ecosystem Impact: biodegradation
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.
Anti-predator Adaptations: cryptic
Based on studies in:
Puerto Rico, Puerto Rico-Virgin Islands shelf (Reef)
USA, Northeastern US contintental shelf (Coastal)
This list may not be complete but is based on published studies.
- Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:19
- Opitz S (1996) Trophic interactions in Caribbean coral reefs. ICLARM Tech Rep 43, Manila, Philippines
Known prey organisms
Based on studies in:
USA, Northeastern US contintental shelf (Coastal)
This list may not be complete but is based on published studies.
- Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:19
Life History and Behavior
Communication and Perception
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
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
Most species live from five to ten years.
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)
- Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
Evolution and Systematics
Discussion of Phylogenetic Relationships
Tree modified from Kerr (2000).
The evolutionary relationships of the major holothuroid lineages were, until quite recently, poorly understood. This was in part due to their lack of an integrated skeleton like that providing the extensive fossil record and numerous morphological characters of other groups of echinoderms. There have been numerous speculations about the relationships within Holothuroidea extending well back into the 19th century. The methods of modern comparative biology had not been applied to these problems until quite recently. Then Littlewood et al. (1997), in an effort to resolve class-level relationships within echinoderms, sequenced two ribosomal genes from a total of four orders. Their analyses consistently supported a close relationship between Dendrochirotida and Aspidochirotida, but they could not resolve the phylogenetic position of Elasipodida and Apodida (Figure 3: A, B). Smith (1997) subsequently argued that the Elasipodida are more closely related to (Dendrochirotida + Aspidochirotida) than the Apodida (Figure 3: C). This hypothesis recalls an early speculation (Semper 1868) whereby Apodida is sister to the remaining holothuroids.
Figure 3. Recent hypotheses about holothuroid relationships.
A. Tree based on complete 18S rDNA sequences (Littlewood et al., 1997).
B. Tree based on partial 28S rDNA sequences (Littlewood et al., 1997).
C. Interpretation of the 18S and 28S rDNA data favored by Smith (1997).
More comprehensive cladistic analyses of morphological and DNA data (Kerr, 2000) agree with Smith's hypothesis. Further, it appears that Dendrochirotida is paraphyletic due to the Dactylochirotida lineage arising from within the Dendrochirotida. This arrangement of the two orders is so far supported by few characters, and an alternate arrangement may ultimately prevail. Kerr (2000) also places Molpadiida as sister to Dendrochirotida plus Dactylochirotida. Together with Aspidochirotida, the aforementioned orders form a group united, most notably, by the presence of respiratory trees. The placement of two rare families currently referred to the Molpadiida, Eupyrgidae and Gephyrothuriidae, is uncertain; they may turn out to be only distantly related to one another and other ordinal level groups of holothurians. Based on the presence of respiratory trees, however, they are unlikely to be closely related to either the Apodida or Elasipodida, which lack such structures. The remaining features of the higher level relationships shown in the figure at the top of this page, though, appear solidly supported and unlikely to change with the consideration of new characters.
Sticky ejectable threads of sea cucumbers adhere underwater and cure quickly because as the outer cell layer is shed, the inner cell layer springs open, elongates, and secretes granules of insoluble proteins that stick together.
"Patrick Flammang of the University of Mons, Belgium, is studying the sea cucumber. The sea cucumber, a relative of the starfish, protects itself from predators by ejecting, in a matter of seconds, fine, sticky threads that entangle an attacker and enable the sea cucumber to sneak away. Before they are ejected, the threads, which consist of an outer and inner layer of cells, are quite short and not sticky. But as they are ejected, the emerging threads shed their outer cell layer, enabling the inner cell layer to spring open and elongate. At the same time, the inner cells secrete granules of insoluble proteins that stick together and adhere to whatever they come in contact with in the water. Such quick-drying, underwater glues may be alternatives to existing marine adhesives which take longer to cure." (Courtesy of the Biomimicry Guild)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimen Records: 6537
Specimens with Sequences: 4570
Specimens with Barcodes: 4413
Species With Barcodes: 413
Public Records: 2202
Public Species: 147
Public BINs: 190
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.
Relevance to Humans and Ecosystems
Economic Importance for Humans: Positive
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
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