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.

Holothruroidea, or sea cucumbers, have around 1100 described extant species.

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 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):

Apodida

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.

Elasipodida

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.

Aspidochirotida

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.

Molpadiida

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.

Dendrochirotida

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.

Dactylochirotida

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

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

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

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.

Known Predators:

• sea stars, Asteroidea 
• gastropods, Gastropoda 
• crustaceans, Crustacea 
• fish
• humans, Homo sapiens 

Anti-predator Adaptations: cryptic

Holothuroidea (Sea Cucumbers) is prey of:
Chondrichthyes
Actinopterygii
organic stuff
Decapoda
Stomatopoda
Anomura
Asteroidea
Echinoidea
Gastropoda
Urophycis chuss
Gadidae
Melanogrammus aeglefinus
Leucoraja erinacea
Leucoraja ocellata
Amblyraja radiata
Macrozoarces americanus
Pleuronectes ferrugineus
Glyptocephalus cynoglossus
Pleuronectes americanus
Mustelus canis
Squalus acanthias
Lophius americanus

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.

Holothuroidea (Sea Cucumbers) preys on:
detritus

Based on studies in:
USA, Northeastern US contintental shelf (Coastal)

This list may not be complete but is based on published studies.

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)

View Holothuroidea Tree

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.

Threads adhere underwater: sea cucumber
 

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.

Collection Sites: world map showing specimen collection locations for Holothuroidea

Barcode of Life Data Systems (BOLD) Stats
                                                             

Specimen Records:	3,325
Specimens with Sequences:	1,437
Specimens with Barcodes:	1,348
Public Records:	1,171
Species:	356
Species With Barcodes:	121

Download FASTA File

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.

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