Holothuroidea

(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

List of Species

Hydrothermal vent sea cucumber

Chiridota hydrothermica

order

Apodida

family

Chiridotidae

taxonomy

Chiridota hydrothermica Smirnov, 2000, Manus Basin at 8,622 ft (2,628 m) depth (3°6.63′S, 50°21.62′E).

other common names

None known.

physical characteristics

Grayish brown, cylindrical holothuroid with thin, translucent body wall to more than 10 in (25.4 cm) long. Tentacles incompletely surround mouth, leaving a ventral gap. Tips of tentacles have round, flat lobes and scalloped edges. Ossicles are typical of the genus Chiridota. Wheel ossicles are concentrated in body-wall papillae and branched rods in the tentacles.

distribution

Manus and Fiji basins in equatorial western Pacific and southeast Pacific rise near the Galápagos Islands between 6,600 and 8,500 ft (2,010–2,590 m) depth.

habitat

Observed only within the immediate vicinity of active hydrothermal vents, sometimes at the base of black smoking chimneys on bare rock or atop communities of sessile vent organisms.

behavior

Often seen with posterior end hidden in a crevice and in small aggregations of up to three individuals per square foot (0.1 m²).

feeding ecology and diet

Suspension and deposit feeder. Feeds on suspended matter by raising the anterior end upward and spreading its tentacles but also has been seen feeding on benthic material.

reproductive biology

Gonad is composed of clusters of short tubercles to 0.6 in (1.5 cm) long. Nothing else concerning reproduction is known.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Giant medusan worm

Synapta maculata

order

Apodida

family

Synaptidae

taxonomy

Holothuria maculata (Chamisso and Eysenhardt, 1821), Marshall Islands, Micronesia. Three subspecies recognized.

other common names

German: Wurmseegurke.

physical characteristics

An unmistakable species, the longest sea cucumber with a maximum length of 10 ft (3 m), although most animals reach only approximately 3–5 ft (1.0–1.5 m) long. This sea cucumber is serpentine and a mottled light and dark brown. There are 20-40 tentacles, which are feather-like. Ossicles are anchors and oblong perforated plates as well as tiny rosettes and rough rods. The sharp tines of the anchor ossicles protrude from the body wall so that when handled the animal seems to stick to the hands.

distribution

Tropical western Indian Ocean to central Pacific Ocean.

habitat

Coral reefs and adjacent sand flats in subtidal areas to approximately 40 ft (12 m) depth.

behavior

A common species that is active during the day, the giant medusan worm moves slowly by peristalsis, using the posteriorly recurved anchor ossicles protruding from its body wall to gain a purchase on the substrate. When attacked by its principal predator, the gastropod Tonna perdix, the giant medusan worm may allow the snail to tear off the posterior most portion of its body without any apparent ill effect.

feeding ecology and diet

The giant medusan worm is a deposit feeder. It feeds by lashing its feathery tentacles over the sediment, rocks, and sea grass blades.

reproductive biology

Like several other members of Synaptidae, the giant medusan worm is hermaphroditic. Eggs are less than 0.004 in (0.1 mm) in diameter. The animal is a broadcast spawner with a feeding auricularia larva that lives as plankton until it metamorphoses and settles to the bottom as a juvenile.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Tiger's tail sea cucumber

Holothuria (Thymiosycia) thomasi

order

Aspidochirotida

family

Holothuriidae

taxonomy

Holothuria (Thymiosycia) thomasi Pawson and Caycedo, 1980, Colombia.

other common names

None known.

physical characteristics

Large holothuroid to 6.5 ft (2 m) long. Mottled light brown with white-tipped papillae and 20 usually light-colored peltate tentacles. Ventral side lighter than dorsum with scattered tube feet. Skin is thick but soft. The large scattered papillae on the dorsal surface give this sea cucumber a somewhat shaggy appearance. Ossicles in the body wall are ellipsoid buttons with two longitudinal rows of holes in pairs. Tower ossicles usually have a squat spire and a square base ringed with 12 holes.

distribution

Most of the Caribbean Sea from the Bahamas to Colombia, eastward to Panama and Mexico.

habitat

Steep forereefs with living corals from 10–100 ft (3–30 m) depth.

behavior

Hides in reef crevices during the day when not feeding and at most extends only the anterior end to feed. When disturbed, the tiger's tail sea cucumber swells its posterior end to prevent dislodgment and retracts quickly into its shelter.

feeding ecology and diet

A nocturnal deposit feeder capable of ingesting large pieces of coral rubble.

reproductive biology

Nothing is known. However, gonads of specimens found in the Virgin Islands appeared ripe in July and consisted of numerous elongate tubules. Other species from the subgenus Thymiosycia have a feeding, planktonic auricularia larva.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Candy cane sea cucumber

Thelenota rubralineata

order

Aspidochirotida

family

Stichopodidae

taxonomy

Thelenota rubralineata Massin and Lane, 1991, Madang, Papua New Guinea.

other common names

None known.

physical characteristics

A large and colorful sea cucumber to 20 in (51 cm) long and trapezoidal in cross section, the candy cane sea cucumber has a pronounced, flattened ventral sole crowded with tube feet. The dorsal side bears numerous large, pointed papillae. The candy cane sea cucumber has a unique crimson herringbone-like pattern of stripes on a white background and bears approximately 20 dull red, peltate tentacles.

distribution

New Guinea, Indonesia, Philippines, Sulawesi, Solomon Islands, New Caledonia, and Guam.

habitat

Forereef slope on sand patches from 20 ft (6 m) to at least 200 ft (60 m) depth.

behavior

Very little is known about the biology of this rare sea cucumber. It crawls exposed on the reef during the day and night and curls up by bringing its anterior and posterior ends together when disturbed.

feeding ecology and diet

A deposit feeder that ingests reef sediments.

reproductive biology

Nothing is known. Probably similar to the reproductive biology of the other two members of the genus, which broadcast spawn and have indirectly developing larvae.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

Harvested incidentally in small numbers with other commercially valuable holothuroids used in the beche-de-mer industry.

Flask-shaped sea cucumber

Rhopalodina lageniformis

order

Dactylochirotida

family

Rhopalidinidae

taxonomy

Rhopalodina lageniformis Gray, 1853, Congo.

other common names

None known.

physical characteristics

Unusual flask-shaped holothuroid to 4 in (10 cm) long. Body covered in plates. Mouth and anus adjacent atop a slender stalk above a globose body. Fifteen to 25 digitate tentacles in two concentric whorls. The doubled-over body gives the appearance of 10 radii along the body, unlike the canonical five of other echinoderms. The radii do not cross the ventral pole of the body. Ossicles are small knobby towers. Cruciform plates are present at the ventral pole. Tube foot plates have an elongate roughened end.

distribution

Atlantic Ocean along western coast of Africa from Senegal to Cabinda.

habitat

Coastal mud bottoms at 7–20 ft (2–6 m) depth.

behavior

Remains burrowed in mud with only its mouth and anus exposed.

feeding ecology and diet

Nothing is known. The tentacle structure suggests this sea cucumber is a deposit feeder.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Sea apple

Pseudocolochirus violaceus

order

Dendrochirotida

family

Cucumariidae

taxonomy

Colochirus violaceus (Théel, 1886), Philippines. Synonyms include P. axiologus, P. arae, P. bicolor, and P. tricolor.

other common names

French: Pomme de mer; German: Seeapfel.

physical characteristics

A large, colorful species to 7 in (18 cm) long. Color is variable, often purple. There are three ventral, longitudinal rows of tube feet. The dorsal side has two rows of papillae and small scattered papillae. The body is curved in life so that the mouth and anus point upward. The 10 tentacles are bushy purple to red with white tips. Body-wall ossicles are rounded, smooth plates with a few holes and occasionally are absent in large animals.

distribution

Indian Ocean to western Pacific Ocean in continental and continental-island areas, such as Fiji through Indonesia north to southern Japan, south to Australia and Lord Howe Islands but absent from true oceanic islands. India west to Red Sea, Madagascar, and South Africa.

habitat

Hard substrates, including coral reefs, to 40 ft (12 m) depth in areas with currents or upwellings.

behavior

Lives partly concealed to fully exposed with tentacles expanded, even during the day.

feeding ecology and diet

The sea apple is a suspension feeder. It can feed continuously, capturing large phytoplankton with outstretched arborescent tentacles lightly coated in mucus.

reproductive biology

Sexes are separate. Females are distinguished by having a gonopore atop a single unadorned tube a few millimeters long. The counterpart structure in males is tipped in numerous papillae. Males and females release gametes into the water column, where fertilization and development of the larvae take place.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

Sea apples are taken in moderate numbers for the marine aquarium trade but are of only minor economic significance, because only approximately 1,000 of these animals are imported annually into North American and European countries.

Slipper sea cucumber

Psolus chitinoides

order

Dendrochirotida

family

Psolidae

taxonomy

Psolus chitinoides H. L. Clark, 1901, Puget Sound, Washington, United States.

other common names

English: Armored sea cucumber, pedal sea cucumber.

physical characteristics

A yellow to pinkish orange and ovoid sea cucumber to 3 in (8 cm) long. Both mouth and anus are upturned. The dorsum is arched and covered in large, flat plates. The bottom is a soft, flattened sole with tube feet concentrated around its perimeter and scattered down the center. The 10 white-tipped, red tentacles are extensively branched. In addition to the large dorsal plates, the ventral wall of this species has smaller flat and oval ossicles with closely spaced holes and, in larger ossicles, knobs or a central reticulated mound.

distribution

Pacific coast of North America from the Aleutian Islands south to central Baja California.

habitat

Most common in intertidal areas such as rocky shorelines, but occurs from 0–800 ft (0–244 m) on hard, inclined surfaces swept by current.

behavior

The slipper sea cucumber firmly attaches itself to smooth rocks but can use its tube feet to creep along slowly. When the sea cucumber is positioned, the body often becomes covered in debris or other organisms, leaving only bright red tentacles in view.

feeding ecology and diet

The slipper sea cucumber feeds in a manner very similar to that of the sea apple, by extending its bushy tentacles into the current to capture passing particles of food.

reproductive biology

Sexes are separate. Spawning occurs from March until May by release of gametes into the water column, where fertilization takes place. Males aid the dispersal of sperm into the water column by waving a tentacle across the gonopore. Eggs are red and approximately 0.02 in (600 µm) in diameter. The larva does not feed while in the plankton but is provisioned with lipid stores, which see the larva through development.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Sea pig

Scotoplanes globosa

order

Elasipodida

family

Elpidiidae

taxonomy

Elpidia globosa Theél, 1879, Western Pacific Ocean below 2,000 ft (610 m).

other common names

English: Sea cow.

physical characteristics

Transparent, rounded sea cucumber 2–4 in long, with 10 tentacles and a small number of large papillae. The dorsal papillae are of two widely spaced antenna-like pairs. The other papillae are arranged in a row around the edge of the somewhat flattened ventrum. The tentacles are discoid with marginal lobes. Ossicles in the body wall are smooth to spiny rods and smaller C-shaped rods nearly identical to those found in some demospongean sponges.

distribution

Nearly cosmopolitan, although apparently absent from the North Atlantic Ocean and the westernmost Pacific Ocean from Central and South America.

habitat

Deep ocean bottoms from 1,800 to 2,400 ft (550–730 m). Lives at the shallow end of its bathymetric range at higher latitudes and colder water.

behavior

Moves above the sediment with the aid of long, locomotory papillae. Sea pigs aggregate, forming large "herds," sometimes in response to the presence local accumulations of relatively fine sediment. This aggregating behavior, the leg-like papillae, and curved dorsal papillae have earned these animals the alternative name "sea cow."

feeding ecology and diet

Feeds on fine surface sediment on the deep ocean bottom by pushing material into the mouth by means of tentacles with flattened ends. On most specimens a sediment-filled gut is easily seen through the thin body wall.

reproductive biology

Very little is known about the reproduction of deep-sea holothuroids, including sea pigs. Maximum egg size is approximately 0.008 in (0.2 mm) in diameter, a characteristic that suggests the larvae are nonfeeding. The ripe gonad may be visible through the body wall near the anterior end.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Pelagic sea cucumber

Pelagothuria natatrix

order

Elasipodida

family

Pelagothuriidae

taxonomy

Pelagothuria natatrix Ludwig, 1894, Gulf of Panama, Eastern Pacific Ocean below 2,000 ft (610 m) depth.

other common names

None known.

physical characteristics

A bizarre, transparent sea cucumber 2–3 in (5–8 cm) long with 12–16 webbed papillae exceeding the body's length and forming a veil around the mouth. This sea cucumber is slender and pale pinkish purple. The tentacles are two-pronged and number approximately 15. Ossicles and a calcareous ring are absent.

distribution

Western Indian Ocean to eastern Pacific Ocean and Atlantic Ocean at subtropical to tropical latitudes.

habitat

Open ocean from near surface waters to at least 10,000 ft (3,050 m) depth.

behavior

The pelagic sea cucumber often is encountered hanging motionless and drifting. It swims by flapping its veil posteriorly and gliding. The veil collapses and is pulled inward and upward, and the stroke is repeated. Because the veil is incompletely webbed, the sea cucumber moves at an angle to its body axis.

feeding ecology and diet

The pelagic sea cucumber is a suspension feeder. It concentrates descending organic matter in mid-water by hanging vertically in the water column and spreading its extensive veil of webbed papillae into an inverted cone. Gut contents suggest that this species never ingests bottom sediment.

reproductive biology

Spawning and a larval form have not been reported for the pelagic sea cucumber. Postlarval juveniles are pelagic and have midventral tube feet that are lost during growth. The ripe gonad sometimes is visible through the body wall as a white or yellowish tuft.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Rat-tailed sea cucumber

Molpadia oolitica

order

Molpadiida

family

Molpadiidae

taxonomy

Chiridota oolitica (Pourtalès, 1851), Florida, United States.

other common names

None known.

physical characteristics

A brownish gray to reddish black, sausage-shaped sea cucumber to 6 in (15 cm) long with 15 digitate tentacles and a small, sometimes indistinct "tail." The body surface is smooth. The tail and oral region usually are somewhat lighter in color than the rest of the body. Body color is age related; older individuals often are darker as a result of the presence in the body wall of microscopic phosphatic bodies that replace the calcareous ossicles as the animal ages. Ossicles in the body wall are few or absent in larger animals. When present, ossicles are more common in the tail in the form of three-spired towers with a tripartite to elongate base.

distribution

Eastern Gulf of Mexico and western Atlantic coast from Florida to Labrador, west to the North Sea and the Arctic Ocean.

habitat

Soft sand, mud or silt in subtidal areas to beyond the continental slope to at least 1,500 ft (457 m) in depth.

behavior

A burrowing sea cucumber that can occur at densities of several animals per square foot (0.1 m²) in some areas. These populations considerably modify bottom topography and the vertical grain-size distribution of sediment. The relatively stable fecal mounds become home to clams, amphipod crustaceans, and tube-dwelling worms.

feeding ecology and diet

Ingests and processes large amounts of fine sediment in a conveyor-belt manner while positioned vertically, oral end down, in the substrate. Feeding is apparently selective for the finer fraction of sediment, which passes through the gut without being pelletized. The anus lies at the surface, where evacuated material forms a large, wide mound.

reproductive biology

Nothing is known. Other species of Molpadia, such as M. intermedia and M. blakei, have large, yolky ova and a nonfeeding, planktonic vitellaria larva.

conservation status

Not listed by the IUCN or under the CITES convention.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Other

Kerr, A. M. "Holothuroidea: Sea Cucumbers." Tree of Life. 1 Dec. 2000 [14 July 2003]. <http://tolweb.org/tree?group=Holothuroidea&contgroup;=Echinodermata>.

Alexander M. Kerr, PhD