Holothuroids are generally cucumber shaped, but some are almost spherical and others are vermiform.
The mouth and anus are at opposite poles.
A tail may be present (which may be postanal).
Sea cucumbers lack arms.
Ambulacral and interambulacral axes are arranged meridianally around the polar axis.
The polar axis is greatly lengthened.
The mouth and anus may be terminal, ventral or dorsal.
Sea cucumbers lie with one side of the body against the substrate. This becomes the ventral surface, which may be flattened into a sole. The mouth and anus may then be dorsal. The dorsal surface may be arched. This ventral-dorsal differentiation imposes bilateral symmetry on top of the axial radial symmetry.
Buccal podia form a circle of 10-30 tentacles around the mouth. These tentacles may all be the same size, or some may be dwarfed. The latter may comprise an inner circlet of tentacles. The tentacles may branch (described as dendritic / arborescent), and they may be pinnate, peltate or digitate. The tentacles are retractile and the body wall can close over them. (The body wall may also form an introvert in Dendrochirota, which can also be withdrawn). A tentacular collar is present.
The skeletal plates reduced to microscopic ossicles.
Interradius CD forms the middorsal line. Radius A is midventral, radius B is on the animal’s right and radius C is on the left.
Genital papilla may lie between or posterior to the tentacles. A hydropore or madreporic plate may occur close to the genital papilla.
Sea cucumbers typically range in length from a few cm to 30-50 cm. Stichopus variegates may be up to 1 m long and 21 cm in diameter. The vermiform (worm-like) Apoda may be up to 2 m long when fully stretched.
Most are dull coloured – gray, brown, olive or black. The sole may be a lighter hue – white, yellow, pink, rose or terra cotta. Deeps-sea Elasipoda are often purple, maroon, or violet. Some holothuroids are transparent, or brightly coloured – rose, pinkish, orange, violet, yellow or red and may have spots or stripes.
The epidermis is non-ciliated and covered by a thin cuticle. It is often thick and leathery and slimy, but it may be thin and transparent. It is often covered by warts, papillae or tubercles.
The podia may be arranged in rows (more or less), but dorsal podia often lose their suckers and become reduced to sensory papillae. If present, the creeping sole embraces three ambulacral radii (E, A, B) and is known as the trivium. The dorsal surface (radii D, C) is the bivium. The tube feet may be radially arranged, or they may cover the sole.
Echinoderms have mutable dermal ('skin') tissue that can change its consistency through a range of rigidities from soft to hard, under the control of the nervous system. The dermal tissue contains collagenous fibres and, in at least some holothurians, a protein called tensilin has been found to cross-link collagen fibres to increase rigidity during transition from the soft to normal state. An enzyme can, when activated, cut these cross-links to make the dermis soft again. At least one additional protein has been found which can increase rigidity further, triggering a transition from normal to hard dermis.
The body wall may possess armoured calcareous endoskeletal plates (ossicles). The anus is terminal or dorsal or ventral and may be surrounded by 5 calcareous teeth, plates or papillae. A calcareous ring of calcareous plates encircles the beginning of the pharynx (analogous to Aristotle’s lantern in echinoids). This supports the pharynx, nerve ring and water vessels and is the point of insertion of longitudinal muscle bands and pharynx retractor muscles. These muscles retract the tentacles and pharynx.
Most sea cucumbers are sluggish, benthic animals that crawl amongst seaweed, rocks, over sand or live inside burrows. Depending on species the time required to excavate a burrow is from 5 min to 4 hours, and is achieved by muscle contractions of the body and digging movements of the tentacles. The maximum rate of burrowing is 2-3 cm / min. Sea cucumbers tend to be nocturnal, outstretching their tentacles to feed at night and exhibit both diurnal and annual activity cycles.
Locomotory podia, or pedicels, may be present. In forms with a flattened ventral sole, pedicels may be present on the sole, but may be lacking elsewhere or reduced to warts or papillae. The creeping sole may also apply suction to the substrate, and these animals can climb vertical glass surfaces. Muscular waves in the sole may assist locomotion. These sea cucumbers may travel 1 m in 15 min. The elasipods (elasipoda) are deep-sea holothuroids with enlarged podia for walking. This helps to keep them clear of the muddy substrate.
Apodous sea cucumbers lack tube feet and have poor locomotory powers. Most are burrowers. They can locomote, however, by using the tentacles to pull the animal along, assisted by muscular contractions of the body.
Pelagic sea cucumbers have floating or swimming devices, comprised of papillae webbed together to form sails and fins. It is estimated that about half of sea cucumber species are capable of swimming. The ends of the body may be thrashed together in a U-shaped motion. Peniagone diaphana floats vertically with the tentacles uppermost.
Dendrochirotes are plankton feeders. The tentacles fan-out into the water or sweep across the substrate. Their mucous secretions collect particles and one-by-one the tentacles bend over into the pharyngeal lumen. The mouth closes and, as the tentacle is pulled out, the food particles are wiped off into the pharynx. The food consists of microorganisms, organic particles, small crustaceans, nematodes, etc.
Non-dendrochirotes use their tentacles to shovel the substrate into their mouths. They pass 6-8 g of matter per hour.
The mouth is in the centre of the buccal membrane, which is encircled by tentacles and equipped with a sphincter muscle. The pharynx leads through the calcareous and water rings and may give rise to a short oesophagus. The pharynx or oesophagus lead into the stomach and hence into the intestine. The intestine is looped within the coelom and is 2-3 times the length of the body. First it descends posteriorly along the middorsal and then ascends anteriorly along the left side. This whole descending and ascending section forms the small intestine. Finally the intestine descends posteriorly along the midventral, as the rectum or large intestine, to the anus. The anus opens into the cloaca, along with the respiratory trees. The gut is supported by 1-3 mesenteries.
Digested food particles are absorbed by amoebocytes, which enter the circulation via the intestinal rete.
A supepidermal plexus innervates the body wall. The main neural centre consists of a circular or pentagonal nerve ring in the buccal membrane close to the tentacle bases. Just inside and adjacent to this nerve ring is a circular coelomic cavity – the peribuccal sinus. The nerve ring sends out a ganglionated nerve into each tentacle and nerves to the buccal membrane and pharynx and puts out radial nerves along the ambulacra in the dermis, just external to the radial water vessels.
The nerve ring and the ectoneural radial nerves constitute the ectoneural system. This system innervates the podia and the body wall. The hyponeural system is mostly, if not entirely, motor and supplies the body wall muscles. There is no aboral system in contrast to the crinoids.
The body wall nerve plexus innervates scattered epidermal sensory cells. Podia are also very sensitive. In synaptids, which lack podia, sensory buds or warts occur over the surface. The tentacle stalks of apodous forms also contain 1-30 ciliated sensory pits per tentacle. Statocysts are found in holothuroids. In Synaptids there is one pair along each radial nerve. These statocysts are hollow spheres of flattened nonciliated epithelium enclosing 1-20 lithocytes. The lithocytes are vacuolated cells containing an inorganic material. Other sea cucumbers may have up to 100 statocysts around the nerve ring.
The dermis is sensitive to light. Some species possess a pair of ‘eyes’ at the base of each tentacle.
The coelom is a spatious fluid-filled cavity between the body wall and digestive tract. Hyponeural sinuses, peripharyngeal sinuses, peribuccal sinuses and the perianal sinus (coelomic ring) are all coelomic cavities. Some synaptids have a pulsatile rosette – rounded coelomic projections near the calcareous ring. These rosettes pulsate a few times every minute. Other synaptids have vibratile clubs (ciliated clubs projecting into coelom) along the longitudinal muscle bands. The functions of rosettes and vibratile clubs are unknown.
Coelomic fluid recirculates at 10-15 mm/s, due to action of the ciliated epithelium.
In the Apoda, ciliated urns or funnels act as excretory organs. These usually occur on mesenteries or on the body wall. Waste laden coelomocytes accumulate here, and at the general body wall. Waste clumps (brown bodies) fall from the funnels into the coelom. In species with no urns the respiratory trees are the principal excretory organs, and laden coelomocytes will accumulate here. The gonads and intestines also act as excretory exits for coelomocytes.
The ciliated water ring (ring canal) encircles the pharynx, posterior to the calcareous ring. It gives off five radial canals and connects to 1-12 expansion chambers, or polian vesicles and typically a single stone canal. The stone canal may open to the exterior via a hydropore or madreporite or may be closed off and terminate in a madreporic swelling that may hang free in the coelom.
The haemal system is well developed and is complex in the larger forms. It consists of a haemal ring around the pharynx, which gives off 5 radial sinuses, which innervate the tentacles and podia. Ventral and dorsal sinuses run along the intestine. The dorsal sinus supplies the gonad and also forms a rete mirabile (‘wondrous blood network’) supplying the intestine. Coelomocytes are manufactured in large haemal channels. The blood spaces are sinuses rather than true vessels, since they lack an epithelial lining, but instead the lumen is lined by connective tissue, backed by muscle, which in turn is backed by coelomic lining.
The dorsal sinus is contractile, pulsing 1-12 times / min. It delivers blood to the anterior of the animal.
This may disappear in adult forms (or is much reduced?).
These arborescent tubes spring from the anterior part of the cloaca. They are digestive tract evaginations that descend anteriorly in the coelom for much of the animal’s length. The respiratory trees are contractile and take-up oxygen. They are in close association with the rete mirabile of the haemal system and probably transfer their oxygen to it.
Tubules of Cuvier
These white, pink or red tubules are attached to the bases of the respiratory trees (especially the left respiratory tree). They have a defensive function. When the animal is sufficiently irritated, these tubules are emitted from the anus (through a rupture in the cloacal wall) towards the source of irritation. They rapidly elongate into sticky or toxic threads and detach from the animal. They then regenerate.
Most holothurians are dioecious. The sexes are indistinguishable, unless the female is brooding her young. Some species are hermaphrodite. Unlike all other echinoderms, holothurians do not have a pentamerous reproductive system. There is a single gonad, situated in the anterior part of the coelom in interradius CD. It opens to the exterior (near the madreporite when this is present) along interradius CD, via a ciliated gonoduct and gonopore. The gonad may consist of numerous tubules, and may be branched.
During spawning, the sex cells leave the gonopore in a slow stream to be dispersed by tentacular movements. Each spawning lasts from 15 min. to 4 or more hours, and there may be several spawnings during spring and summer.
Some holothuroids brood their young, using one of the following strategies:
The ovaries may incubate the eggs (viparity).
The young hatch and adhere to the external surface of the mother, for example on the creeping sole.
The eggs may be stored on the tentacles and then later ejected.
The young may develop in pockets or depressions on the body surface (on the back or sole, sometimes beneath large dorsal scales).
The eggs may be caught by the tentacles, which form a mesh, and transferred to the dorsal surface of the animal. Two very extensile podia near the gonopre may help this transfer and dorsal podia transport the eggs to brood pockets.
The eggs may rupture from the gonads into the coelom. Sperm may enter the coelom through the anus and then through pores in the intestine.
Holoblastic, radial cleavage gives rise to a coeloblastula. Gastrulation gives rise to a gastrula. The flagellated embryo later escapes either as a doliolaria larva (cf. crinoids) or it develops (after about 3 days) into an auricularia free-swimming, pelagic larva (0.5-1 mm across). The auricularia is characterised by the possession of continuous flagellated bands (including the preoral and anal loops). The auricularia develops into a doliolaria.
The doliolaria possesses a gut (vestibule, or oral region) and 3-5 flagellated rings. The doliolaria undergoes torsion – the vestibule rotates to the anterior end – resulting in a pentactula larva, which posses 5 primary tentacles and 1-2 podia. The pentactula develops into a young sea cucumber.
Asexual reproduction may occur by transverse fission. Constriction, twisting and pulling breaks the animal into 2-3 separate pieces, which regenerate.
Regeneration is known to occur after evisceration and transverse fission.
Although some float or swim, most holothuroids are benthic animals. They inhabit all seas and all depths. They are found from the littoral zone down to the Hadal depths. They are generally sluggish and may not locomote at all if conditions are suitable.
Holothurians dominate Hadal depths below 8000 m, where they comprise 98% of animals and more than 99% of wet weight animal samples. Deep sea holothuroids are sometimes seen in herds, aggregating perhaps to reproduce or gathering around particularly rich feeding sites.
Holothurians, as sluggish animals, support a wide range of commensals and parasites, including protozoa, rotifers, turbellarians, polychaete annelids, crustaceans and gastropod and bivalve molluscs. Some bivalve parasites have been found in the descending intestine, robbing the host of food. Some parasitic snails are still recognisable as such. Some have no shell, but some parasitic snails of holothuroids are changed beyond recognition. They may be tubular or worm-like organisms, with no shell, attached at one end to a haemal sinus or intestine. Some may reach 130 cm in length and hollow, lacking all systems except the reproductive organs. These snails penetrate the host by boring through the body wall with their proboscis, during which an anterior fold grows over the visceral hump of the snail as a pseudopallium, which forms a brood chamber. Body systems, except the reproductive system, degenerate. Gasterosiphon deimatis attaches at one end to the inner wall of the sea cucumber and at the other to the host intestine and has a sac-like pseudopallium containing embryos.
The pearl fish, Carapus, is a small, very slender fish up to 15 cm long, with a pale body and long, slim tail. It occupies the main stem of one of the respiratory trees, with its head protruding from the anus. They enter through the anus, and although the sea cucumber may resist by closing the anal aperture, it must eventually open the anus for respiration. The fish finds its own food (small crustaceans) when it leaves its host at night and only users the host for shelter and refuge.
Holothurians include such forms as the sea cucumbers and sea apples. They exhibit a remarkable diversity of forms, from sac-like spherical or cylindrical to worm-like. They have pentaradial symmetry (with 5 axes) apparent from the 5 tracts of podia or tube-feet. However, there is usually some bilateral symmetry superimposed, with tentacles at the anterior end, which are often branching and tree-like, and a flattened ventral surface whose tube feet function in locomotion. Holothurians are the dominant animal forms on the deepest ocean beds in the Hadal zone. Sea pigs have podia modified into large limbs on which they walk, holding themselves above the soft ooze as they trawl for food.