Sea cucumbers are aclass ofechinoderms, theHolothuroidea. They have a longish body, and leathery skin. Sea cucumbers live on the floor of the ocean. Most sea cucumbers arescavengers. There are about 1500species of sea cucumbers. Sea cucumbers have a uniquerespiratory system, and effective defences againstpredators. TheChinese people eat them.
Like all echinoderms, sea cucumbers have anendoskeleton just below the skin,calcareous structures that are usually reduced to isolated ossicles joined byconnective tissue. These can sometimes be enlarged to flattened plates, forming an armour. Inpelagic species the skeleton is absent.[1][2]
Sea cucumber, Coconut Island,HawaiiSea cucumber in theSeychelles ejects sticky filaments from the anus in self-defence.
Sea cucumberscommunicate with each other by sendinghormone signals through the water.
A remarkable feature of these animals is thecollagen which forms their body wall. This can be loosened and tightened at will. If the animal wants to squeeze through a small gap, it can undo the collagen connections, and pour into the space. To keep itself safe in these cracks, the sea cucumber hooks up all its collagen fibres to make its body firm again.[3]
The animals have an internal respiratory tree which floats in the internal watery cavity. At the rear, water is pumped in and out of thecloaca,[4] so gaseous exchange takes place with the resiratory tree in the gut.[5]p80
Some species ofcoral reef sea cucumbers defend themselves by expelling stickycuvierian tubules to entangle potential predators. These tubules are attached to the respiratory tree in the gut. When startled, these cucumbers may expel the tubules through a tear in the wall of the cloaca. In effect, this squirts sticky threads all over a predator. Replacement tubules grow back in one-and-a-half to five weeks, depending on the species.[6] The release of these tubules can also be accompanied by the discharge of atoxic chemical known asholothurin, which has similar properties to soap. This chemical can kill any animal in the vicinity and is one more way in which these sedentary animals can defend themselves.[3] Other cucumbers, lacking this device, can split their intestinal wall, and spew out their gut and respiratory tree. Theyregenerate them later. Zoologists who experience this believe it to be an impressive deterrent. "The mess one individual can make must be seen to be believed".[5]p81
The existence of these defences explains why the holothurians were able to do without the strong skeleton of their ancestors.
Highly modified tube feet around the mouth are always present. These are branched and retractiletentacles, much larger than the regular tube feet. Sea cucumbers have between ten and thirty such tentacles, depending on the species. There is a ring of larger ossicles round the mouth andoesophagus to which the muscles of the tube feet are attached.[7] With their sticky tentacles the animal collects detritus and small organisms.
