Holothuroidea

General Description

As their name suggests, many holothurians have a long, cucumber-like appearance. Most species have five rows of tube feet called podia which they use to walk along the sea floor, though two rows of these are not used in locomotion. Around the mouth, a ring of modified tube feet form tentacles used for feeding. Sea cucumbers are considerably diverse, and display many shapes, colours and sizes. Size ranges from just a few millimetres up to five metres long.

Biology

Sea cucumbers are most closely related to sea urchins and sand dollars. Like their relatives, they are radially symmetrical . Sea cucumbers are also bilaterally symmetrical. Like other echinoderms, holothurians retain an endoskeleton underneath their skin made up of calcium carbonate plates. In most cases, these plates have been reduced to microscopic structures called ossicles. Though most diversity occurs in coral reefs, sea cucumbers make up a large percentage of the animals found in the extreme deep sea. Though most spend their life walking along the sea floor, a number of deep sea species have developed the ability to swim through the water column, and one species is known to be entirely pelagic (Pelagothuria natatrix). These swimming sea cucumbers have very gelatinous bodies that are naturally very buoyant. Most adult sea cucumbers are also able to move through the water to escape a threat, but do not spend time actively swimming. Sea cucumbers are mainly scavengers that feed on plankton and decaying organic matter. This may be done by digging through the sea floor with shovel-like tentacles, or extending specialised tree-like tentacles into the ocean currents. As sea cucumbers ingest large amounts of sediment, they play an important role in bioturbation (the reworking of sediments), improving the quality of shallow marine and reef habitats. Reproduction is generally achieved by broadcasting sperm and eggs into the surrounding water, which then develop into free-swimming larvae. A number of species brood their young in pouches. Sea cucumbers belonging to the orders Aspidochirotida and Dendrochirotida can eject their digestive and respiratory organs through the body wall in a spectacular process called evisceration. This is often in response to a threat, however is also known to occur seasonally in some species. The ejected body parts are able to quickly regenerate. Some species are harvested for food and considered to be a delicacy.

Distribution

Worldwide.

Habitat

Most diverse in shallow-water reefs, but range from intertidal platforms to deep ocean trenches. One species is known to be pelagic.