Characteristics of sea cucumbers
Unlike other echinoderms, holothurians don't have a distinct radial
symmetry but are bilateral (distinct dorsal and ventral side).
Holothurians are also called sea cucumbers. As their name suggests,
they are cucumber shaped with an elongated, muscular, flexible body
with a mouth at one end and the anus at the other. Around the mouth
there is a number of tentacles (modified tube feet) used in food
collecting. Sea cucumbers come in many sizes, from small species
only a few centimeters in length to long snakelike animals which
may stretch up to 2 meters!
Ecology and range of sea cucumbers
Rubble, rocks and sand. Also seen on some sponges in large
aggregations.
Behavior of sea cucumbers
Most species feed on the rich organic film coating sandy surfaces.
The crawl over the bottom ingesting sand. The edible particles
(organic matter such as plankton, foraminifera and bacteria) are
extracted when passing through their digestive tract and the
processed sand is expelled from the anus (as worm-like
excrements).
Sea cucumbers move by means of tube feet which extend in rows from
the underside of the body. The tentacles surrounding the mouth are
actually tube feet that have been modified for feeding.
Other holothurians feed on current-borne zooplankton. They bury in
sand extruding their featherlike tentacles (Pseudocolochirus
violaceus, Neothyondium magnum or Pentacta crassa ). The tentacles
have the same shape as soft corals or some anenemones. Large
congregations of some small species are found on sponges. They
apparently feed on substances secreted by the sponges as well as
detritus from the surface.
Some species of holothurians have separate sexes others are
hermaphrodites. The sea cucumbers hold on to exposed rocks or
corals, raise their body to a upright position, rock back and forth
and release the sperm and eggs into the sea.
Sea cucumbers have a remarkable capacity for regenerating their
body parts. When attacked they shed a sticky thread like structure
which is actually parts of their guts. The so called Cuverian
threads are toxic (the poison is called holothurin) and can
dissuade many potential predators. These structures quickly
regenerate.
Holothurians host a variety of symbiotic organisms: crabs, shrimps,
worms and even a very unusual fish. The pearlfish (Encheliophis
homei and mourlani / Onuxodon margaritiferae) has a long slender,
transparent body and lives in the gut cavity of the sea cucumber
(Boshida argus, Thelanota ananas, Stichopus chloronotus). They also
inhabit some starfish as well as pearl oyster shells. The fish
leaves and enters (tail first) through the holothurian's anus. They
probably feed on the gonads and other tissues of its host. It is
said to leave at night to feed on small fishes and shrimps. Sea
cucumbers are used in Asia as a base for soups.
