Sea cucumber

Sea cucumber

Scientific classification
Kingdom:	Animalia
Phylum:	Echinodermata
Subphylum:	Echinozoa
Class:	Holothuroidea de Blainville, 1834

Where seen?
Sea cucumbers are commonly seen on many of our shores. On sandy patches large sea cucumbers may move about on the surface, while smaller ones burrow. In coral rubble areas, they may hide under or cling to the rubble, seaweeds or other animals such as sponges and ascidians. On the rocky shore, some hide under the rocks. Many lie buried in the sand, only their feeding tentacles emerging above ground.

What are sea cucumbers?
Sea cucumbers are animals and NOT vegetables as their common name suggests. Often mistaken for worms, sea cucumbers are related to sea stars and belong to Phylum Echinodermata. Class Holothuroidea has about 1,200 known species. Some are colourful and easily seen, others are well camouflaged or hidden under stones or in the sand. Sea cucumbers can be round as balls, long and worm-like, or even U-shaped. They are found almost everywhere from shallow areas to the deep sea, from tropical to the Arctic and the Antarctic.

Wormy Echinoderms:
Sea cucumbers look quite different from their echinoderm cousins such as sea stars. Sea cucumbers don't have arms, obvious spines or a hard skeleton. They are instead soft and worm-like. But like other echinoderms, they are symmetrical along five axes. Instead of lying on their mouths like sea stars, brittle stars, sand dollars and sea urchins, sea cucumbers lie on their sides with their mouths on one end and backsides on the other.

Feeding with their feet!
Most sea cucumbers have tube feet. The tube feet around the mouth are modified into branched or feathery feeding tentacles. There can be 10-30 such tentacles, which can be completely retracted into the body. Most have tiny tube feet which are used to cling to surfaces and things. Only a few use their tube feet to creep on. In some, tube feet emerge in rows concentrated on the flattened lower half that faces the surface (called the sole). In others, the tube feet may be scattered all over their body. Sea cucumbers that lack tube feet include synaptid sea cucumbers and those sea cucumbers that burrow.

Morphing sea cucumbers:
Instead of a hard skeleton, the bodies of sea cucumbers are mostly made up of mutable connective tissue that they can rapidly change from soft to rock hard. This tissue plus well-developed layers of muscles (around the body and along its length) helps them to move about; flow into narrow places to hide; or disuade predators from taking a bite out of them. They also have ossicles (hard pieces of calcium carbonate), but these are microscopic and widely distributed in their mutable connective tissue. In some sea cucumbers, their ossicles can give their skin a stiff and rough texture. Sea cucumber ossicles take on a wondrous variety of delicate shapes and are used to identify sea cucumber species!

Breathing through their backsides!
A unique feature of some sea cucumbers is an internal breathing system of branching tubes along the length of their bodies. Called respiratory trees, most large sea cucumbers have a pair of these, each connected to the opening on the backside. To breathe, the sea cucumber pumps water in through its backside and up through the respiratory trees. The water is then flushed out through the backside again. With this constant flow of water, some tiny creatures find the backside of a seacucumber a cosy and safe place to be! These include pea crabs and the Pearlfish. Small or thin-walled sea cucumbers, however, simply breathe through their skins.

Dust Busters of the Sea:
Most sea cucumbers eat detritus, collecting this in their sticky, mucus-covered branched or feathery tentacles by sweeping the sea bottom or waving their tentacles in the water. The tentacles are then inserted one by one into the mouth and sucked clean. Some simply shovel sediments into their mouths with their tentacles and process the edible bits, leaving behind them a trail of sausage-like lumps of processed sediments. Some sea cucumbers have been estimated to process 130kg of sediments per year!

Cucumber babies:
Most sea cucumbers have separate genders and are usually either male or female. Their reproductive organs are near the front of their body. In most species, sperm and eggs are released simultaneously for external fertilisation. Some spawning sea cucumbers raise their front end in a cobra-like posture when releasing their eggs and sperm. Observations suggest spawning is synchronised, sometimes more than one species spawning together.

Sea cucumbers undergo metamorphosis and their larvae look nothing like their adults. The form that first hatches from the eggs are bilaterally symmetrical and free-swimming, drifting with the plankton. They eventually settle down and develop into tiny sea cucumbers.

Credit: http://www.wildsingapore.com/wildfacts/echinodermata/holothuroidea/holothuroidea.htm