Sea cucumbers Class Holothuroidea
Thorny sea cucumber Colochirus quadrangularis

On Chek Jawa, sea cucumbers can be found almost everywhere. Some are colourful and easily seen, others are well camouflaged or hidden under stones or in the sand. There are about 900-1,200 species of sea cucumbers, ranging from 3cm to 1-2m long. 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. On sandy patches in the seagrass lagoon, large sea cucumbers may move about on the surface, while smaller ones burrow. In the coral rubble, they may 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 on the sand bar, only their feeding tentacles emerging above ground. Wormy Echinoderms: Sea cucumbers are echinoderms but look quite different from their cousins. They don't have arms or spines; and are soft and worm-like. But like other echinoderms, they are symmetrical along five axes along the length of the body. 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 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. 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! Repulsive vomiting: Being soft and slow, sea cucumbers protect themselves by hiding or being unpleasant to deal with. To repel and distract predators, some sea cucumbers vomit their entire digestive system and other internal organs such as the respiratory tree and even their reproductive organs. Depending on the species, these can emerge from the front or back end of a sea cucumber. Some species eject toxic or sticky strings (called Cuvierian tubules) from their backsides. These immobilise the predator in a gummy mess or release toxins. The sea cucumber eventually regrows its innards and arsenal of Cuvierian tubules. Please don't make sea cucumbers expel their intestines. Not all do this. Those that do cannot eat until they re-grow their innards. 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. 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. Here is a fascinating photo of a sea cucumber larva on Image Quest 3-D Marine Library Role in the habitat: Sea cucumbers can make up 90% of the deep sea ecosystem. Since this ecosystem makes up about 70% of the surface of the earth, sea cucumbers can be considered the dominant organisms on earth! Being so numerous, sea cucumbers are important to the ecosystem. Their larvae are probably important in plankton-based food chains. The constant processing of sediment by countless sea cucumbers possibly plays a role in nutrient recycling. Human uses: Some large sea cucumbers are considered delicacies and harvested for food. Others may be collected for the live aquarium trade. Scientists are also studying the toxins of sea cucumbers for possible medical and other applications. Status and threats: Some edible sea cucumbers are globally threatened by over-collection. In Singapore, the main threat is habitat loss due to reclamation or human activities along the coast that pollute the water. Many of those found on Chek Jawa are on the list of threatened animals in Singapore.

Sea apple sea cucumber Pseudocolochirus violaceus Sandfish Holothuria scabra Ball sea cucumber Phyllophorus sp. Synaptid sea cucumber Family Synaptidae Synaptid sea cucumber Family Synaptidae Unidentified sea cucumber Unidentified sea cucumber Unidentified sea cucumber

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• Pechenik, Jan A., 2000. Biology of the Invertebrates. McGraw-Hill Book Co., Singapore. 578 pp.
• Hendler, Gordon, John E. Miller, David L. Pawson and Porter M. Kier, 1995. Sea Stars, Sea Urchins, and Allies: Echinoderms of Florida and the Caribbean. Smithsonian Institution Press. 390 pp.
• Schoppe, S., 2000. Echinoderms of the Philippines. Times Edition, Singapore. 144 pp.
• Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore Science Centre, Singapore. 160 pp.
  

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