Chek Jawa is among the few places in Singapore where you can see many sea stars, sand dollars and sea cucumbers. These animals are all echinoderms. 'Echinodermata' means 'spiny-skin' in Greek. There are about 6,500 species of echinoderms. Echinoderms have several unique features. Give me five! An echinoderm has a five-point (pentamerous) radial symmetry. In contrast, a human has a 2-point (bilateral) symmetrical; you can draw a line from head to toe and have a mirror image of most of our body parts on the left and right. An echinoderm, however, can be divided into five equal parts (or multiples of five) along a circle; much like cutting up a birthday cake into five equal slices! Although echinoderms can take on different shapes, they are all symmetrical along five axes. Here are some diagrams explaining this in greater detail. Splendid spines: Most echinoderms have a spiny skin. The spines are most obvious in sea urchins. Brittle stars and some sea stars also have prominent spines along their arms. Sand dollars and most sea stars have tiny spines. Sea cucumbers generally lack hard spines. Morphing marvels: Echinoderms have an internal skeleton made up of an arrangement of ossicles (plates made mostly of calcium carbonate), connected with muscles and a special kind of connective tissue called 'catch connective tissue'. This tissue is made of collagen and echinoderms can rapidly control the consistency of this tissue from rock hard to almost liquid within seconds. By changing the consistency of this tissue, echinoderms can move, feed, defend themselves and reproduce asexually. A sea urchin can move and lock its spines; a brittle star and sea star can bend or even purposely break off an arm; sea cucumbers can move or flow into narrow places then harden when it is in a safe place. Awesome ossicles: All echinoderms have ossicles, but these have different shapes and are arranged differently. Sea cucumbers have microscopic ossicles and most of their bodies are made up of catch connective tissue. This is why they appear softer than their cousins. The ossicles in sea stars and and especially brittle stars are larger. In brittle stars these articulate with each other to form flexible arms. In sea urchins and sand dollars, the ossicles are large and fused to each other, forming a rigid skeleton which is the spherical body in sea urchins, and flattened into a thin disk in sand dollars. Water instead of blood? Another unique feature of echinoderms is the water vascular system, a hydraulic system of internal canals pumped mainly by seawater. By expanding or contracting chambers in the system, the water pressure in the canals can be directed and changed. Sea stars pump up and move their tube feet to move, brittle stars pump up and move their entire flexible arms, sea cucumbers move their entire body in a more worm-like manner. Sea urchins and sand dollar move using their spines, more than by using their hydraulic system. Handy Feet: Most echinoderms have tube feet. These are connected to the hydraulic system which pumps them up. Tube feet have muscles to retract them, but no muscles to extend them. These tube feet stick out through holes in the skeleton and have many uses. In sea stars and sea urchins, tube feet are used to move, gather food and breathe. Sea stars appear to have special glands in their tube feet that secrete a glue so the feet stick to things, and another substance to release the tube feet. In brittle stars, tube feet are tiny and used mainly to gather food and breathe. Brittle stars use their highly flexible arms to move, and not their tube feet. In sea cucumbers, tube feet around the mouth are modified into feeding tentacles to gather food. In sand dollars, the minute tube feet on the underside are used to gather food, while tube feet emerging from the upperside are used to breathe with. Tube feet in general may also be used to excrete wastes, and to sense chemicals with. An echinoderm can have as many as 2,000 tube feet! Heartless and brainless! An echinoderm doesn't have a heart. It has a simple nervous system but no brain. Echinoderm babies: Most echinoderms practice external fertilisation, releasing their eggs and sperm simultaneously into the water. Most 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 miniatures of their parents. Some echinoderms can reproduce asexually by purposely dividing themselves or budding off a part of their body. Human uses: Uses of echinoderms are listed under the various headings for the major classes of echinoderms. Status and threats: Echinoderms in Singapore are mainly threatened by 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 urchin Biscuit sea star Sandfish sea cucumber Brittle star Synaptid sea cucumber

• Barnes, Robert D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
• 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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