Sea stars are found mainly in on the sandy parts of Chek Jawa. Smaller sea stars are sometimes also seen among the seagrass. 'Asteroidea' means 'star-like' in Greek. There are about 1,800 species of sea stars, ranging from 1cm to nearly 1m across. Asteroidea is the second largest class of echinoderms after the brittle stars. Although often called starfish, these creatures are not fish at all! So we prefer to call them sea stars. Almost everyone knows what a sea star looks like! They are among the most familiar of the Phylum Echinodermata. Like all echinoderms, sea stars are symmetrical along five axes, have a spiny skin and tube feet. All About Arms: Sea stars have arms that blend into each other before joining the central disk, unlike brittle stars. Some have long arms, others short ones. Some have arms which are so short that they look like pentagons! Each arm is usually tipped with one or more sensory tube feet, and an eye spot that detects light and dark but does not form an image. 3-armed Common sea star Handicapped Stars: Most species of sea stars have five arms, although some may have more. However, sometimes, you might come across a sea star with fewer than five arms. Sea stars are famous for their ability to regenerate lost arms. But this takes time and resources. Some species take up to a year to replace a lost limb. In the meantime, the sea star is probably disadvantaged. If the central disk is damaged, the sea star may die. Only a few species of sea stars are known to regenerate from a piece of an arm. So please don't purposely mutilate sea stars. Water of Life: Like other echinoderms, sea stars use seawater instead of blood to pump up their bodies and move their tube feet. They suck water into their bodies through the madreporite: a sieve-like structure that usually appears as a spot on the upperside near the centre. By expanding or contracting chambers in the internal system, the water pressure in the canals can be directed and changed. This is how they move their tube feet. As they rely on seawater, it is stressful for sea stars to be left out of water for too long. Please return sea stars quickly to where you found them. Tube feet of Common sea star Stomach is also everted out of the mouth Fancy footwork: A sea star has many tube feet that stick out of the groove along the centre of its arms. It may use its tube feet to walk and to handle food. 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 some sea stars, the tube feet ends in suckered disks. These act like suction cups when pressure is applied by the sea star. Some sea stars also breathe through their tube feet! Unlike brittle stars, sea stars move mainly with their tube feet and not by bending their arms. Skin and bones: Sea stars have skin that covers all of the body, including the spines. Sea stars have no bones or a skeleton. Like other echinoderms, instead of a hard skeleton, the Common sea star's body is mostly made up of a tissue that can rapidly change from almost liquid to rock hard. This tissue is called 'catch connective tissue'. This connective tissue allows them to slowly bend and move their arms to climb, right themselves and clasp prey. Sea stars can also autotomize (drop off) an arm when stressed or attacked, by rapidly changing the consistency of the tissue. Ossicles (tiny pieces of calcium carbonate) may be embedded in this connective tissue. Dead or Alive? All the sea stars that you see are probably alive. You are unlikely to come across a skeleton of a sea star. Dead sea stars disintegrate quickly and do not leave behind whole skeletons. A live sea star also has moving tube feet. When removed from the water, however, sea stars will retract their tube feet and may appear dead. Star Food: Most sea stars are scavengers or carnivores, 'sniffing' out their meal by the chemicals released by the prey or dead animals. Among the more common prey are snails, bivalves, crustaceans, worms and other echinoderms. Some sea stars specialise in a certain prey. Some sea stars feed on sponges, sea anemones and corals. An outbreak of the notorious Crown-of-Thorns sea star (Acanthaster planci) can devastate entire reefs. The prey of sea stars have developed ways to escape these predators; bivalves may leap or burrow away quickly, snails may somersault, sea cucumbers and sea anemones may swim away. Some sea stars, however, just grub around on the sand for detritus. Some carnivorous sea stars eat detritus when there's nothing better to eat. The mouth of a sea star is on its underside. Some sea stars have jaws made up of five or more teeth arranged in a star around the mouth. Part of the digestive system of a sea star extends into its arms. Not all sea stars have an anus. Those that don't, spit out indigestible bit through their mouth. In those that do, the anus is on the upper surface of the central disk. Some sea stars also have tiny structures called pedicellariae which look like jaws. The main function of these is to keep the body of the sea star free of debris and parasites. They may also be used to collect food. Stomach Turning Table Manners: Some sea stars, especially those with long arms, can evert their stomachs. This ability is particularly useful for carnivorous sea stars that feed on bivalves. How does it do it? A carnivorous sea star uses its tube feet to hold the bivalve againsts its central mouth. It then pushes out its stomach through its mouth and inserts its stomach into the bivalve's shell through imperfections in the fit of the two shells. If there are no such imperfections, the sea star simply pulls the shells apart to create a tiny gap! Once inside the shell, digestive juices are poured on the hapless victim. Digested material is moved by cilia (minute hairs) on tracks into the sea star. Thus the prey is partially digested in its own shell! The Shape of Life page on the PBS website has a cool video clip of a sea star digesting a mussel in its shell! The Crown-of-Thorns sea star pushes its stomach out of its mouth to digest coral polyps in their skeletons. Sea stars that eat detritus may push out their stomachs to mop up whatever is on the surface. However, sea stars with short arms usually don't push out their stomachs and simply swallow their prey whole and digest them in their stomachs. Star Babies: Sea stars have separate genders and are usually either male or female. Eggs and sperm are stored in their arms. Most species practice external fertilisation, releasing eggs and sperm simultaneously into the water. Some can produce lots of eggs; a single female may produce millions! Sea stars undergo metamorphosis and their larvae look nothing like the 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 stars. Here is a fascinating photo of a sea star larva on Image Quest 3-D Marine Library. Human uses: Sea stars are not eaten or used for other purposes. They are also not that popular for the live aquarium trade as they tend to eat their tank-mates. In some coastal areas, sea stars are harvested and chopped up as fish meal or fertiliser. Some sea stars are considered pests on mussel, oyster and scallop farms. Status and threats: Sea stars have become uncommon in Singapore mainly because of 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.

Common sea star Archaster typicus Sand star Astropecten sp. Biscuit star Goniodiscaster scaber

• 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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