The familiar starfish (sea star) and its relatives are members of a group of spiny-skinned organisms, the echinoderms. Close relatives include brittle stars, basket stars, sea urchins, sea cucumbers, feather stars, and sea lilies. All of the animals in the starfish family have five or more arms radiating from a central body, shown in the lower color insert on page C-5. The mouth of an echinoderm is located on the underside and the anus on top of the body. Echinoderms may be carnivores, detritivores, or herbivores, depending on the species.
Tentacle-like structures called tube feet, visible in Figure below, help echinoderms move slowly across the reef. The tube feet act like suction pads, grasping and releasing surfaces. A hydraulically controlled vascular system supplies water to each tube foot through small muscular tubes. As the feet press against an object, water is withdrawn, creating suction. When water is returned to the cups, the suction is broken, and they release.
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| On its ventral sides, a starfish, or sea star, has a centrally located mouth surrounded by five arms with tube feet. (Courtesy of Dr. James P. McVey, NOAA Sea Grant Program) |
To reproduce, echinoderms release sperm and eggs into water. These fuse to form zygotes that develop into larvae. After some time in the plankton, a larva settles to the bottom and takes on typical echinoderm features. Most echinoderms can also reproduce asexually. If part of the animal breaks off, it may grow into a complete, new organism. All are capable of regenerating missing limbs, spines, and in some cases, intestines.
Starfish are common on the reefs. Most are carnivorous, feeding on sponges and small invertebrates. One species, the crown of thorns (Acanthaster planci), eats live coral polyps. Crown of thorns can reach 20 inches (50 cm) in width and may have 10 to 20 spiny arms. As is the case for all starfish, it eats by extruding its stomach over its prey, then drenching it in digestive juices. After the prey tissues liquefy, the crown of thorns pulls its stomach and the partially digested food back into the body.
A brittle star has a small central disk from which radiate five snakelike arms. Unlike starfish, brittle stars lack an anus, so waste products are eliminated through the mouth. On the underside of a brittle star, there are 10 slitlike openings at the base of the arm that function in breathing, as well as releasing eggs and sperm. Brittle stars often hide in the crevices in coral reefs, emerging at night to feed on plankton. If a predator happens to find and grab a brittle star, it has trouble hanging on. As its name suggests, this animal’s arms break off easily,
allowing for easy escape. A broken arm quickly regenerates.
Feather stars, or crinoids, are cup-shaped animals that have five to 200 feathery arms projecting upward from a central disk. The arms are coated with sticky material that helps them catch planktonic food. Feather stars cling to coral or rocks but can move by rolling, walking, crawling, swimming. They often establish symbiotic relationships with other animals including some species of shrimp, lobsters, and clingfish.