Most people call them starfish, but marine biologists are working to change the name to "sea star." The obvious reasons for the change is that starfish are not fish. Fish have gills, scales and fins, and sea stars do not. The approximately 2,000 species of sea stars all have a rough or spiny external skeleton and belong to the phylum Echinodermata, Greek for spiny skin. They have as few as five or as many as 40 arms and, depending on the species, can have arms longer than four feet or shorter than one inch.
Most sea stars are carnivores and exist on a diet of clams, oysters and other bivalves. They use suction cups on the bottom of their arms to pry open the shells of their prey. When the shell is pulled apart the sea star pushes its stomach out of its body and into the shell where stomach enzymes digest the soft tissue and suck it into the sea star. Their ability to eat about a dozen clams or oysters daily damages clam and oyster beds. One species specializes in eating coral reefs and has decimated sections of Australia's Great Barrier Reef.
Sea star predators include by sharks, sea gulls, sea turtles, manta rays and otters. They have a significant advantage over even their most voracious predators. They can regenerate a lost arm if it is pulled off and can purposely sever an arm to keep from being eaten. Some species regenerate the entire body from as little as one-fifth of an arm. This makes them difficult to control around commercial oyster farms, shell fish beds and coral reefs.
Female sea stars release as many as 2.5 million eggs directly into the water where they are fertilized by sperm from a nearby male. To increase the chance of fertilization some species gather into reproductive groups during the spring breeding season. The fertilized eggs become free-swimming larvae and feed on phytoplankton until settling to the ocean floor and transforming into adults. Adults sea stars can live 35 years.