Starfish, considered echinoderms and classified as Astreroida, are a simple life form without a brain and sensory organs. Starfish are best known for their distinctive star-like shape and their ability to regenerate limbs. There are approximately 2,000 species of starfish in the world, ranging from tropical regions to colder sea habitats. They have a spiny, calcified skin and come in a variety of colors.
Starfish are fairly peaceful life forms in aquariums. They are slow moving and can be kept with most species of peaceful fish without harm. More aggressive fish, however, such as puffers, may attack starfish. Allies include other forms of echinoderms, such as sea urchins, sand dollars, sea cucumbers and sea lilies.
Starfish are scavengers and carnivores, willing to eat most anything found on the ocean floor. They help to keep an aquarium clean. Starfish feed upon slow moving prey, such as mussels, mollusks, barnacles, small sponges, clam and oysters. They are able to push their stomach out to surround and ingest prey too big to swallow. They use their suction-cupped feet to pry open small shellfish.
Predators of starfish include birds such as seagulls, large fish and sea otters. Some shells, including Harlequin shrimp (Hymenocera elegans), Triton trumpet (Charonia tritonis) and reef crab (Trapezia), also prey upon starfish. Due to their regenerative powers, a starfish can often survive an attack by a predator that does not completely consume it.
Other threats to starfish besides predators include water pollution, such as oil spills. An oil spill from a supertanker off the coast of Britain in 1996, for instance, wiped out an entire population of green rockpool starfish in the area. Petroleum products and other chemicals dumped into coastal waters may also be detrimental to starfish.