Starfish, or more accurately sea stars, fill a niche as predator and prey in marine ecosystems close to shore. With more than 18,000 species in the world, they range from the tropics to the Pacific Northwest and further north. They can grow larger than a foot across and can have as many as 40 arms. While sea stars are known for their ability to regrow lost arms, they have an unusual digestive system, too.
On the Menu
While they’re omnivores, sea stars’ diets vary according to species. Some extremely predatory types, such as the crown of thorns, eat almost any living creatures they can, including coral and even other sea stars. The Forbes’ sea star and most other species dine on bivalves, such as clams, mussels and oysters. Others, such as bat sea stars, eat cucumbers, sea urchins, shrimp, tubeworms and fish, as well as algae and sea grasses. Bat sea stars vary their diet even more by scavenging dead animals from the sea floor.
Hunting and Digestion
After a sea star locates potential prey by smell, it usually traps the animal or plant beneath its body. The sea star might use an arm to carry the prey to its mouth, which is located on the underside of its body. More commonly, it squeezes its stomach out through the mouth opening so it can feed on large prey or reach into otherwise inaccessible crevices. Sea stars that eat bivalves use their powerful arms to force the shells open, then extend their stomachs into the shells and release digestive enzymes that help them absorb the prey.
Sea stars fall prey to many of the same creatures they hunt. Crabs, predatory fish and mollusks often add sea stars to the menu. During low tide, seagulls and other shorebirds feed on stranded marine animals, including sea stars. Some species, such as the bat star, release potent chemicals to deter predators.
As Pets: Eat or Be Eaten
If you want to keep a sea star in a reef aquarium, research various species to find the best fit. Predatory fish sometimes kill and eat sea stars, while aggressive sea stars might gobble up your coral, crabs and snails. Many species, such as chocolate chip sea stars, help keep your aquarium clean by consuming algae, and coexist well with nonaggressive fish, but prey on bivalves who share the tank. The crown of thorns can catch and eat docile fish, and most invertebrates, including other sea stars, but often gets along with lionfish or moray eels. If you ever have questions about which invertebrates and fish you can house safely with your sea star, talk to a vet who specializes in aquarium life.
- Shedd Aquarium: Explore by Animal: Sea Stars
- National Geographic: Animals: Starfish (Sea Star)
- Georgia Aquarium: Explore the Aquarium -- Interact: Georgia-Pacific Cold Water Quest: Bat Star
- Indianapolis Zoo: Oceans Animal Facts -- Bat Sea Star
- Marine Parks WA: Sea Stars
- University of Rhode Island: Environmental Data Center -- Sea Star (Asterias forbesii)
- Comstock Images/Comstock/Getty Images