Starfish

Asteroidea

Did you know a starfish is not a fish? Starfish, or sea stars, are actually invertebrates. This means they don’t have backbones. (And fish do have backbones.)

Starfish

There are about 2000 species of starfish. All starfish live in saltwater and have arms (or rays) that radiate out from a central body. If they happen to lose an arm, it will regrow. Most sea stars have five arms, but one species, the Antarctic sun starfish, can have over 40!

StarfishStarfish are covered in spiny skin, which gives them their phylum name, echinoderm, which is Greek for “hedgehog skin”. Beneath this skin is a skeleton made of pebbly growths called ossicles. Made of the mineral, calcium carbonate, the ossicles are arranged in a diamond pattern, which allows the skeleton to be strong, in order to give the sea star protection, but also flexible, so it can eat.

Starfish do not have blood. On the top of a starfish is a tiny circle called a madreporite. Water is pulled through this spot and pumped through their body to deliver nutrients.

On the underside of the starfish are two important features: their mouth and hundreds of hollow tubes. The tubes create suction, which helps sea stars move and eat.

What Starfish Eat

Starfish primarily eat bivalve mollusks, like clams and oysters. They will also eat snails, fish, and urchins.

Starfish eating

Sea stars don’t have eyes like we do, but they do have eyespots at the tips of their arms, which can sense changes in light. They use these eyespots to detect prey. Then, the starfish will grasp the prey in their arms and push their stomach out of their mouth to dissolve and eat their prey. When they grab onto a shelled creature, they pull it open to insert their stomach.

Starfish Reproduction

In general, starfish reproduce in the spring and summer. The males release sperm and the females release eggs into the water. Eggs are fertilized by chance and hatch as larvae. After a period of about 60 days, they metamorphose into their adult form.

But some starfish can also reproduce when part of their central disc and an arm break free. The “parent” starfish will regrow a new arm, and the segment that broke off will generate new arms and become a new starfish on its own.

Starfish can live for over 30 years in the wild. Sea stars are hunted by fish, crabs, and shore birds.

Conservation Status

Starfish are often killed by shell fishermen because they don’t want starfish eating all the shellfish. Starfish are also threatened by pollution, acidification of oceans, and rising temperatures.

The main threat to sea stars is a mysterious disease called sea star wasting syndrome, which causes their arms to get twisted and their body to disintegrate, and leads to death. The disease is linked to rising ocean temperatures and has been catastrophic for starfish populations. In fact, one starfish species, the sunflower sea star, had 90% of its population decimated by sea star wasting disease from 2013-17 and is now considered critically endangered by the IUCN Redlist.

Starfish Range

Found in every ocean in the world, sea stars can live deep in the ocean, in shallow tide pools, or on rocky shores.

What You Can Do to Help

You can help starfish by writing to your politicians urging them to take action on climate change. You can also donate to Oceana, an international advocacy organization dedicated to ocean conservation.

Starfish Resources

Blog Posts about the Starfish

About the Author

P.A. Smith is a middle school Language Arts teacher. He is also a contributing editor of My House Rabbit.