Invertebrates

Sea star

The animal that can regrow a lost arm

It’s true—to a point. Sea stars can regrow a lost arm as long as some part of the central disk, or the middle of their bodies, remains intact. If any of a sea star’s arms are injured, bitten or broken off, tissues at the injury site seal themselves off, then special cells migrate to the area and slowly begin to regrow the arm. This process can take a year or even longer.

At the Aquarium

Five arms and then some

A sea star’s body consists of a central disk and five or more radiating arms. How many more? That depends on the species. The rose star has between eight and 14 arms, and the sunflower star can have up to 24 arms at maturity!

Eyes? But where are they?

Sea stars don’t have brains, but they do have eye “spots” at the end of their rays that help them detect light. They also have a water vascular system that supplies the water pressure needed to operate their tube feet.

An unusual way of eating

Sea stars eat a variety of foods including bivalves, barnacles, crabs, fish, plankton, sea anemones, other sea stars, and more—different sea star species prefer different kinds of foods. But it’s how they eat that’s really interesting: after gripping their prey with their tube feet (and, if needed, opening the shell), they bring their stomachs outside of their bodies to engulf the food and digest it with a special fluid.

What’s that fuzz?

You may be surprised to find out that sea stars have gill structures—although they don’t look like the gills seen on fish. The fuzzy-looking stuff on top of their bodies is actually their gill structures. Some sea star species can extend and retract these structures through their skin.

So happy together

Sea stars are often found in groups—or in close proximity to one another. One reason for this may be because of the way they reproduce, by spawning: females cast eggs into the water, and males cast sperm into the water; it mixes and creates sea star embryos. Sea stars may also feed from the same piece of food, another potential reason that they stay close together.