The Plight of the Sea Star

by Gerald Shaffer

Sea stars

Sea stars are some of the most unusual creatures you can find in a tide pool. Did you know they can spit out their stomachs, "see" with their feet, grow back most of their body if damaged, and move around using water pressure?

Sea stars used to be called "starfish," but they're not fish at all. They're echinoderms, members of the same club as sea urchins, sea cucumbers, sea lilies and sand dollars. Echinoderm comes from the Greek meaning "hedgehog skin"--quite an accurate description for these animals, with their spiny skin covering their skeleton. A sea star's skeleton is made up of many plates that move like flexible joints. (In sea urchins and sand dollars, their skeletal plates are fused.)

When sea stars are born, they're symmetrical. That means their right and left sides are exactly the same. But as they grow, they eventually develop five similar parts. Some sea stars, like the purple sea star of the Gulf of Maine, have nine or ten arms. They are incredibly resilient, capable of regenerating (re-growing) parts of their body. If they lose an arm, or even most of their body, they are can grow those sections back. So long as one fifth of the central disk and at least one arm remains, the sea star can completely recover. Regeneration can take up to a year. The brittle star, a relative of the sea star, is named for its fragile appendages, which can easily break off if caught by a predator or under a rock tossed by the crashing tides.

On the end of each arm is a small pigment eye spot that is sensitive to light. Some stars are drawn to the light, while others would rather stay in darkness.

When most sea stars reproduce, they release their swimming larvae to the sea. But the blood star (Herica) holds on to her eggs, forming a pouch with her arms to protect her young. She holds them there, hunched over, until they are fully formed and ready to be released.

 

From "The Intertidal Zone." Courtesy of Bullfrog Films
and the National Film Board of Canada.
If you've ever picked up a sea star and turned it over, you probably noticed the hundreds of tube "feet" lining its arms. It is these suction-bottomed tubes that the sea star uses to move about. It draws in water and channels it to canals that run throughout its body, usually ending in the tube feet. By changing the pressure of water in its body, the sea star can move the tubed feet, and thus move forward.

How a sea star moves (~1.6M)

 

(From "The Intertidal Zone." Courtesy of Bullfrog Films and the National Film Board of Canada)

This water is taken in through a tiny spot on the upper part of its body, called the madreporite. You can see the madreporite quite clearly on the common sea star, Forbes' Asterias (photo above).

 

But these tube feet are capable of more than just movement. They also play a role in respiration and in collecting food. One of the sea stars favorite foods to dine on is bivalves--dual-shelled mollusks with strong muscles that keep their shells shut. That doesn't stop sea stars, however. They can grasp a mollusk and pull the shells apart slightly using their strength and the suction of the tubed feet. All sea stars need is to get the shells open just enough to spit their stomachs out through their mouth and right into the mollusk! Digestive juices can turn the mollusk's body to liquid inside the shells, making for a tasty snack. Cilia lining the arms of the sea star guide the liquidy mollusk into its mouth, which located on the underside of its body.

 


(Icon: Food Web Alert)

Sea stars dine out (or in, if you're a mollusk) on mussels, snails, oysters, worms and crustaceans. Some sea stars scavenge for decaying matter (or detritus) on the surface of the mud. The blood star, with its bright red (or orange or yellow) color, feeds on sponges. Most of the other tide pool animals can eat very tiny sea stars, but no tide pool animal can eat an adult sea star. Sea stars do have to watch out for hungry birds, and even very big snails in the open sea.