Hello, I Am a… Sea Star

A sea star on sand, in shallow water.

A sea star. Image: Philippe Gingras © Philippe Gingras

Sea stars belong to a large group of animals called echinoderms. That word breaks down to mean “hedgehog” (from the Greek word for echinos) and “skin” (from the Greek word for derma). Knowing that, if you are imagining the skin of the sea star to feel prickly, then you would not be all that wrong. Their skin is full of spiny bits called ossicles, which form their exoskeleton.

The appearance of a sea star will change from species to species. Most of the time, they will have the classic five-arm shape that makes them resemble the star of a sheriff’s badge, but some have 10 to 15 arms. The sea star Labidiaster annulatus, which is found in the Antarctic, clocks in with 50. That’s a lot of arms.

So with all these arms, how do they move about? We rely upon our muscles to move our skeleton. Feel like walking? Your brain then has to tell your leg muscles to start contracting and relaxing. Sea stars lack legs, not to mention a centralized brain, but what they do have are tube feet. These tube feet run along the underside of the sea star’s arms and resemble little suction cups. By pumping water in and out of these feet, the sea star can creep about. They are not the speediest of animals.

Tube feet also help out in another important function: eating. But first, here’s a question. Look at a sea star stuck to the glass of an aquarium, such as in our RBC Blue Water Gallery. See that little hole in the middle of their underside? If you think that’s their mouth, you’ve got it. So with such a small mouth, how do they eat their dinner of clams or oysters?

We rely on our mouth to rip apart and chew our food into smaller bits that we can swallow and send down to our stomachs. While some sea stars are big enough that they can eat their prey whole, smaller sea stars have a different dining trick. If the food can not go to their stomach, their stomach will go to their food.

Here’s how it works. A sea star comes across its clam of choice, and it wraps its arms around the clam. Now, the clam will try its hardest to keep its shell closed, but the strength of the tube feet in the sea star’s arms is incredible. Eventually the two halves of the clam’s shell will crack apart, just a bit, and that’s enough for the sea star to push out its stomach and shove it into the clam shell. When we eat something, our stomach produces acids that help to break down our food so that it can be absorbed. The sea star’s stomach—even outside its body—does the exact same thing. In the end, the sea star will mosey along, leaving behind the empty shells of its dinner.

Sea stars: There is definitely more to them than just looking pretty.

References:

http://en.wikipedia.org/wiki/Starfish#cite_note-3

http://animals.nationalgeographic.com/animals/invertebrates/starfish

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