Starting Dec. 12, we’ll be blogging the “12 Days of Fishmas.” Each day we’ll feature a different species that you can see during your visit to Birch Aquarium at Scripps. Love this species? Consider our Adopt-A-Fish program and help us keep these animals thriving!
Sea stars are some of the most unusual creatures you’ll see in a tide pool. There are approximately 1,600 species currently found around the globe. Although sometimes called a “starfish,” aquarists prefer the term “sea star” since these animals are not actually fish.
Sea stars are known as echinoderms from the Greek word for “hedgehog skin.” Like sea urchins, sea cucumbers, sea lilies, and sand dollars, sea stars are known for their rough, prickly, or spiny skin that covers a bony skeleton.
A Perfect Star
The sea star’s skeleton is made up of many plates that move like flexible joints. This allows them to move fluidly, unlike the less mobile sea urchins and sand dollars, whose plates are fused. As a larva, sea stars are “bilaterally symmetrical”; that is, they could be folded down the middle and reveal two equal sides, but only along one axis. Sea star larvae come in a variety of surprising shapes and sizes, often not resembling their final form at all.
As they mature, the sea star develops five arms and “radial symmetry,” meaning any straight fold through their middle point will give you two similar parts. Some species have more than five arms, such as the purple sea star in the Gulf of Maine that has nine or 10. On the end of each arm is a small pigment eye spot that is sensitive to light that helps the animals determine their location. Each arm also has hundreds of tiny, sticky disks on the underside called tubefeet, or “podia,” which is what sea stars use to hold onto surfaces (like the rocks in our tide pools!) and prey, or for burrowing in sand.
What’s for Dinner?
Sea stars typically like to dine on mussels, snails, oysters, worms, and crustaceans. Different sea star species use different methods to feed. Some employ tiny hairs (cilia) and mucus to trap small organisms and living particles suspended in the water column. Others swallow their prey whole and digest it within. Still others use the suckers on their podia to pry open bivalves, evert their stomach out of their mouth into the opening, and digest the soft parts of the animal right within its own shell. Most tide-pool animals can eat very tiny sea stars, but no tide-pool animal can eat an adult sea star. They do, however, have to watch out for hungry birds and even very big snails in the open sea.
- Not all sea stars live in tide pools along the ocean’s surface. While there are some sea stars that are drawn to the light, there are others that prefer to stay in darkness at much deeper depths.
- If a sea star loses an arm, or even most of its body, the animal can completely regenerate any section as long as one-fifth of its central disk and at least one arm remain.
Come see nine sea stars sticking in our tide pools this holiday season
Sea Stars are just a few of the 3,000 fish and invertebrates that thrive at Birch Aquarium at Scripps. By adopting one today you can be part of the team that provides essential care to these special ocean inhabitants. Find out more about the Aquarium’s Adopt-a-Fish program.