Sea stars are invertebrates related to sea urchins, sea cucumbers, and sand dollars, which are all echinoderms. Echinoderm means spiny skin, a reference to their hard, calcified skin, which helps to protect them from predators.
These brainless and bloodless creatures actually use sea water, not blood, to pump nutrients through their system. While they lack brains, they have special sensing cells that send signals through their network of nerves.
Sea stars have rows of tiny tube feet extending from the grooved surface on their underside. These tube feet allow them to crawl along the ocean floor via suction created by an internal water-driven hydraulic system. Sea stars contract their muscles, which forces water into the tube feet, causing them to extend and attach to the ground. When they relax their muscles, the tube feet retract. By alternately contracting and relaxing these muscles, pumping water through their feet, they are able to slowly move around.
Sea stars have an amazing ability to regenerate arms when they are severed, or even a new body in some species. All of their vital organs are located in the arms, so a portion of an arm could potentially grow a whole new sea star.
There are close to 2,000 species of sea stars in the world’s oceans. Most species have five arms, but some have many more—even as many as 40! At the Aquarium, you can see 10 species of sea stars throughout the exhibits. Look for the sun sea stars in the Kelp Forest, which sport 20 arms each!
Sea stars are mostly carnivorous and prey on mollusks such as clams, mussels, and oysters, which they pry open with their suction-cupped feet. At the Aquarium, we feed some of our sea stars pieces of fish and shrimp, but the majority of our sea stars feed on detritus, which helps to keep our exhibits clean.
Sea stars have a peculiar way of eating. They actually digest their prey outside of their bodies by extruding their stomach out through their mouth and enveloping the prey item. When the meal is digested, the stomach is drawn back into the body.
The smallest sea stars are less than an inch in diameter, while the largest sea stars can reach up to 3 feet in diameter.
Sea stars live in salt water and are found in all of the world’s oceans, from warm, tropical waters to the cold sea floor.
Many different animals eat sea stars including fish, sea turtles, snails, crabs, shrimp, otters, birds, and even other sea stars. Though the sea star’s skin is hard and bumpy, if a predator has a large enough mouth it can eat it whole. Predators with smaller mouths can flip the sea star over and eat the softer underside.
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