10 Facts About Starfish (Sea Stars)

Learn About This Popular Echinoderm

Starfish (sea stars) are beautiful animals that can be a variety of colors, shapes and sizes, although all resemble a star. While some appear smooth, they all have spines covering their upper surface and a soft underside. If you gently turn over a live sea star, you'll see its tube feet wiggling back at you. These iconic marine animals are fascinating creatures. Learn more about them below.
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Sea stars are not fish.

Red Mesh Starfish Feeding on Coraline Algae
Jeff Rotman/The Image Bank/Getty Images

Although sea stars live underwater and are commonly called "starfish," they are not fish. They do not have gills, scales, or fins like fish do and they move quite differently from fish. While fish propel themselves with their tails, sea stars have tiny tube feet to help them move along (see more on that below).

Echinoderms: Starfish and purple sea urchin
Starfish and purple sea urchin. Kathi Moore / EyeEm / Getty Images

Sea stars belong to the Phylum Echinodermata. That means they are related to sand dollars (yes, they are a real animal), sea urchins, and sea cucumbers. All echinoderms have five-point radial symmetry, which means that their body plan has five sections (or multiples thereof) arranged around a central disk. Next time you're in a beach-themed store, see if you can find a dried sea star, sand dollar and sea urchin and find the 5 sections in each. More »

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There are thousands of sea star species.

Galapagos, closeup of seastar on colorful sand.
Colorful sea star in the Galapagos. Ed Robinson / Getty Images

There are about 2,000 species of sea stars. Some live in the intertidal zone, some in deep water, some in tropical areas, some in cold water.

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Not all sea stars have 5 arms.

Diver and Sun Star, Crossaster sp., Monterey Bay, California, USA
Sun star with many arms. Joe Dovala / Getty Images
While the five-armed varieties of sea star are the most well known, not all sea stars have 5 arms. Some have many more. Take the sun star for instance, which has up to 40 arms!
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Sea stars can regenerate a lost arm.

Comet Starfish regenerating
Sea star regenerating four arms. Daniela Dirscherl / Getty Images

Amazingly, sea stars can regenerate lost arms. This is useful if the sea star is threatened by a predator - it can drop an arm, get away and grow a new arm. Sea stars house most of their vital organs in their arms, so some can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. It won't happen too quickly, though. It takes about a year for an arm to grow back.

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Sea stars are protected by armor.

Crown-of-Thorns Starfish / Borut Furlan / WaterFrame / Getty Images
Crown-of-Thorns Starfish (Acanthaster planci) on Coral Reef, Phi Phi Islands, Thailand. Borut Furlan / WaterFrame / Getty Images

Depending on the species, a sea star's skin may feel leathery, or slightly prickly. Sea stars have a tough covering on their upper side, which is made up of plates of calcium carbonate with tiny spines on their surface. A sea star's spines are used for protection from predators, which include birds, fish and sea otters. One very spiny sea star is the crown-of-thorns starfish, which is shown here. 

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Sea stars do not have blood.

sea star
Closeup of the arms of a sea star under a pier, showing its tube feet. pfly via Flickr (CC BY-SA 2.0)

Instead of blood, sea stars have a water vascular system, in which the sea star pumps sea water through its sieve plate, or madreporite, into its tube feet to extend them. Muscles within the tube feet retract them.

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Sea stars move using their tube feet.

Tube Feet of Spiny Starfish / Borut Furlan / Getty Images
Tube Feet of Spiny Starfish. Borut Furlan / Getty Images

Sea stars move using hundreds of tube feet, which are located on their underside. The tube feet are filled with sea water, which the sea star brings in through the sieve plate, or madreporite, on its top side. Sea stars can move more quickly than you might expect. If you ever get a chance, try visiting a tide pool or aquarium and take a moment to watch a sea star moving around. The sea star's tube feet also help the sea star hold its prey, which includes bivalves like clams and mussels.

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Sea stars eat with their stomachs inside-out.

Sea Star Eating a Bivalve
Sea star eating a bivalve. Karen Gowlett-Holmes / Getty Images

Speaking of prey, sea stars have a rather unique way of eating theirs. A sea star's mouth is on its underside. They prey on bivalves like mussels and clams, as well as small fish, snails, and barnacles. If you've ever tried to pry the shell of a clam or mussel open, you know how difficult it is. Sea stars wrap their arms around the animal's shell and pull it open just enough. And then it does something we could never imagine - it pushes its stomach through its mouth and into the bivalve's shell. It then digests the animal and slides its stomach back into its own body. This unique feeding mechanism allows the sea star to eat larger prey than it would otherwise be able to fit into its tiny mouth.

Common Sea Star, Showing Eye Spots / Paul Kay, Getty Images
Common Sea Star (visible eye spots circled). Paul Kay / Getty Images

While they can't see as well as we do, sea stars have an eye spot at the end of each arm. This is a very simple eye that looks like a red spot. The eye doesn't see much detail, but can sense light and dark. More »