12 Surprising Facts About Starfish

Starfish (or sea stars) are beautiful animals that can be a variety of colors, shapes, and sizes. All of them resemble a star, which is how they received their most commonly used name.

While some sea stars 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 and there is a lot that you can learn about them.

Close-up of orange starfish on sand
Carlos Agrazal / EyeEm / Getty Images

Although sea stars live underwater and are commonly called "starfish," they are not true fish. They do not have gills, scales, or fins like fish do.

Sea stars also move quite differently from fish. While fish propel themselves with their tails, sea stars have tiny tube feet to help them move along. They can move very quickly, too.

Because they are not classified as fish, scientists prefer to call starfish "sea stars." More »

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, sea cucumbers, and sea lilies. Overall, this phylum contains over 6,000 species.

Many echinoderms exhibit radial symmetry, meaning their body parts are arranged around a central axis. Some sea stars have five-point radial symmetry because their body has five sections or multiples thereof.

This symmetry also means that they do not have an obvious left and right half, only a top side and bottom side. These organisms also usually have spines, which are less pronounced in sea stars than they are in other organisms like sea urchins. More »

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 while others live in the deep water of the ocean. While many species live in tropical areas, you can also find sea stars in the cold waters of the Earth, even the polar regions.

Not All Sea Stars Have Five Arms

Diver and Sun Star, Crossaster sp., Monterey Bay, California, USA
Sun star with many arms. Joe Dovala / Getty Images

While you might be most familiar with the five-armed species of sea stars, not all of them have just five arms. Some species have many more arms. For instance, the sun star can have up to 40 arms.

Sea Stars Can Regenerate Arms

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

Amazingly, sea stars can regenerate lost arms, which 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. This means that some species 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.

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 it may be 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 aptly named crown-of-thorns starfish. 

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 circulatory system made up primarily of sea water.

Sea water is pumped into the animal's water vascular system through its sieve plate. This is a sort of trap door called a madreporite, which is often visible as a light-colored spot on the top of the starfish.

From the madreporite, the sea water moves into the sea star's tube feet and that is how it extends an arm. Muscles within the tube feet are used to retract the limb.

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 madreporite on its top side.

Sea stars can move quicker than you might expect. If you get a chance, visit a tide pool or aquarium and take a moment to watch a sea star moving around. It is one of the neatest sights in the ocean.

The tube feet also help the sea star hold its prey, which includes clams and mussels.

Sea Stars Eat With Their Stomachs Inside-Out

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

Sea stars 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. However, sea stars have a unique way of eating these creatures.

A sea star's mouth is on its underside. When they catch their food, a sea star will wrap its arms around the animal's shell and pull it open just enough. Then it does something amazing.

The sea star 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

It may surprise you that starfish do have eyes. They're just not where you might expect.

While they can't see as well as we do, sea stars have an eye spot at the end of each arm. This means that a five-armed sea star has five eyes with the 40-arm sun star has 40 eyes.

Their eyes are very simple and look like a red spot. The eye doesn't see much detail but it can sense light and dark, which is just enough for the environments they live in. More »

A Child's Hand Touching A Starfish
Marcos Welsh / Design Pics / Getty Images

Starfish are classified in the Class Asteroidea. All asteroids have several arms arranged around a central disk.

Asteroidea is known as the classification for "true stars." These animals are in a separate class from brittle stars and basket stars, which have a more defined separation between their arms and their central disk. More »

Sea Stars Reproduce Two Ways

Mating starfish amongst mussels.
Doug Steakley / Getty Images

Male and female sea stars are hard to tell apart because they look identical. While many animal species reproduce by only one method, sea stars are a little different.

Sea stars can reproduce sexually. They do this by releasing sperm and eggs (called gametes) into the water. The sperm fertilizes the gametes and produces swimming larvae which eventually settle on the ocean floor, growing into adult sea stars.

Sea stars can also reproduce asexually through regeneration, such as when they lose an arm.

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Kennedy, Jennifer. "12 Surprising Facts About Starfish." ThoughtCo, Aug. 4, 2017, thoughtco.com/facts-about-sea-stars-2291865. Kennedy, Jennifer. (2017, August 4). 12 Surprising Facts About Starfish. Retrieved from https://www.thoughtco.com/facts-about-sea-stars-2291865 Kennedy, Jennifer. "12 Surprising Facts About Starfish." ThoughtCo. https://www.thoughtco.com/facts-about-sea-stars-2291865 (accessed April 21, 2018).