Starfish (or sea stars) are beautiful marine animals found in a variety of colors, shapes, and sizes. All starfish resemble stars, and though the most common have only five arms, some of these animals can grow up to 40 arms. The amazing sea creatures—part of a group of animals known as echinoderms—travel using their tube feet. They can regenerate lost limbs and swallow large prey using their unusual stomachs.

Sea Stars Are Not Fish

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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.

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

Sea Stars Are Echinoderms

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, sea urchins, sea cucumbers, and sea lilies. Overall, this phylum contains approximately 7,000 species.﻿﻿

Many echinoderms exhibit radial symmetry, meaning their body parts are arranged around a central axis. Many sea stars have five-point radial symmetry because their body has five sections. This means that they do not have an obvious left and right half, only a top side and a bottom side. Echinoderms also usually have spines, which are less pronounced in sea stars than they are in other organisms such as sea urchins.

There Are Thousands of Sea Star Species

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, sea stars can also be found in cold areas—even the polar regions.

Not All Sea Stars Have Five Arms

Sun star with many arms. Joe Dovala/Getty Images

While many people are most familiar with the five-armed species of sea stars, not all sea stars have just five arms. Some species have many more, such as the sun star, which can have up to 40 arms.

Sea Stars Can Regenerate Arms

Sea star regenerating four arms. Daniela Dirscherl/Getty Images

Amazingly, sea stars can regenerate lost arms, which is useful if a sea star is injured by a predator. It can lose an arm, escape, and grow a new arm later.

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. This 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 (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 aptly named crown-of-thorns starfish.

Sea Stars Do Not Have Blood

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

Instead of blood, sea stars have a circulatory system made up primarily of seawater.

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

From the madreporite, seawater moves into the sea star's tube feet, causing the arm to extend. 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

Sea stars move using hundreds of tube feet located on their underside. The tube feet are filled with seawater, 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 most amazing sights in the ocean.

Tube feet also help the sea star hold its prey, including clams and mussels.

Sea Stars Eat With Their Stomachs Inside-Out

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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 it catches its food, the sea star will wrap its arms around the animal's shell and pull it open just slightly. 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.

Sea Stars Have Eyes

Common Sea Star (visible eye spots circled). Paul Kay/Getty Images

Many people are surprised to learn that starfish have eyes. It's true. The eyes are there—just not in the place you would expect.

Sea stars have an eye spot at the end of each arm. This means that a five-armed sea star has five eyes, while the 40-armed sun star has 40 eyes.

Each sea star eye is very simple and looks like a red spot. It doesn't see much detail but it can sense light and dark, which is just enough for the environments the animals live in.

All True Starfish Are in the Class Asteroidea

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Starfish belong to the animal class Asteroidea. These echinoderms all have several arms arranged around a central disk.

Asteroidea is 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.

Sea Stars Have Two Ways to Reproduce

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Male and female sea stars are hard to tell apart because they look identical. While many animal species reproduce using 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, which is what happens when the animals lose an arm.