Like a Star on the Ocean Floor

Starfish! This marine animal has always been fascinating because of its radial symmetry with five or more arms. The starfish (or "sea star" as scientists prefer to call it) gets its name from its star-shaped body.

A glowing starfish on the ocean floor
A starfish rests on the ocean floor, its star-shaped body instantly recognizable.

Starfish Species

About 2,000 species of this marine animal are found in every ocean across the world. Most of them live in the Indian and Pacific oceans. Starfish are generally found in shallow waters.

Crown of thorns starfish
The crown-of-thorns starfish is one of the most distinctive species, covered in sharp spines.

Because they share a body plan that is symmetrical by five points (just like sea urchins) starfish belong to the phylum Echinodermata.

Tube Feet: Built for Movement and Feeding

If you have ever picked up a starfish and turned it over, you will find hundreds of tiny projections on its underside. These are called tube feet. They serve two main jobs: moving the starfish through the water, and helping it pry open scallops or clams to eat.

Close-up of starfish tube feet on the underside of a sea star
Hundreds of tube feet cover the underside of a starfish, they grip surfaces and force open shells.

Two Stomachs

One of the most interesting facts about the starfish is that it has two stomachs. The cardiac stomach can reach outside the body to start digesting food before it is even swallowed. When the cardiac stomach comes back inside, the partially digested food is passed to the pyloric stomach.

Starfish on the ocean floor
A starfish on the seabed, beneath that calm exterior is a surprisingly complex digestive system.

Mouth of a Starfish

Have you ever tried to find the mouth of a starfish? It is located on the ventral surface, the underside. The sea star with five arms is the most commonly spotted type. However, some species have 10, 20, or even up to 40 arms!

Eye Spots at the Tip of Each Arm

The starfish has an eye spot at the end of each arm. It cannot see fine details, but the eye spot lets the starfish detect movement and tell the difference between light and dark.

Red starfish on a bed of seaweed
At the tip of each arm, a small eye spot helps the starfish sense its surroundings.

Protective Armor

Most starfish have a spiky or leathery surface, depending on the species. The crown-of-thorns starfish is famous for its thorny spines covering its whole body. This tough outer layer is made of calcium carbonate plates with tiny spines, it acts as armor against predators.

Take the Starfish Fact or Fiction Quiz!

5 quick questions. How much do you really know about sea stars?

A Unique Anatomy

In addition to having no brain, these sea animals do not have a single drop of blood in their body. Instead, they use a water vascular system, seawater is pumped in through a small opening called a sieve plate. The anus of this invertebrate is located on top of its body.

Starfish on the seabed showing its body structure
A starfish has no brain and no blood, seawater powers its movement and body functions.

Regeneration: Growing Back from One Arm

Amazingly, if a starfish loses an arm, it can grow a new one back in about a year. This is very useful when threatened by a predator, the sea star can shed the arm and escape. Sometimes, an entirely new starfish can grow from just one detached arm. This happens because most of the vital organs are stored inside the arms.

Starfish arms shown in detail
The arms of a starfish hold most of its vital organs, which is why a single arm can sometimes grow into a whole new animal.

Frequently Asked Questions

Is a starfish actually a fish?

No, starfish are echinoderms in the class Asteroidea, not fish at all.

How long can starfish live?

Starfish can live for up to 35 years in the wild.

How many species of starfish are there?

About 2,000 species of starfish are found in every ocean around the world.

Can a starfish regrow a lost arm?

Yes, a starfish can regenerate a lost arm in about a year. In some cases, an entirely new starfish can grow from just one arm.

Do starfish have blood?

No, starfish do not have blood. They use a water vascular system, pumping seawater through a sieve plate to move and feed.

Feeding Technique

These marine animals have a very unusual way of eating prey that is larger than their mouth. The starfish wraps its arms tightly around a clam or shell and forces it open. Then it pushes its stomach out through its mouth and into the shell, digests the prey there, and pulls the stomach back in. This lets ocean invertebrates like starfish eat food much bigger than their mouths could normally handle.

Starfish resting on a stone on the ocean floor
A starfish at rest, when it spots a clam, those arms spring into action to force it open.

Flexible Reproduction System

An interesting fact about starfish is that they can reproduce both sexually and asexually. In the mating season, females release eggs and males release sperm into the water.

Starfish eggs
Starfish eggs, a single female can produce around a million eggs in one mating season.

A female starfish is capable of producing around a million eggs at one time. These facts about starfish give us a glimpse of the kind of life that exists beneath the sea, complex, beautiful, and full of surprises.

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