Is a Starfish a Fish? Why Scientists Say Sea Star

What is a starfish, then?

Starfish belong to the phylum Echinodermata, a name that means “spiny skin” in Greek. Echinoderms are a group of roughly 7,000 or more living species found only in the sea. They include five main groups: sea stars, brittle stars, sea urchins and sand dollars, sea cucumbers, and sea lilies and feather stars.

Sea stars form their own class, the Asteroidea, with about 1,900 known species. They live in every ocean, from tropical coral reefs to icy polar seas, and from tide pools down to about 6,000 m (20,000 ft) deep. A familiar example is the ochre sea star of North America’s Pacific coast, the purple or orange star of countless tide pool visits.

Most sea stars have five arms around a central disc, but some have many more. The sunflower sea star of the North Pacific usually has 16 to 24 arms, and one Antarctic species can have more than 50.

Why is it called a starfish?

The “fish” in starfish is a leftover from older English, when people used the word for almost any animal that lived in water. That is why we still say jellyfish, shellfish and cuttlefish, even though none of them are fish. You can read about two of these in Are jellyfish fish? and Is a cuttlefish a fish?.

Many aquariums, marine scientists and teachers now prefer the name sea star, so that nobody assumes the animal is a kind of fish. Several European languages already use that idea: German calls it Seestern, French étoile de mer and Spanish estrella de mar, all meaning “star of the sea”.

How is a sea star different from a fish?

Fish are vertebrates, animals with a backbone, while sea stars are invertebrates. Beyond that, almost every part of their bodies works differently.

A sea star’s skeleton is made of calcite, a form of calcium carbonate, and sits just under the skin. The plates are linked by a special connective tissue that can stiffen or soften quickly, so the animal can be flexible one moment and rigid the next.

No brain and no blood? How a sea star works

A sea star has no central brain. Instead, a ring of nerves around the mouth connects to a nerve running down each arm. If one arm senses food, it can take the lead and the others follow.

It also has no blood in the usual sense. Sea stars run on a hydraulic system called the water vascular system. Seawater enters through a small sieve plate on the upper surface, called the madreporite, and flows through canals to the tube feet. Muscles squeeze small bulbs at the base of each foot to stretch it out, and sticky chemicals, rather than pure suction, let it grip. The fluid in the body cavity, along with this water system, moves oxygen and food around the body.

Breathing happens through thin, finger-like skin gills on the upper surface and through the tube feet. At the end of each arm sits an eyespot made of dozens to a few hundred tiny light-sensitive units. These eyes are simple, but studies show some species can use them to find their way back to a reef.

Feeding is just as unusual: many sea stars push their stomach out of their mouth to digest prey outside the body, as explained in How do starfish eat?.

Are sea stars related to fish at all?

Distantly, and more closely than you might expect. Animals with backbones, including fish and people, belong to a large group called the deuterostomes, named after the way their embryos develop. Echinoderms are deuterostomes too. That means a sea star is more closely related to a fish than to a crab, an octopus or a jellyfish.

The family resemblance shows up in the young. Sea star larvae are tiny, see-through and bilaterally symmetrical, with left and right sides like a fish or a human. Only when they settle on the seabed do they rearrange their bodies into the five-part star shape.

Sea stars are also an ancient group. Their fossil record goes back to about 450 million years ago, although complete fossils are rare because the small skeletal plates fall apart after death.

Where do sea stars live?

All sea stars live in salt water. They cannot control the salt content of their body fluids, which stay as salty as the sea around them, so there are no freshwater species and few can cope with the mixed water of estuaries.

They live on almost every kind of seabed: rocky shores, coral reefs, sand, mud, kelp forests and the deep-sea floor. Many are predators of clams, mussels and snails, some scavenge dead animals, and a few deep-sea species filter food particles from the water with long, thin arms.

So the next time you see a “starfish” in a tide pool, you are looking at a spiny-skinned, brainless, five-armed relative of sea urchins, and a very distant cousin of the fish swimming above it.