Is a Cuttlefish a Fish? What Cuttlefish Really Are

What is a cuttlefish, then?

Cuttlefish belong to the class Cephalopoda, which means “head-foot” in Greek, because their arms grow directly from the head. Cephalopods are mollusks, the same large group of invertebrates as snails, slugs, clams, mussels and oysters. Over millions of years most cephalopods lost or shrank the outer shell their ancestors carried, and they became fast, intelligent predators.

There are roughly 120 species of cuttlefish, all in the family Sepiidae. They live in the coastal seas of Europe, Africa, Asia and Australia, but none live in the Americas. The best known in Europe is the common cuttlefish (Sepia officinalis) of the eastern Atlantic and the Mediterranean, which grows to a body length of about 30 cm (12 in), and up to 45–49 cm (18–19 in) in the coldest parts of its range.

Why is it called a fish?

The “fish” in cuttlefish 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 starfish, jellyfish, crayfish and shellfish, even though none of these are fish either.

The Latin and Greek name for the cuttlefish is sepia, which is also the scientific name of the main cuttlefish genus. The word lives on in the color sepia, named after the dark brown ink of the common cuttlefish.

How is a cuttlefish different from a fish?

Fish are vertebrates, animals with a backbone, while cuttlefish are invertebrates. The two groups share a few features, such as gills and good eyesight, but their bodies are built in completely different ways.

A cuttlefish has two gill hearts that pump blood through its gills and a third heart that sends it around the body, a design shared with octopuses and described in How many hearts does an octopus have? Its blood is blue because it carries oxygen with a copper-containing protein rather than iron.

The cuttlebone: a shell, not a bone

The cuttlebone, the white oval object often found on beaches, is not a bone at all. It is an internal shell, a much-modified version of the outer shells of the cuttlefish’s ancient ancestors. It is made mainly of aragonite, a form of calcium carbonate, and divided into many thin chambers.

By moving liquid in and out of these chambers, the cuttlefish changes how much gas they hold and so adjusts its buoyancy, letting it hover or sink without effort. The nautilus uses a similar system in its coiled outer shell, a reminder that the two animals are relatives.

Arms, tentacles and W-shaped eyes

A cuttlefish has eight arms lined with suckers, which it uses to hold and handle food, and two longer feeding tentacles with sucker-covered clubs at the tips. The tentacles stay hidden in pockets below the eyes and shoot out in a split second to grab a shrimp or small fish. In the middle of the arms is a beak like a parrot’s, strong enough to crack crab shells.

Its eyes are among the most distinctive in the animal kingdom. The pupil is shaped like a W, and the eye is a camera-type eye with a lens and retina, remarkably similar to a fish eye. This is a classic case of convergent evolution: cephalopods and vertebrates evolved camera eyes independently. One difference is that a cephalopod eye has no blind spot, because the nerve fibers leave from behind the retina rather than passing in front of it.

Unlike fish, a cuttlefish can change the color, pattern and texture of its skin almost instantly, as explained in How do cuttlefish change color?

Cuttlefish, squid and octopus: how to tell them apart

Cuttlefish, squid and octopuses are all cephalopods, but each has its own body plan:

• Cuttlefish have a broad, flattened oval body edged all around by a narrow fin, an internal chalky cuttlebone, eight arms plus two tentacles and a W-shaped pupil. They usually live close to the seabed.
• Squid have a long, streamlined body with fins near the rear, a thin flexible internal pen called the gladius, and eight arms plus two tentacles. Many are fast open-water swimmers, and the group includes giants such as the giant squid.
• Octopuses have a soft, rounded body with no internal shell or only tiny remnants of one, eight arms and no tentacles. Most live on the seabed and crawl as much as they swim.

The differences between the last two are covered in more detail in octopus vs squid.

Are any cephalopods closer to fish?

No. Even the most fish-like squid are mollusks through and through. Their streamlined bodies, fins and keen eyes are adaptations to the same way of life as fast-swimming fish, not signs of a close relationship. The last common ancestor of cuttlefish and fish was a simple worm-like animal that lived more than 500 million years ago, long before either group appeared.