Are Jellyfish Fish? What Jellyfish Really Are

Why jellyfish are not fish

Fish are vertebrates, animals with a backbone and a skull. They breathe with gills, swim with fins, have a brain, a heart that pumps blood, and a gut that runs from mouth to anus. All of that puts fish much closer to frogs, birds and people than to jellyfish.

Jellyfish have none of these features. A jellyfish such as the moon jellyfish is built around a soft, umbrella-shaped bell with a single opening underneath that serves as both mouth and anus. It has no skeleton and no gills, and instead of a brain it has a loose net of nerve cells (see Do jellyfish have brains?).

The two groups also split very early in animal evolution. The ancestors of jellyfish branched off from the line leading to vertebrates more than half a billion years ago, before the first fish appeared.

What are jellyfish, then?

Jellyfish belong to the phylum Cnidaria, a group of more than 11,000 species of mostly marine animals. The name comes from the Greek word for nettle and refers to their stinging cells, called cnidocytes, which fire tiny harpoon-like threads to catch prey or defend the animal.

Several cnidarian groups produce jellyfish:

• True jellyfish (Scyphozoa): about 200 known species, including the moon jellyfish and the lion’s mane.
• Box jellyfish (Cubozoa): about 50 species with cube-shaped bells and well-developed eyes, including the venomous Australian box jellyfish.
• Stalked jellyfish (Staurozoa): about 50 species that attach to seaweed or rocks instead of swimming.
• Hydrozoans (Hydrozoa): a large group that includes small hydromedusae, the “immortal jellyfish” and colonial forms.

In many of these groups, the jellyfish is only one stage of life. The same animal also spends time as a tiny polyp fixed to the seabed.

What is a jellyfish made of?

About 95 percent of a jellyfish is water. Most of the bell is mesoglea, a stiff, transparent jelly of water, collagen and other protein fibers, sandwiched between two thin layers of living cells. The outer layer forms the skin, and the inner layer lines the stomach and canals.

Because the living tissue is so thin, oxygen can pass straight in from the surrounding seawater, so jellyfish need no gills or lungs. The jelly also acts as a skeleton: water pressure and the springy fibers keep the bell in shape and help it bounce back after each swimming stroke.

Jellyfish relatives: corals, anemones and hydras

Jellyfish have close cousins that look nothing like them. Sea anemones and corals are cnidarians that spend their whole lives as polyps attached to the seabed, with tentacles facing upward (see Is coral a plant or an animal?). A jellyfish is roughly a polyp turned upside down, with its mouth and tentacles hanging below a free-swimming bell.

The Portuguese man o’ war looks like a jellyfish but is a colony of many specialized hydrozoan individuals working together (see Is a Portuguese man o’ war a jellyfish?). Comb jellies are another look-alike: they belong to a separate phylum, Ctenophora, swim with rows of beating hairs and most lack stinging cells.

How long have jellyfish existed?

Jellyfish are among the oldest animal groups still alive today. Because their soft bodies have no hard parts, fossils are rare, but the oldest clear fossil of a swimming jellyfish, found in the Burgess Shale of British Columbia, Canada, is about 505 million years old. Genetic studies suggest that cnidarians as a group are even older. Jellyfish were pulsing through the seas long before the dinosaurs, and before most of the fish groups alive today had evolved.

Why some people say “sea jellies”

Because “jellyfish” suggests a fish, many public aquariums in North America, such as the Monterey Bay Aquarium in California, label them “jellies” or “sea jellies.” Some scientists use the same terms, or call the swimming stage a medusa, named after the snake-haired Gorgon of Greek myth.

The old name is not going away, though. “Jellyfish” remains the everyday word in English, and scientific papers still use it widely.

Other “fish” that are not fish

Jellyfish are in good company. Many sea animals carry “fish” in their names without being fish:

• Starfish are echinoderms, related to sea urchins (see Is a starfish a fish?).
• Shellfish is a food term for mollusks and crustaceans such as clams, mussels, shrimp and crabs.
• Cuttlefish are mollusks, relatives of octopuses and squid.
• Crayfish are freshwater crustaceans.

The reverse mistake also happens. Whales and dolphins look like fish but are mammals that breathe air and nurse their young, while seahorses look nothing like fish but are true bony fish.