No—jellyfish are not fish. Familiar jellyfish are invertebrates in the phylum Cnidaria, the group that also includes corals and sea anemones. “Jellyfish” is a common name, not a biological classification; “jellies” is a more accurate everyday term. Comb jellies look similar but belong to a separate phylum, Ctenophora.
What is a jellyfish, scientifically?
When people say “jellyfish,” they usually mean a gelatinous animal in the cnidarian group commonly called Medusazoa. Cnidarians also include corals and sea anemones. Smithsonian Ocean reports more than 10,000 cnidarian species and fewer than 4,000 Medusazoa, the group it describes as animals commonly thought of as jellyfish; these are source-reported group counts, not a settled count of jellyfish species (Smithsonian Ocean, circa 2013).
The common name can cause confusion: jellyfish are not related to fish simply because “fish” appears in the word. Monterey Bay Aquarium recommends “jellies” as a more accurate everyday term. In broader usage, “jellies” may also refer to other gelatinous animals, such as salps and tunicates, so the meaning depends on context (Monterey Bay Aquarium).
Jellyfish and comb jellies are different animals
Comb jellies, or ctenophores, are not true jellyfish. They belong to Ctenophora, while true jellyfish belong to Cnidaria. Both can have soft, transparent bodies, but that resemblance does not make them close relatives. Comb jellies move using rows of tiny comb-like plates; cnidarians are distinguished by stinging cells. Traits can vary among species, so these are group-level distinctions rather than a claim that every individual looks or behaves alike (Smithsonian Ocean, circa 2013).
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| Feature | True jellyfish | Comb jellies |
|---|---|---|
| Phylum | Cnidaria | Ctenophora |
| Notable feature | Stinging cells | Rows of comb-like plates used for movement |
| Relationship | Separate animal groups; similar gelatinous appearance does not mean close relationship. | |
How are jellyfish built?
Jellyfish have a simple body plan organized around a central axis, a form of radial symmetry. NOAA describes three main tissue layers: an outer epidermis, a middle layer called mesoglea, and an inner gastrodermis. The mesoglea gives the body its gelatinous character.
They have no brain, heart, or blood. Instead, a nerve net helps them respond to stimuli. Their digestive cavity serves as both stomach and intestine and has a single opening (NOAA National Ocean Service, last updated September 23, 2026).
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About 95 percent of a jellyfish’s body is water, according to NOAA’s National Ocean Service, which states: “Only about five percent of the body of a jellyfish is solid matter; the rest is water.” That figure is NOAA’s general description, not a measurement of every species or individual.
What is the jellyfish life cycle?
Many familiar jellyfish alternate between a free-swimming medusa and an attached polyp stage. The bell-shaped medusa is the form most people recognize, but it is only part of the life cycle. The sequence below describes a common pattern; details differ among species.
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- Adult medusae reproduce. Adults release eggs and sperm; fertilized eggs begin developing.
- Planula larvae swim. The developing animal becomes a free-swimming planula larva.
- Larvae settle and become polyps. A planula attaches to a surface and develops into a polyp.
- Polyps produce young. Polyps can reproduce asexually. In some species, they release juvenile jellyfish called ephyrae.
- Ephyrae grow into medusae. The young develop into the familiar adult form (Smithsonian Ocean, January 2013; Monterey Bay Aquarium).
Where do jellyfish live?
Jellyfish occur in oceans around the world, from shallow water to the deep sea. A few species live in freshwater, but that does not mean each jellyfish species occupies all of these habitats. Where a particular species lives depends on that species (Smithsonian Ocean, circa 2013; Monterey Bay Aquarium).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What role do jellyfish play in the ecosystem?
Jellies are consumers and prey: they eat plankton, and animals including sea turtles eat them. When jellyfish gather in large numbers, or form a bloom, they can affect fisheries, swimming, and power plants that rely on seawater. The causes of blooms are debated, and the cited sources do not establish that jellyfish are increasing everywhere or that ocean warming will produce a universal increase (Smithsonian Ocean, circa 2013; Monterey Bay Aquarium).
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