Is a jellyfish a fish?

Most certainly not (and before you ask, neither is a starfish, but that’s another story). A jellyfish is a creature belonging to the phylum Cnidaria, a zoological category of aquatic animals known as coelenterates or cnidarians, which also includes corals and sea anemones.

Jellyfish live in water, and have bell- or saucer-shaped bodies made of a jelly-like substance, which is often transparent or semi-transparent. Around the edge of the body are stinging tentacles, which the jellyfish uses to catch its prey and which can also cause painful injuries to swimmers if they come into contact with them.

One of the main differences between jellyfish and fish is that, while fish are vertebrates, having a backbone, all coelenterates are invertebrates. Jellyfish don’t have a skeleton, a brain, or a central nervous system, and unlike fish, they reproduce both sexually and asexually at different stages of their life cycle.

Typically, a jellyfish goes through four main phases of development: planula, polyp, ephyra, and medusa. A jellyfish looks very different when it’s in the earlier stages of its life cycle: it’s the later, medusa form that we think of when we call a typical jellyfish to mind. A jellyfish begins as a planula, a larval stage, having a flattened, solid body. The next phase is a polyp, which can reproduce asexually, a process known as budding. An ephyra (a small medusa) is also produced asexually, from a polyp. The ephyra then matures into the medusa stage, which can reproduce sexually, from an egg of one individual and a sperm of another.

The confusion between fish and jellyfish probably comes from the name that they share and the fact that they both live in water, but that, essentially, is where the similarities end.

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