What Are Jellyfish (Jellies)?


Anyone who has seen a jellyfish’s dome-shaped body pulsating through the water, tentacles dangling behind, can testify that they are both an intimidating and intriguing sight. Watching them can make you wonder, what are they?

Jellyfish are not really fish, of course, because a fish’s anatomy is centered around its backbone, whereas the jellyfish is a dome-shaped invertebrate. Therefore, it’s more accurate to refer to them simply as “jellies.” Believe it or not, these roving creatures, with their umbrella shape and hanging tentacles, are most closely related to corals , sea anemones, sea whips, and hydrozoans. Why? They share a distinctive body part – a harpoon-like stinging cell used to capture prey. Generally, these cells are called cnidocytes (hint: don’t pronounce the “c”), which comes from the ancient Greek word for nettle. Therefore, animals in this phylum are called cnidarians.

The cnidocytes on jellies’ tentacles discharge venom from a sac called a nematocyst. These help them to capture floating prey in the water column. Jellies have limited control over their movement, using a muscle to propel themselves short distances by expanding and contracting their bell. Therefore, they drift in currents and often appear in large masses called a “bloom,” a “swarm,” or a “smack.”

Purists consider the only “true jellies” to be members of one specific class of cnidarians, but many similar looking animals with dangling tentacles are referred to as jellyfish. For example, the Portuguese man o’ war (aka. blue bottle) is often mistaken for a type of jellyfish but is in fact a different type of cnidarian that inflicts a nasty sting. Comb jellies, despite the word “jelly” in their name, are not related to cnidarians. This is because they lack stinging cells, which makes them harmless to humans.

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