So When You Say Jellyfish? Three Definitions, Explained

When I am giving a presentation at outreach events, I usually start with a slide of a sea nettle in the center of a black screen. Sea nettles have that stereotypical look that the word “jellyfish” most folks conjure up in their mind: large, round bell, tentacles coming out from the edges and semi-transparent with maybe a hint of red or blue. Most of the 200 species in the class Scyphozoa, the “true jellies,” have this morphology, called a medusa, at some point in their life history. They come in various colors and sizes, from the 2 meter diameter bell and 40 meter long tentacles of the Lion’s Mane Jellyfish Cyanea capillata to the smallest jellies at just a few centimeters in length.


Pacific Sea Nettle, Baltimore Aquarium (

After I show my slide with a single sea nettle, the next click reveals a collage of hydroids, siphophores, sea anemones, stalked jellies, box jellies, bubber jellies, and anything else I think looks cool on that day. Scyphozoans are just one class of the Phylum Cnidaria, which includes more than 11,000 species. What unites all the animals in this group is the possession of stinging cell organelles called cnidae, which gives a jellyfish it’s sting.

True jellyfish, along with box jellies (Class Cubozoa) and some hydrozoans (Class Hydrozoa) go through a complex life history, where they start as benthic, asexual budding polyps that will (given the right conditions) undergo a process called strobilation, a kind of metamorphosis where these polyps produce ephyrae, or immature jellyfish. These ephyrae grow into swimming medusa, which are sexually reproductive and release eggs and sperm into the water to produces swimming planula that eventually settle to produce a new polyp. We will get into the nitty-gritty of this process in a different post, but the immediate message is that characteristic medusa shape that many know to be a jellyfish is just one part of a complicated life cycle.

Reproductive life cycle of moon jellyfish (NSF)

But not all cnidarians have a medusa stage. So are these jellyfish? What about those gelatinous animals that are not cnidarians, like comb jellies of the Phylum Ctenophora or the urochordates and pelagic worms of the Phylum Chordates (the same phylum as humans).

Here are three often used definitions for jellyfish:

  • Jellyfish – all pelagic living animals that have a gelatinous body composition
  • Jellyfish – members of the phylum Cnidaria
  • Jellyfish – members of the class Scyphozoa

Of course, these are not end-all-be-all definitions. These are simply based on what I find are are the terminology that can be the most confusing in when talking to the public. They can be restructured based on context, such as Dr. Claudia Mills (2001) by defining jellyfish as medusae of the phylum Cnidaria and pelagic (open water) members of the phylum Ctenophora. Environmentally and ecologically, these animals play a similar role as gelatinous, carnivorous animals that live in the water column, so it makes sense for some purposes to alter these definitions. Within folks that study cnidarians, jellyfish is often a reference to the medusa life stage, which would indulge hydromedusae, or hydrozoans at the medusoid stage.

This seems like a strong argument for definition one; similar functions would warrant a single definition. But the reasoning, at least for the confusion specifically between Cnidaria and Ctenophores, is that scientists were also unsure about their evolutionary relationship until fairly recently. Previously, both of these groups were placed in a group called Coelenterata, sister to the rest of the bilaterians. But this term is now obsolete, thanks to recent sequencing of comb jellies (Pacific sea gooseberry, Wartycomb jelly) and a few cnidarian genomes (Nematostella, Hydra). Though the debate continues of which is more ancient, comb jellies or sponges, we can pretty safely say Cnidaria is sister to the rest of us bilaterians. But the confusion between Cnidaria, Ctenophores, and other gelatinous zooplankton will likely continue when talking to members of the public.

In some ways, it is usually just better to go with the intuition of the audience than try and be obstinate about exact terminology. A book titled “Jellyfish” by Lisa Ann-Gershwin, a veritable queen of jellyfish systematics, includes comb jellies and other gelatinous zooplankton in her book, but uses a section to described the differences between these groups. The Smithsonian combines their information sections on “Jellyfish and Comb Jellies” so that when you learn about one, you automatically learn the similarities and differences of the other.

As many science communicators come to learn, there is a fine line between too much information, such that it makes the experience unpleasant, and trying to convey as accurate as information as possible. And this is not just in talking about jellyfish. As I alluded to in the Venom vs. Poison post, the difference between venomous and poisonous is something I encounter almost every time I talk to the public, be they adults or children. As a young “student of learning” and even now as a graduate student, I like to learn when I am wrong (and it happens a lot), but I know how easy it is to be turned off to actually listening to information if the educator appears mocking or exasperated or bog (see meme below) or presents an overwhelming amount of information. That is also part of why making a blog like this is important to me, so I can see how my own understanding of the subject grows over time, and figure out how to convey information that I repeat over and over in a way that everyone can learn and enjoy learning about it.


(I do not take credit for this masterful representation of sci comm fears)

I must say, I never think I will get tired of hearing the “ooohs” and “ahhs” and occasionally the audible gasp when I present my slide full of different jellies, because I know, even if they do not hear anything else, they found out something cool that day.

And if it makes a difference at all, when I hear the word jellyfish I go with definition number three. When I am referring to cnidarians, I will say jellyfish and their relatives. If I am talking about all gelatinous zooplankton I say… gelatinous zooplankton. Whatever you consider a jellyfish to be, these are undeniably beautiful creatures that deserved to be celebrated (3 November people!).

This video by from Steve Haddock at Monterey Bay Research Institute gives a great summary of the group Cnidaria (along with AMAZING footage) as well as ctenophores and other gelatinous critters.


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