Is there such thing as a Jellyfish?

by Camila Lupi '19

​Have you ever asked yourself the question “what is a jellyfish?” Probably not. We don’t have time to be asking ourselves such rudimentary questions when we have orgo problem sets to finish and deadlines to meet. But is this actually as simple a question as it seems to be on the surface? To find out, I asked a handful of Brown students what they thought a jellyfish was, and these are the answers I got:

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• “A squishy sea organism that is often clear or pink and has tentacles and can sting humans and comes in many different types.”
• “A squishy animal that lives in the ocean. May or may not sting.”
• “An aquatic animal that swims and has stinging tentacles.”
• “A sea animal that’s squishy and protects itself with poison.”
• “I honestly don’t know what a jellyfish is, other than a blob in the ocean that will sting you.”

 
These responses were mostly correct in naming jellyfish as gelatinous and marine. However, a recurring theme in all the answers, and a widespread misconception in general, is that a jellyfish is a single type of animal or group of animals that sting. In fact, “jellyfish” is really just a general term used to describe any gelatinous sea creature [1], and these sea creatures actually fall into several distinct clades—or groups—of organisms, some that aren’t all that closely related and many of which don’t sting. The phylogenetic tree—used to describe genetic similarity of organisms through lineage—depicted below shows the different clades under which “jellyfish” fall. 

[2]

According to the Monterey Bay Aquarium Research Institute, the most commonly seen jellyfish, such as sea nettles, are cnidarians [2]. They’re characterized by unique cells that release harpoon-like organelles called nematocysts that sting predators and prey. Within the cnidarians, there is vast variation with respect to lifecycles, number and position of tentacles, movement, and shape. Many people know of the Portugese man-o'-war, but few people know that it is just as much a “jellyfish” as the common sea nettle.

Mollusks and worms are definitely not the organisms that come to mind when thinking of jellyfish. However, many mollusks and polychaete worms are marine gelatinous creatures and, in fact, one of the most abundant deep sea creatures is a “jellyfish” polychaete. Some have distinct bristles, others are parasitic, and some have the ability to produce and emit light, making them bioluminescent.
Another crucial misconception is that a jellyfish, by definition, is an animal that stings. While the cnidarians are known for their stinging capabilities, mollusks and polychaete jellies lack the ability to sting. Ctenophores—the comb jellies—have sticky tentacles that don’t sting. Instead, they are defined by gorgeous bioluminescence and combs of vibrating cilia. Similarly, urochordates lack the capability to sting, and are instead characterized by the way they move by essentially squirting water out of themselves.

Whether they’re groups within one clade, or groups amongst different clades, the animals commonly lumped together as “jellyfish” are much more varied than the stereotypical medusa-shaped stinging jellyfish. They come in all shapes, sizes, colors, lifecycles, and ways of living.

For more detailed information, check out the Monterey Bay Aquarium Institute’s video.
 
Sources:
[1] the definition of jellyfish [Internet]. Dictionary.com. 2016 [cited 17 November 2016]. Available from: http://www.dictionary.com/browse/jellyfish
[2] There's no such thing as a jellyfish [Internet]. YouTube. 2016 [cited 17 November 2016]. Available from: https://www.youtube.com/watch?v=3HzFiQFFQYw