The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change

The Jellyfish Are Back


I am in Maine this summer, on the coast. I know, I know, it’s not polite to brag, but my point is I’ve started seeing jellyfish. One washed in, among the seaweed, my first week here–a moonjelly, I think, dead, and so battered from the waves that its tentacles were gone and its margins looked chewed. Then, last week, I went down to the shoreline and spotted another jelly–and another and another, all floating among the wavelets of the bay on a rising tide. I couldn’t stay long enough to do a real survey, so I don’t know how many there really were, but I counted well over ten in the small patch of water right in front of me. A few days after that, a woman I’d just met told me about having gone out in a boat and finding herself entirely surrounded by hundreds of jellies.

Last year, the Gulf of Maine was visited by huge numbers of jellyfish and this year it seems to be happening again. Nor in Maine alone–many parts of the world are seeing huge blooms of jellies, and nobody really knows why.

As you might have guessed, climate change is one of the primary culprits.

About the Jellies

Jellyfish are one of those subjects that everyone thinks they understand–and they are wrong. Obviously, the animals are not really fish. There are those who want to call them “jellies” instead, to avoid confusion, but since the animals clearly are not jelly, either (they do not come in grape or strawberry flavor), that doesn’t seem like much of an improvement. A better term is medusae (singular, medusa), meaning any of those gelatinous bell-shaped animals with stinging cells. The stinging cells mark them as cnidarians, a phylum that also includes corals, sea anemones, and several other groups (a “phylum” is a very large group of distantly related living things. To put it in perspective, all vertebrates belong to the same subphylum).

Two other groups of animals are often mistakenly lumped in with jellyfish, since they, too, have jellylike bodies–the Portuguese man o’ war and its relatives (which are actually siphonophores, another type of cnidarian); and comb jellies, which actually comprise a whole different phylum (Ctenophora) because they lack stinging cells. All three groups, medusae, siphonophores, and ctenophores, sometimes turn up in articles about the recent swarms of jellyfish–I’ll talk only about medusae, but much of this may apply to the others, too.

Jellyfish can’t swim purposefully–not because they lack brains, many brainless animals do move purposefully, but because they are adapted to a very passive lifestyle. A few sit on the bottom, but most float where the current takes them, pulsing their bells to keep water and food floating past their tentacles. Thus, when jellyfish “blooms” occur, it isn’t because the jellies have swum anywhere. Instead, either currents have pushed them together or the the jellies in one area have reproduced very quickly for some reason. All medusae sting, but very few are actually a concern for humans–although I have heard some hints that people who are allergic to bee stings may also be unusually sensitive to jellyfish stings. Personally, I am unusually sensitive to both.

How Is Climate Involved?

Jellyfish breed faster in warmer water, so climate change benefits them directly. Also, since medusae can eat pretty much anything small enough to fit in their mouths, and since they can tolerate poor water quality, anything that renders the seas unlivable for other animals clears the way for jellies. In this sense, the act rather like certain invasive plants in that their proliferation is a symptom of larger ecological collapse. Global warming (and the ocean acidification that goes along with it), overfishing, pollution, and other problems may all be contributing to a situation that is very good for jellyfish precisely because it is bad for everything else.

But all of the above only increases the likelihood of large jellyfish blooms–because their populations vary anyway, it is hard to pin any specific bloom on climate. They are rather like weather, that way.

So, How Much Do We Really Know About This?

Um, nothing. Or, next to nothing. Scientists do seem pretty clear on what conditions are good for medusae in general, and while the “jelly ocean” hypothesis (the idea that environmental collapse could result in an ocean dominated by medusae) is still a guess, it is an educated guess. But as to specifics?

We still know next to nothing about where jellies normally occur and in what numbers, so it’s hard to say whether they are really increasing. For example, there is no such thing as an expert in Maine jellyfish–the closest we’ve got are scientists willing to do the work to become experts, except they can’t do that because there is no money available for research. At present, they are asking members of the public to at least call in any jellyfish sightings in the region.

But the jelly blooms do seem odd, especially to locals–and while such hunches are hardly conclusive, when people who understand an area well think something is unusual, they are probably right.

What’s Wrong with Jelly?

You mean, aside from the “ick” factor? Personally, I find medusae fascinating and lovely, but they are made of wiggly slime and I have serious texture aversions to anything that wiggles. But aside from that, is there anything really wrong with a whole bunch of jellyfish?


Big jellyfish blooms can clog nuclear power plant water intakes, shut down swimming beaches, clog up fish nets and traps, and kill huge numbers of fish in aquaculture pens. Some blooms are so intense that the sea can become almost solid jelly–there is no room for anything else. And, of course, if the blooms are indeed a symptom of larger problems,  hundreds of miles of jellyfish are a major cause for concern.

Basically, we’re looking at another example of the “new normal” of an imbalanced world–and just like all the other examples, we face the problem with virtually no solid information to work with.


Author: Caroline Ailanthus

I am a creative science writer. That is, most of my writing is creative rather than technical, but my topic is usually science. I enjoy explaining things and exploring ideas. I have one published novel and another on the way. I have a master's degree in Conservation Biology and I work full-time as a writer.

2 thoughts on “The Jellyfish Are Back

  1. Pingback: Jellyfish Tide | The Climate Emergency

  2. Pingback: Looking Back | The Climate Emergency

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