How do jellyfish perceive the world?

This post is part of a new series on my recent paper: “Evolution and development of scyphozoan jellyfish.” I try really hard to make my research accessible, and when I can, publish everything open access. However, that’s not always enough. And I want to especially share my work with the sci-curious, and all the taxpayers out there who support me through the US National Science Foundation. Expect more posts to come, and thank you!!

I’ve often wondered how jellyfish sense their world. I even asked a Zen master once if she thought jellyfish were self-aware and she looked at me in amazement like, “you’re the expert, how would I know??” It’s true, as someone who studies jellies, I’d probably be one of the better folks to ask. Though of course there are folks out there with way more knowledge than me of how jellies perceive and process information (folks like Richard A. Satterlie and Nagayasu Nakanishi). Still, I felt like it was my time to give understanding the world through a jellyfish a try. In my recent review, I talk about the ways jellyfish sense their world.

There are several major clues that can help us detangle the jellyfish approach to life, and how aware they might be of their inner and outer surroundings. First, there is the bizarre way they sense their environment. Next, there are two different kinds of nervous systems that help them process that strange information.

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Photo of a rhopalium (finger-like structure in the middle of the ‘orchid-like’ flower. Photo by Alexander Semenov

A jelly’s ability to sense the world is due in large part to a specialized sensory structures, that sort of combine the eye, middle ear, cerebellum, and possibly the nose, all in one place. These little structures hang from the bell margins like earrings, and are called “rhopalia”. Each ropalium is shaped like a finger, and is packed with microscopic crystals at its tip. These crystals help the jelly sense up and down, by bending in the direction of gravity, similar to our inner ear. They also have a small pigment spot, which likely helps the jelly sense basic light and dark. So far, we’ve got an animal that can tell which way it’s pointing in space, and see rough light and shadow. Next we’ve got a few mystery structures, like the little bonnet-like structure surrounding the rhopalium above, which may act like a jelly nose, helping it sense chemicals in the water, but no one knows for sure. Each rhopalium also acts like a pacemaker, helping coordinate jelly movement, similar to the way our cerebellum coordinates ours. The neurons in the rhopalia communicate all this info to the rest of the nervous system, which is sort of broken into two parts.

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Photo by Alexander Semenov

The two different nervous systems in the rest of the jelly’s body are spread over the animal like a net. Jellyfish don’t have brains, after all, so where would all these neurons go? Instead of connecting to one central spot, they interconnect to each other, no one set of neurons in this net being more important than any other (at least, that we know of).

The first nervous system is the “Large Nerve Net.” Why large? Well, the nerve cells that compose it are HUGE. See, jellyfish don’t have a specialized kind of fatty cell (oligodendrocytes), which humans and many other animals have in abundance. In many animals, these fatty cells wrap around neurons and make the nerves extra conductive, like wrapping a wire to protect and shield it. So human neurons don’t have to be very big to be really powerful. Because jellyfish don’t have these fatty cells in order to conduct lots of information quickly the neurons have to get massive. The Large Nerve Net is packed around the muscle that lines the underside of the jelly, and basically this nervous system coordinates puling, turning the whole animal into a big beating heart.

The second nerve net is the “Small Nerve Net.” You can see pictures of it from three different species below. This nerve net I find really fascinating because it coordinates all the non-swimming behaviors. These behaviors are finer-tuned than pulsing, where the jelly has to, for example, move it’s mouth to only one tentacle in order to slurp up prey. Somehow this nerve net helps the jelly figure out where all the different parts of its body are and act accordingly. In other words, it helps one part of the body be aware of, and respond to, the needs of another part.

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Image from the paper “Organization of the ectodermal nervous structures in jellyfish: scyphomedusae.” by Satterlie and Eichinger.

These nervous systems also work together, creating some semblance of behavior and choice. For example, jellyfish do have a flight response–they move away from things that damage them, and they can move towards things that they like—like food. Some box jellies even have courtship and mating behavior.

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Photo by Alexander Semenov

Do these behaviors mean jellyfish are self-aware? They don’t have any type of ‘centralized’ nervous system. As far as anyone knows the two nerve nets and rhopalia do all the heavy lifting as far as ‘thinking’ goes. Ten years ago I would have said that no brain is evidence enough that they’re not really aware. But, of all things, cloud computing has made me rethink my assumption. Do you really need a centralized hub to profess information? What if their whole body works as a kind of brain?

We may never know. It’s difficult to understand the world through another organism, not just because we have to study its biology, but because all we know is what it’s like to be human: our imagination is limited by our own experience. We may never completely understand the world according to a jelly.

Personally, in my own lab, I assume that jellies are aware (if only in a rudimentary way) and treat them accordingly, even if I’ll never be able to prove it.

And while we may never know what it’s like to be a drifter: to float through the ocean with no brain, we can still respect them, and afford them the reverence such alien and wondrous animals are due.


10 thoughts on “How do jellyfish perceive the world?

  1. I really appreciate your last 3 paragraphs! I wish all scientists (and non-scientists) would approach living things this way – with wonder and respect.

  2. I completely agree! These organisms are so advanced and unique, we can only begin to understand them. But, I think you totally nailed the article! Couldn’t have said it better myself.

  3. Thank you for this very interesting post. I’m not a scientist, only an artist…and I have wondered about how jellies perceive the world. Have you read “Other Minds” by Peter Godfrey-Smith? It’s sort of a philosophical musing on consciousness and cephalopods.

      1. I definitely recommend it, even tho it’s isn’t about jellies!
        I found it a very interesting exploration about the possible meanings of consciousness (we don’t really have a clear definition of that yet…or perhaps ever?), and perception of the world around us is part of that.

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