Sea Jelly

The Stingy Biology of Jellies & Sea Combs

Today, things are getting more and more colorful with sea jellies of all shapes and forms, with rainbow-light sea combs, and more. Sea jellies are usually called jellyfish, though they are far from fish, and sea combs are usually called comb jellies, though they aren’t jellies. Oh, the fun naming of taxonomy.

Until very recently, sponges were thought to be the oldest animal species. Currently, scientists think that Ctenophora were potentially first. The tree of life is always changing, so it’ll be interesting to see where this leads. Sea combs, ctenophores, are the pretty rainbow-light things at the aquarium. The ones next to the sea jellies that you always thought were sea jellies. Well, spoiler alert: they are not.

Okay, anyway, what is the current version? That’s not as straightforward as it sounds, but it looks something like this:

Last time, the sponges were the first of the Animalia to branch off. Though, as I said, that’s still up for debate. Anyway, that—for now—leaves us with the so-called Eumetazoa with their four branches: the sea combs, the nettle animals, the bilateral animals, and yet more boring blobs called Placozoa. First, let’s cover the rainbows of color that look so much like jellies, but definitely aren’t.

Ctenophora: The Rainbow-Colored Not-Jellies

Sea combs have eight ribs with cilia that move particles toward the mouth. That’s the rainbow combs themselves. To move those, ctenophores have actual muscle cells. But much unlike the sea jellies, ctenophores don’t sting. They have sticky cells called colloblasts instead. The sea combs used to be lumped together with the nettle animals but as they don’t have nettle celles it is a little ridiculous to lump them into the nettle animals, isn’t it.

One of my favorite things about ctenophores—well, apart from their rainbow colors—is that they are probably the largest animals to use cilia to swim. You might remember that cilia are those tiny eyelash-like whips. It’s as if we started swimming by moving the hair on our bodies. Weird, right? But, I guess, you just need enough of them to move.

And with that, we are adding a layer of complexity to our animals—almost literally.

Diploblasty: Two Germ Layers Means Organization

If you want internal organization in a body, you need real tissues. Sponges have only one germ layer, so they can’t really close anything off. They lack epithelial tissue, the tissue type used to line inner and outer surfaces. Your own skin is an example of an epithelium, the epidermis.

Diploblastic animals have two germ layers. During both evolutionary and embryonic development, you first have a blastula, so one layer with a cavity inside. The cavity of the blastula is called the blastocoel, which is at least in line with the sponge’s cavity being called a spongocoel.

This blastula folds its skin inside to form a second layer with a single opening to the outside. In the simplest sense, we now have a primordial mouth (Urmund) and a primordial digestive tract (Urdarm) formed out of that second germ layer, the entoderm. Entoderm more or less means inner skin. And with all its new fancy features, the blastula has now turned into a gastrula, a diploblastic organism.

We will talk more about bilateral animals later—after all, they are the most interesting to me—but they take this whole thing a little further and their biggest distinguishing feature is a third germ layer, making them triploblasts.

But, for now, let’s return to the diploblastic animals. I want to talk about sea jellies after all.

Cnidarians: Stinging Goo Sacs?

As I also said, the cnidarians are diploblastic, so they only have two germ layers, the endoderm and the entoderm, the inner and the outer “skin.” In between those two skin layers is the mesoglea, jelly-like goo that makes up the actual body of the cnidarians.

Two germ layers already provide a lot more options, as the epithelial cells allow closing off the inner spaces from the outer spaces, something the Porifera can’t do. While cnidarians don’t have real muscles (which would come from the third germ layer, the mesoderm), they have epithelial muscle fibers. It’s essentially just contractile myofilaments, so fibers than can expand or contract.

I know, I know, still not super exciting. Let’s move on.

The distinguishing feature of the Cnidaria is the cnidocytes, the stinging cells. It’s also what gives them their name and also what excludes the ctenophores from this realm. They work kind of like tensioned springs that move different organelles inside the stinging cells. There are organelles for tensioning, for penetrating, for injecting venom, even for sticking to prey or building the tubes of tube anemones. There are more than thirty types, so we won’t go into more detail. The actual discharge takes a few microseconds, but holds quite the punch. Most of them are not dangerous to humans, though, as their cells can’t penetrate our thick skin—and other creatures are immune to them like the clownfish. More on that another time. Symbiosis is ducking cool.

The life cycle of the cnidarians varies a bit with the groups but they generally have three life stages: floating larvae called Planula larvae, sessile polyps, and free-swimming medusae. Medusae are what jellies look like most of the time while polyps are what corals look like most of the time.

There are a couple of groups within the cnidarians, all of which we’ll talk about over the next weeks. There are the anthozoans, the group that includes the corals, the cubozoans, cube jellies, the hydrozoans, small predatory cnidarians, myxozoans, parasites, polypodiozoans, more parasites, staurozoans, the stalked jellies, and the scyphozoans, the true jellies.

We’ll talk about all the jellies today, and about the anthozoans next time. We’ll ignore the parasites for now and leave them for an episode on parasites. I also won’t get into the stalked jellies, as there is just one member and the only special thing about is that it skips the polyp stage. Otherwise, they are just small umbrella-like jellies that float around upside down. Not very exciting. And the only hydrozoans we’ll discuss are the Man o’ War, which are ducking cool. Let’s start with the true jellies.

Scyphozoans: The True Jellies

The true jellies are exclusively marine, so there are no representatives in freshwater environments. The name comes from the Greek name for a kind of cup that they apparently look like. Cool, cool.

They can be pretty tiny with a diameter of 2 cm but also pretty impressive with diameters of up to 2 meters (roughly 6.5 feet). The Lion’s Mane jelly, the Cyanea capillata, According to the BBC, the largest recorded specimen was found in Massachusetts Bay in 1870. The animal had a bell of more than 2 meters and tentacles of almost 40 meters. That’s just impressive.

By W.carter – Own work, CC0,

I’ve gone diving with jellies and their bells were nowhere near even a single meter, so I can’t even imagine what it would be like to see one of those giants. And, as I just found out, they can even be found in the North Sea and the Baltic (though they can’t breed in the Baltic because of the low salinity). Mostly they live in the Arctic, Atlantic, and Pacific, though. There’s a very similar jelly that might actually be the same species around Australia and New Zealand.

Of course, they aren’t this impressive for their entire life. Even giant Lion’s Mane jellies start out very small. They, like all cnidarians, start off as planula larva which settles to the bottom and attaches there. Once attached, they become polyps, kinda like the main life stage of a coral. In this state, they can reproduce asexually by producing smaller polyps in a process called budding. When they no longer feel like doing that, they can pull of a pretty cool magic trick called strobilation. Not all cnidarians can do this, but it’s pretty weird and cool, so naturally, I have to tell you about it.

Strobilation is a way to reproduce asexually, so without a partner, in which the body is sliced into multiple segments. After some preliminary morphing, lines appear on the body. These lines deepen and furrow down to create actual segments of the body. The segments, now called ephyrae, then morph again to discard the secretions needed for strobilation. One ephyra usually stays where it was before and regrows the body while the rest goes their separate ways and develops into proper medusae.

Another example of a true jelly is the very common moon jelly, Aurelia. Moon jelly is also the name used for the type species Aurelia aurita, but as it’s pretty hard to distinguish all the Aurelia without genetic analysis, I prefer the vaguer version that includes all of the Aurelia. They, too, use strobilation for asexual reproduction.

What you can see pretty well in Aurelia is the rhopalia, small sensory structures found in some scyphozoans and cubozoans. If you see a jelly where the bell has indentations on regular intervals, you’re probably looking at the light and gravity sensors of the jelly.

These rhopalia develop during that magic trick of strobulation, which means that they probably have to use different senses before. We don’t know enough, as so often. But, as those sensor cells don’t exist in the planula larval stage or the polyp stage, I think it’s reasonable to assume they don’t completely survive without senses before.


All I knew about hydrozoans was that one particular example, the freshwater polyp Hydra, is very annoying once accidentally introduced to an aquarium. They are small polyp things that are way too good at reproducing, which they do both asexually through budding, but also in sexual reproduction. The weird thing is that they never send the medusa out on their own, but keep them as reduced medusoids instead. That explains why I never saw any swimming around my aquarium. Hmm.

Anyway, I thought hydrozoans were all as boring as the hydra. But, once I looked into them, I found out they have at least one more interesting member: the Man o’ War. And that one’s definitely pretty cool!

As I said before, the life stages of the cnidaria differ between the different kinds. This is especially true for the hydrozoans. Some hydrozoans such as the Hydra don’t have a medusoid stage, so they stay as polyps, while some others like the Liriope don’t have the polyp stage. The hyrdozoans also include the only freshwater jelly, the Craspedacusta. The peach blossom jellyfish, Craspedacusta sowerbii, originally was only found in the Yangtze basin of China, but is now an invasive species found throughout the world. They, like me, like it warm, but they, very unlike me, like to live in large colonies. Their life stages are pretty much those of the true jellies, so planula larvae, polyp, medusa.

The Man O’War are essentially living sailboats. If you’ve never seen one, they look like a balloony elongated body with a sail on top floating on the ocean surface, and incredibly long tentacles below.

But the thing is, they aren’t one animal. Each specimen is a colony of multiple organisms called zooids working together. They are always a combination of a medusa and a few polyps. The inflatable body is used to stay afloat, so as a buoyancy device, while the sail on top (also inflatable) catches the wind and propels the man o’ war forward. Below, they sport pretty impressive tentacles that can reach up to 47 meters, which they use to eat a hundred or more small fish each day. There’s a cool BBC video about them on Youtube with David Attenborough. 

The different species of Man O’ War have since been united into a single species thanks to genetic analysis: Physalis physalis. It can be found essentially all over the world.

They carry a pretty heavy punch and their nettle cells can actually harm—and kill—a human. Most people never see them in the water, but instead find them washed up on the beach. If you find these pretty blue things on the beach, take your fingers off them. Their toxic nematocysts (that’s the organelles inside the cnidocytes responsible for injecting the toxin) can stay toxic for months if the conditions are right. I can’t find the supposed source of my teacher’s slide, but if he’s right then you’d need 0.00002 grams of the Portuguese Man O’ War’s toxin to kill a human weighing as little as I do. And it’s a pretty versatile toxin that attacks the nervous system, the heart, the lungs, as well as leading to an allergic immune response and being corrosive. Yeah, don’t touch them.

The Physalis aren’t the only siphonophores, but definitely the best-known ones. As WHOI said so nicely, they are all more than the sum of their parts. Each zooid wouldn’t be able to deal on their own—or at least not as well—but together they can do a lot.

The Cubozoa: Cube-Shaped Jellies with Sometimes Deadly Toxins

One of the places I really want to visit is the Great Barrier Reef. I’d love to see the Australian North-East. When I tell people, they always tell me how scared they are of all the things that can kill you there. What most of them don’t even think about is deadly things in the water.

Chironex fleckeri, the Australian box jelly, is also called the sea wasp. Sea wasps, like other jellies, like to hunt small fishes and invertebrates like crabs or prawns. Their life cycle is essentially the same as for the true jellies, so they have a planula larval stage, a polyp stage, and a medusa stage. But what’s so interesting about C. Fleckeri is that they can kill a human in three minutes. Granted, that’s a pretty extreme case, but their stinging cells, which cover each of their roughly sixty three-meter tentacles, are pretty potent. Fun fact: they are named after a dude called Hugo Flecker, boring, but the rest of their name, Chironex, means essentially hand murderer.

When you learn about box jellies, this is usually the only one mentioned, but I am much more fascinated by the common kingslayer. The Malo kingi was named kingslayer after killing a dude named Robert King. King swam off Port Douglas and got killed by its sting—the second recorded fatality of these little monsters. Usually, they only induce Irukandji syndrome which will land you in the hospital with excruciating pain, and some other fun symptoms like vomiting and muscle cramps. In the serious cases, they lead to heart failure or fluid in the lungs. Seriously, for something that doesn’t get larger than a human fingernail, they are pretty dangerous.

Another cool feature of the box jellies is that they are more developed than other jellies. They have actual true eyes, as opposed to the light-sensing receptors in true jellies. While the rhopalia of the true jellies are simple receptors, the box jellies have a pair of lenses in each rhopalium. In addition, box jellies have twenty ocelli, which can’t see but detect light and dark. So, technically, that means that these jellies have 24 eyes—though most of them are not really eyes. But either way, it gives the box jellies visual guidance, which means they can swim faster and avoid obstacles—to kill you faster?

Anyway, that’s all for today. We’ll get into the anthozoans in one of the next episodes. As you might know, part of this series is actually already published, and this is one of the early episodes that I had to re-record as it was audio-only before. At this point, there are only the episodes on corals and bilateral animals left to redo, so we’re getting there. I already know I want to get into more depths on some of the episodes, but we’ll do that over time.



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