Cnidaria: Pretty Stingy Things (Ep. 60)

Today, things are getting more and more colorful with sea jellies of all shapes and forms, with rainbow-light sea combs, and more.

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.

The tree of life is definitely a tree I wouldn’t want to climb around on. The branches change as often as the staircases of Hogwarts. In our lectures, we get taught one thing, but the scientific world already agrees on another version. Or, more often than not, argues about another version.

Porifera were long thought to be the oldest animal species, but it’s a little less certain, and it might also be the ctenophores. Our teacher calls Porifera the simplest animals, but that could also be the placozoans, flat multicellular blobs. I vote for the blobs as the simplest animals. And what does simple even mean?

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

The Messy Taxonomy of the Ctenophores and Cnidaria

We’ve got the kingdom Animalia, the animals. Then the ctenophores, sea combs, and the Porifera, sponges. Most people will still tell you the Porifera were the first to branch off, but genetic analysis says it’s probably the ctenophores. Some people would even move the ctenophores one up and move them out of the Animalia into their own group. For now, they are treated as sister branches.

(Image credit: Photo by Jonathan Diemel on Unsplash)

The part of this branch that gets more complex is the clade of the ParaHoxozoa. If you are going through the taxonomy mnemonic now (Dear King Phillip Came Over For Good Soup), you might realize that there is no clade to be found. Yeah, I know. It sucks. But that simple division into Kingdom, Phylum, Class, Order, Family, Genus, and Species just isn’t detailed enough to deal with every level, so people have been adding sublevels. Clade is a very generic term for one of those sublevels. You might also see things like Subkingdom (e.g. the eumetazoans). The branch that includes us and all animals, except for the ctenophores and Porifera, is currently commonly referred to as the clade of the ParaHoxozoa. Yep, catchy, I know…

Despite the ridiculous name, I really want to move down that branch, so let’s cover those ominous sea combs quickly.

Ctenophora: The Rainbow-Colored Not-Jellies

Sea combs have eight rips 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. But what’s way cooler is that they are probably the largest animals to use cilia to swim.

(Image credit: Photo by Naja Bertolt Jensen on Unsplash)

Okay, so we’ve got the Animalia; the ctenophores and the Porifera have branched off, and we are on that weirdly labeled clade of the ParaHoxozoa.

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 promised you corals and sea jellies, after all.

The ParaHoxozoans—seriously, who decided on a capital H here?—are divided into the nettle animals, the Cnidaria, and the bilateral animals. The Cnidaria used to be grouped up with the sea combs, but that makes little sense, as we already established.

Cnidarians: Stinging Goo Sacs—but seriously, some are pretty cool

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, the jelly-like goo that makes up the actual body of the cnidaria.

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 cnidaria 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 that 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.

Cnidocytes are tensioned cells with one large organelle inside called a cnidocyst. The cnidocyst can deliver a sting (and a toxin) to other organisms. The actual discharge takes a few microseconds, but holds quite the punch.

There are over 30 types of cnidocysts, which can roughly be divided into three groups: penetrant cnidocysts (stenoteles), which pierce the skin or exoskeleton of the prey and inject a venomous fluid; glutinant cnidocysts (ptychocysts) that stick to prey but are also used to create the tubes of some cnidarians such as the burrowing tube anemones; and the volvent cnidocysts (desmonemes) which are kinda like a tensioned spring. When discharged, it coils around the prey in a lasso-like string.

There are three basic forms of cnidarians: floating larvae, sessile polyps, or free-swimming medusae. Medusae are what jellies look like most of the time, while polyps are what corals look like most of the time. So, in general, the female and male gametocytes fuse to create a fertilized egg, a zygote, which then develops into the planula larvae. What happens from there, depends a lot on the type of cnidaria.

So, now that we have way-too-thoroughly established what Cnidaria have in common, let’s finally talk about the animals.

Types of Cnidaria: From Boring to Pretty Cool

The Cnidarians are divided into categories, but again there is a lot of flux here. I’ll go with the current classification as provided by the World Register of Marine Species, WORMS.

There are the anthozoans, the corals, the cubozoans, the cube jellies, the hydrozoans, small predatory cnidarians, myxozoans, obligate parasitic cnidarians, polypodiozoans, a specific type of parasitic cnidarians sometimes included in the myxozoans, the scyphozoans, the true jellies, and the staurozoans, the stalked jellies.

Scyphozoans: The True Jellie

Let’s start with the so-called true jellies, the scyphozoans. 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.

Image Credit: 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.

Image credit: By © Hans Hillewaert, CC BY-SA 4.0,

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.

Hydrozoans: Small Predatory Stingers—and then there’s the Portuguese Man O’ War

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 more interesting members, like the Portuguese man o’ war. And that one’s definitely pretty cool! More on them later.

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.

Craspedacusta: Freshwater Jellies

The only freshwater jelly, the Craspedacusta, is a hydrozoan. 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.

Obelia: Sea Furs

Then there’s the sea furs, the Obelia, which grow like branched fur on rocks and other organisms like kelp. It’s all over the photos I took while diving in the Pacific. Looking through those, I think it’s everywhere.

Image credit: By Ernst Haeckel - Upscaled from using AI program and edited to remove artifacts from original, CC BY-SA 4.0,

Physalia: The Man O’ War—a Living Sailboat

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.

Image Credit: By Image courtesy of Islands in the Sea 2002, NOAA/OER. - U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Public Domain,

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.

Anyway, there’s multiple species of man o’ war, some of which may be the same species, so let’s not get into that. The most common one is the Physalis physalis, so the Portuguese Man O’ War, which can be found in the Atlantic and Indopacific. There’s a very similar smaller version, the Pysalia utriculus, so the Pacific Man O’ War, and a few other local representatives 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 a little heavier than the tiny person I am. 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 Physalia 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.

Most siphonophores live in the deep sea, so we don’t encounter them on a regular basis, but all of them are colonies of specialized organisms working together to be more than they would be alone.

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 3-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 in Greek/Latin.

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 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.

Okay, I’ve talked for about 3,000 words now, I’ll stop talking here and move my favorite Cnidaria, the corals into their own episode, so you don’t fall asleep here.

We’ve learned that ctenophores, sea combs, are not sea jellies, that there are a few pretty cool species in ever group of the Cnidaria. We’ve also learned that there are some that are pretty deadly, and that all Cnidaria have nettle cells (thus making them Cnidaria). That’s not to say that all jellies are dangerous to humans. Only a small portion of the Cnidaria is capable of getting through our thick skins, so don’t freak out every time you see a jelly or get close to a coral.

And the sea combs just look fucking cool with their rainbow lights. No stinging cells, just a bit of sticky stuff.

Next time, we’ll give the corals a closer look, as they deserve more than an end note in a 3,000-word essay.

There’s so much to tell you about the natural world. We’ve barely reached organisms with tissues, and I already want to squeeze in episodes about the environmental impact of and on these species, but we’ll do that in another series when this one is over.


I study Marine Ecology at the University of Hamburg, so a lot of this knowledge comes from hours of research and sitting through lecture after lecture.

Going through the lecture slides from school is a process that involves a shit-ton of fact-checking, as a lot of what we learn is pretty outdated. So, all semester, I google things to death, read papers and essays, ask a million questions, and discuss things with friends and classmates.

Where the source isn’t our lecture slides or unidentifiable sources from hours of late-night knowledge hunts, I have linked them in the text.

Kate Hildenbrand

Kate Hildenbrand

Kate Hildenbrand is the writer behind the essays here, author of fiction novels, the creator of the Kate Hildenbrand podcast, and a student of marine ecology. At least, that's her on the surface.