Cnidarians: What are they?

Everyone (I hope…) has heard of jellies (jellyfish…but they aren’t fish), corals, sea anemones, and Portuguese man-o-war’s, but do you really know what they are?

Here are some things you probably didn’t know about them:

For starters, they are all cnidarians. The word, “cnidarian” (the “c” is silent), literally means “nettle-like” (from roots knide and aria). They get this name from the stinging cells on their tentacles. These cells are called cnidocytes (also nematocytes), and they contain organelles called nematocysts. The nematocyst is a bulb that contains several things:

  1. Toxins: these are what makes jelly stings HURT. (…and can sometimes kill you, depending on how big you are and how much of the toxin is injected)
  2. Barbs: these pierce skin and prevent nematocyst from detaching.
  3. A cnidocil: this is the trigger that causes the nematocysts to shoot out and pierce the “skin” of the target.
  4. Thread: this is coiled around the nematocyst when it’s still in the cnidocyte. When fired, the thread detaches from the nematocyst and can entangle small prey and keep away predators, as it also usually barbed and contains toxins.
  5. Operculum: the cap that seals an unused cnidocyte.
Hydra eating Daphnea

Photo I took of Hydra eating a Daphnia

So, how exactly do you get stung?

If you get stung, it is because you bumped up against the cnidocyte-covered tentacles and triggered the cnidocils to activate nematocyst-firing. Here’s what happens:

  1. A tentacle with cnidocytes (stinging cells) touches your skin and is triggered to fire by the cnidocil.
  2. Once triggered, the operculum (cap) bursts open and water rushes in, causing the nematocyst (the organelle with the toxins) to shoot out extremely fast.
  3. The nematocyst pierces your skin with barbs (and latches on) and injects toxins into your body; this is what hurts.
  4. After a nematocyst is activated and fired, it cannot be reactivated and fired again.

Note: Nematocyst-firing is one of the fastest processes in the animal kingdom, and the nematocysts shoot out with an acceleration of up to 40,000 g’s. (that’s 40,000 times the acceleration of something falling due to gravity…really freaking fast)

           Also, this whole process is a mechanical process, which means the animal has no control over it. {It can’t decide to not sting you, even if it really, really, really likes you…if it bumps into a rock, the nematocysts will fire.}

What do you do if you’re stung?

Really, you should just find some pain-relief while your body takes care of the toxins, because home remedies don’t “deactivate” or “neutralize” them. (exception: vinegar may help with one species (Portuguese man-o-war), but exacerbates for the majority of species)

If you were stung a lot, or by a box jelly, you should go to the hospital. There might be too much toxin (or too potent of toxin) for your body to handle.


Portuguese man-o-wha?


Most of us have heard of and/or seen Portuguese man-o-wars, and most of us say, “Jellyfish!”.

Well, it’s not a jellyfish. In fact, “it” isn’t even one animal, it’s a colony of hydrozoan polyps (order Siphonophora), and is more closely related to Hydra than true jellies.

The Portuguese man-o-war polyps are highly specialized, and there are three different types:

  1. Gastrozooids: feeding polyps-they capture and digest prey, then distribute the nutrients to other polyps via the connected gastrovascular canals.
  2. Gonozooids: reproductive polyps (can reproduce both sexually and asexually)
  3. Dactylozooids: stinging polyps-they don’t do any feeding, they just sting

The “float” or “sail”, which makes the colony look like a jelly, is actually a modified polyp filled with gas. It keeps the colony afloat and allows the colony to be carried along by the wind.

Note: Since the Portuguese man-o-war is a colony, if parts of the stinging polyps are separated from the rest of the colony, THEY CAN STILL STING, because they are still alive. So, don’t touch them. However, after a few weeks without water, the nematocysts lose their ability to fire.


Why don’t clown fish get stung by sea anemones?

anemone clown fish

Clown fish (or anemonefish) and anemones seem to be on friendly terms with each other, and they are; they have a mutualistic relationship. (both animals benefit from the relationship)

  • The anemone gets more food from the clown fish creating water circulation, and the clown fish’s presence itself attracts would-be predators that the anemone can snack on.
  • The clown fish also gets food, as it snacks on food particles floating around the anemone. In addition, protection from predators is provided by the stinging tentacles of the anemone.

But, why don’t they get stung?

Well, actually, they do get stung, but they have a thick mucous coating that prevents the stinging cells from penetrating their skin…the anemone also has a thick layer of mucous to prevent from being stung itself.

How do clown fish acquire this mucous?

It is believed that the clown fish both produce the protective mucous, as well as get some from the anemone. In order to produce more mucous, the clown fish brushes against the tentacles, as shown in Finding Nemo. Every once in a while, they get a little sting, and this causes their bodies to produce more mucous.

However, this relationship is very specific. For example, if a sea anemone dies, the clown fish associated with it will need to build up resistance to another anemone, as the toxins may be different. If they don’t, the anemone will kill them like any other fish.

Fun fact: Sea anemones and corals are in the same class, Anthozoa.



Pink anemonefish, Fiji

Anemone and anemonefish

Sea anemones (order Actinaria) are solitary polyps that are mutualistic with clown fish, and they can reproduce sexually or asexually. Asexual reproduction happens via pedal laceration:

As the anemone creeps along, it receives small cuts on its base (aboral side). Sometimes the cuts are expanded until the animal is now in two pieces.                                                                                                                                                                                             Now you have two anemones.

Corals (order Scleractina) are colonial polyps that produce calcareous (calcium carbonate) skeletons, and are mutualistic with types of single-celled algae.

So when you go to a shop that sells “coral”, you can tell them it’s technically a coral skeleton.

…I wouldn’t, though.


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