Cnidaria Part 2: Sea Anemones

So, it has been a while, but the craziness of exams has died down now, so, back to posting!

Last time was an overview of Cnidaria, and this post will be about the members of this phylum (group) which are sessile (stationary).

The animals we will cover are in the class Anthozoa, which is from the Greek Anthos (flower), and zoa (animals).  Before I got into biology, I, like many people, thought of corals, or sea anemones as a type of plant, and so it is easy to see where the class name comes from.

Within this class, I find corals particularly interesting, and so will cover those over several posts in the days to come, but this post is about Sea Anemones, and I hope I can show you that they are also extremely cool!

In common with other Cnidaria, Sea Anemones contain cells known as cnidocytes, which are cells that produce nematocysts,which are a bit like miniature harpoons, which fire threads containing toxins, this is what gives jellyfish their sting, however, in Sea Anemones, the threads are unable to penetrate human skin, so the result is merely a tickling sensation. The video  linked below shows the mechanism of these cells firing (it states jellyfish, but it is the same for all Cnidarians.


Sea Anemone, Image from  image from  Check out the site for more awesome pictures!

Ok, so, you can see how they look a bit plant-like, especially in this image below:

Another sea anemone, this one definitely looks more plant-like. Image from

So, how do these animals work,where do they live, and what do they eat?

Well, they are immobile in their adult form, and they attach themselves to rocks, shells, or pretty much anything they can grab a hold of.  They do not have the medusa stage that was mentioned in part 1 of Cnidaria, but instead, the fertilized eggs develop into larvae, as shown below

Sea Anemone larvae, from

The larva shown above has a mouth (the brown section in the centre), and so is likely a planktonic larval form, which means that it feeds on plankton whilst drifting through the ocean.  There are larva from sea anemones which feed on the yolk from the egg whilst developing, and these are known as lecithotrophic larva (from the greek for yolk lecitho and trophic meaning food or feeding). These differences in how the larva feed have an effect on how far they can travel before they mature.  If they are planktonic larva, they can travel further, as food is always available, whereas lecithotrophic larva are limited in how far they can travel by how much yolk is available from the egg sac.

These larva may spend weeks travelling through the ocean whilst they develop into juvenile anemones.  This form has no tentacles, and instead has cilia (fine hairs) around it, which may be used for movement, or for extracting plankton from the water for feeding.  Once the juvenile anemone settles onto a suitable surface, it develops into the adult form.

Although I said earlier that these are non-moving animals once settled, some species do in fact have the ability to move a small distance by using the muscles within their foot (pedal disc) to detach themselves from the rock which they are attached to, and “walk” a short distance to a more suitable location.

Now, how do the adults actually look?

Sea anemone structure. Image from University of Hawaii

Don’t worry about all the scary sounding words in there, they are actually quite straight forwards (Us sciencey types like to use long Latin or Greek sounding words for things to make it look complicated!).  If we start at the top, the oral disc is simply a flat bit at the top of the anemone, which has the tentacles attached to it.  This leads into the pharynx, which is the top end of the stomach cavity.  As these animals have a simple digestive system, they use the pharynx for both taking in nutrients in the form of food, and for excreting waste products (yes, technically, they poop out of their mouth).

The perforations at the top are for allowing water to circulate better through the pharynx, allowing for easier transport both in and out.  On the blown up section to the top right, the pharynx is the hole in the centre of the cut-away section, and the siphonoglyph labelled at the right of it is a groove covered in fine hairs, this aids the movement of water into the pharynx.

Nearer the base, is the gonad.  Sea Anemones are hermaphrodites (having both male and female organs), so they produce sperm and eggs, which are then fertilised by other anemones.

The video below shows sea anemones feeding, which I find completely fascinating, and utterly gross at the same time!

Next post will be about the  feeding mechanisms and symbiosis some animals have formed with anemones!


2 thoughts on “Cnidaria Part 2: Sea Anemones

  1. Pingback: More Sea Anemones « skepticalsquirrel

  2. Pingback: Recap « skepticalsquirrel

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s