Wednesday 5 October 2011

Sea Anemone

Image: Rosario Beach Marine Laboratory
Sea Anemones are named after the flowers due to their wonderful, vibrant colours. The fact that one is a predatory, stinging animal and the other is mainly just pretty serves as no bar to this whimsy.

One wonders why exactly Sea Anemones come in such a variety of colours. Perhaps they saw a load of jelly and thought "hey, I can do that."

Of course, they're not really made of gelatin, but they are cnidarians, which means they're relatives of jellyfish. Jellyfish aren't made of gelatin either. And they're not fish.

Sea Anemones DO come from the sea, though. So at least we got something right!

Sea Anemones are sessile polyps. Sessile means they're stuck to the floor, perhaps on sand or rocks, using what's called a 'basal disc' to keep a sticky foothold. Polyps are the cnidarians that are cylinder shaped with the tentacles facing upward. The other option is the medusa, being bell or umbrella shaped with tentacles facing down just like the jellyfish. A lot of cnidarians go from one to the other in their lifecycle, but Sea Anemones are purists and stick with being polyps.

Image: Wikipedia
Sea Anemones are most numerous in shallow, tropical reefs but there are some in colder waters, the deep sea and all the way up to intertidal waters. Their diameter can be as tiny as just 4 mm (0.16 in) or as much as 1.8 metres (5.9 feet), with a few dozen to several hundred tentacles.

You mustn't forget those tentacles! As is customary for cnidarians they are covered in nematocytes, also known as cnidocytes. These fire a tiny harpoon at anything that touches them, getting into the skin and injecting toxins. They all seem to be harmless to humans, giving nothing more than a sticky sensation.

It's a whole other story for prey, of course. Small Sea Anemones will eat tiny planktonic creatures, bigger ones will eat fish and crustaceans. The toxins paralyse their food so they can't get away. I mean, you wouldn't want your smoked salmon running off your plate now, would you? Then again, I prefer my meals to actually be dead rather than just motionless...

Next, the tentacles are used to manoeuvre prey into the mouth, situated on the upper surface and handily surrounded by those same tentacles. Once inside, digestive enzymes are released and anything that can't be eaten is simply spat out the same way it went in. Personally, I just try not to eat indigestible things.

Retracting at low tide
Image: Malcolm Storey
Many Sea Anemones can retract their tentacles if necessary, like if the tide goes out. Then they really do look like a lump of gelatin. It's a bit disgusting, really. A Sea Anemone without the sea certainly isn't an anemone, it's an eyesore.

If times get REALLY bad and the Sea Anemone just wants to get away, they swim! They can detach themselves from their surface and swim off using their tentacles or bending their body left and right. It looks like a lot of work, so it's probably kept for really urgent circumstances.

With toxic tentacles and the peculiar ability to swim, it's no wonder that some algae see Sea Anemones as a sanctuary. Many Sea Anemones keep this plant life in various parts of their body, receiving oxygen and nutrients as by-products of photosynthesis. It's like having a garden with apples and potatoes, except it's inside their body. Which... is an odd thought.

Sea Anemone budding
Image: Wikipedia,
Sea Anemones have a few options for reproduction. Some can clone themselves but they even options here, too; one can split into 2 separate individuals; bits of the basal disc can break off and grow; or they can go through budding, where a youngster starts to grow on the adult and is released when big enough.

They can also give good, old sexual reproduction a try, whereby they release eggs and sperm via the mouth (it's their only option, I'm afraid). Some species have males and females, others start out female and then become hermaphrodite.

It's remarkable how many animals find this male-female thing an unnecessary contrivance. Why choose? Try 'em both out, or be both at the same time. Whatever. Fun!


Shannon said...

How big of a fish can a sea anemone eat?

Joseph JG said...

As far as I can see they only eat quite small ones.

It's not like with snakes, where some of them eat things that barely fit and you can watch a deer or something going down its throat and making a huge lump in its belly.

Sea Anemones have a stretchy mouth so you might be surprised at how wide it can get, but they don't eat anything so large that they bulge with so much food in their stomach.