Crinoid

Image via Wikipedia

Crinoid. Attack of the Crinoids. The Intergalactic Crinoid Empire. It sounds like the baddies in a computer game where you destroy an entire civilization of barbaric, technologically advanced, Earth threatening aliens using nothing but a single, rather small space ship, all to the soundtrack of toe-tapping techno which makes you nod your head but with a down-turned mouth because you're enjoying it in a serious and urban way.

If you don't like the Latin name for the entire class, or you just like a bit of variety in your nouns, then you can go for the more wistful and whimsical common names. There are 2 kinds of Crinoid: the ones that have a stalk are called Sea Lilies, the ones without are called Feather Stars. Thoroughly delightful names if you ask me! But what exactly is a Crinoid?

Firstly, it's an echinoderm, related to starfish and the like. In fact, it's rather like an upside down starfish, so its mouth faces upwards to the wide open sea rather than down at the mud and rocks and stuff. This mouth is at the top of a little cup called the calyx, and it's here where all the digestive and reproductive organs are. It's actually really, really tiny. It seems that Crinoids are comprised almost entirely of food gathering organs, which brings us to what we might call the "arms and legs".

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From the calyx, the beautiful Lily and Feather bits spread out and look all lovely. They are somewhat like starfish arms but each one usually splits into 2 parts so that there are 10 rather than 5 of them. Each one will often branch out further, giving up to 200 branches. All along these branches are little... twigs, I suppose, called pinnacles, which is what makes them look so delicate and feather-like. The real point of all this is to passively gather plankton from the water current. Crinoids still have those little tube feet running all along the arms and pinnacles. These are covered in a sticky mucus so that, as they wave their tube feet in the air like they just don't care, crumbs of food are caught before they are flicked down into the deep groove from which the tube feet extend. Cilia in this groove can then make a current to pass the food down into the mouth. Once there, it does almost nothing but go up and down; Crinoids have a U-shaped gut and no stomach, so food goes from mouth to throat to intestine and then into a rectum which is hopefully powerful enough to shoot that stuff out of there quick time. A Crinoids mouth is right next to its anus, a system that other echinoderms understandably shun.

In Sea Lilies all this occurs at the top of a stalk which ends in a sucker to provide a strong grip on a surface. In deep sea species these can be up to a metre long, but they are usually much shorter. Some go a step further and have a kind of root structure to really dig their nails in. A lot of them can still move around about half a metre per hour but... well, I suppose that's quite normal for an echinoderm isn't it.

One particularly vigorous Sea Lily was seen dragging itself along the sea floor in 2005. It was probably the creepiest, crawliest scene mankind has ever witnessed. In any case, it looks like at least some of them can really get up and go if they want to, but then take a look at the Feather Stars! Without that stalk it seems that Crinoids are positively unleashed! These guys can be seen swimming along with a paddling motion of their arms. It seems like all those branches provide them with enough surface area to get right up off the floor and see whole new sights... if only they had eyes. It's quite haphazard and graceless, it looks like it takes a huge amount of energy to do not very much but still, for an echinoderm it's extremely impressive.

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With the glorious adventure that comes with travel it's no surprise that most modern Crinoids are of the stalkless Feather Star variety. The fact remains that most Crinoids have a stalk for at least a portion of their life, it's just that some eventually break away while others never do. Before that, they were larvae. At this point they are little barrel shaped things with rings of cilia and tufts of sensory hairs. It is these that eventually stick to the ground and metamorphose into an adult with a stalk. Before that? An egg of course! Male and female Crinoids can simply release their gametes into the sea and leave them to it.

Image via Wikipedia

But what came first, the Crinoid or the egg? Probably the Crinoid. Crinoids are extraordinarily ancient. The very first ones came along in the Ordovician period 490 to 445 million years ago. They were extremely successful and, as the first plants were starting to make it on land. a huge diversity of Crinoids were spreading across the seas. Loads of them died out in mass extinctions, almost all of them, but a few hardy souls have managed to survive to watch with horror or inscrutable acceptance as fish, shrimp and the like rose up to eat all their food.

Their glory years may be long gone but I'm glad to see the world still has a place for the Crinoid. And who knows? Maybe that Crinoid inter-planetary invasion fleet is on its way even as we speak...