Wednesday 21 March 2012

Brittle Star

Image: Moorea Biocode
Brittle Star? Or a close-up of a star?
I'd long thought that Brittle Stars were basically really thin starfish. Turns out I wasn't doing them justice at all. And it's not just because they're even thinner than I thought.

Brittle Stars are composed of a central disc from which 5 long arms sprawl out. These snaking limbs give them the name ophiuroid, which means Serpent Star.

There are going on 2,000 members of the class Ophiuroidea and they're found throughout the ocean, from the Arctic to the Antarctic and from rock pools to the deepest ocean floors.

Image: neptunecanada
Starfish, sea urchin and lots of Brittle Stars
enjoy the sun dark, sea and sand mud.
There are actually huge numbers of them in the deep sea. Pretty much every picture of the deep ocean floor features at least a few of them.

Their success in this grim environment is partly because a lot of Brittle Stars are detritivores, so they eat bits of food in the abyssal mud. The miles and miles of abyssal mud.

Others are predators of small creatures or suspension feeders, where they use mucus on their tube feet to capture plankton from the current. They might even mix it all up a bit for a healthy, balanced diet.

The tube feet in Brittle Stars are a little different from the ones that starfish have in that they don't end in little suckers. Brittle Stars don't use them for getting around at all, and many will only use them for respiration and sensing their surroundings. Some, especially the suspension feeders, will use them for passing food on toward the mouth...

Image: Simon Coppard
A Bristle Star's underside showing its 5 jaws.
Do you think he's smiling?
Which is probably not a Brittle Star's most attractive feature. It's a 5-fold symmetrical mouth! Surrounded by 5 jaws! That's just weird.

To make matters worse, there could be a little bit of stomach sticking out of it. Yeh, Brittle Stars can do that nasty thing of pushing their stomach out through their mouth if food is too big to go the other way. If Muhammed won't go to the mountain, the mountain must go to Muhammed.

This is all quite a lot like starfish, but one big difference is that Brittle Stars have no anus. It means that everything has to be ejected through their mouth, no doubt leading to a complex, psychological relationship with food. Or it would if they had a head and a brain.

Image: Bill Bouton via Flickr
I imagine that part of the reason for this simple, "in one end, out the same end" digestive system is a simple lack of space. For most Brittle Stars, the entirety of their viscera is contained in that tiny central disc. Starfish have internal organs extending into their arms, Brittle Stars cram it all into that small space. It's different to starfish, but just like Crinoids.

And, like Crinoids, those arms are scarcely more than skin and bone. Echinoderms have a strange, internal skeleton made of calcium carbonate plates called ossicles. Brittle Stars have huge ossicles in their arms connected together with ball-and-socket joints for excellent flexibility. It's like they have 5 backbones. Quite a coup for an invertebrate!

As the arms squirm about, spines sticking out from the sides give them traction. Brittle Stars can get around with surprising speed by slithering or dragging themselves along in a rowing motion. I have this strange image of people rowing in rowing boats across the water's surface and Brittle Stars doing the exact same thing across the water's bottom. It's odd to think, but these must be some of the fastest moving of all echinoderms. Especially when they have all 5 of their limbs...

Image: Moorea Biocode
'Tis but a scratch!
You ever had back problems? Brittle Stars get back problems. And, in a way, they have 5 backs. Their problems tend to be a little different from ours, like a fish nibbling on the end of one of them.

Brittle Stars are rather quick to break off an arm, hence the name "Brittle". They soon regenerate, though. It's not so easy for us humans, so you shouldn't consider hacking bits of yourself off as a first port of call.

And so, by hiding among rocks, living in environments few others can and taking advantage of a casual "on again, off again" relationship with their limbs, the Brittle Star survives and thrives.

Most Brittle Stars have males and females, although a few are hermaphrodite. Some have a free-swimming larval stage but many brood their young within their body and give birth to miniature Brittle Stars.

There are some oddities that have 6 arms and can split in two. Each side regrows half the central disc and the remaining 3 arms. Again, try not to split people in half without asking your doctor first. Results may vary.


Crunchy said...

It's the beast with five backs (four if someone's been nibbling)!

Joseph JG said...

Almost the most terrifying thing in the world, ever!


TexWisGirl said...

makes me very grateful i have a one-way digestive tract (at least most times!)

Joseph JG said...

I know what you mean! A place for everything and everything in its place.

Emily said...

The mouth picture is amazing-- its jaws are beautiful and intricate, but rather disgusting at the same time.

Joseph JG said...

Yes, I don't know whether to be impressed or repulsed!

Drhoz said...

they've also been known to catch larger fish, by raising their disc off the bottom, waiting for a fish to swim underneath, then twisting around to close the trap

Joseph JG said...

Wow, really? I hadn't heard that! It's amazing what echinoderms are able to without having a brain. They can teach jellyfish a thing or two!

Unknown said...

Gorgonocephals are best brittle stars :

Joseph JG said...

They sure are: