Sea Urchins are almost 1,000 species of echinoderm that can be found from the intertidal zone all the way down to the deepest, darkest depths of the sea. Their main claim to fame is their spines. It sort of has to be, what with them looking like nothing other than a ball of spines. Or, to be a little more charitable, a permanently rolled-up hedgehog. That's why they're called urchins, after the Middle English word for 'hedgehog'. Their famous spikiness also gave rise to the name of the entire phylum. Echinos is Greek for hedgehog, so echinoderm means 'hedgehog skin.'
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Book titles: they don't make 'em like they used to.
That man was Carl Linnaeus, and the book was big enough and important enough to earn its ludicrous title.
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Edible Sea Urchins are quite common throughout northern Europe, which would be why they're also known as Common Sea Urchins, and they reach about 16 cm (6 in) across. Linnaeus, a Swede, probably knew them well. I guess that's why they're first in his list of Echinus. On page 663, as it happens.
As Sea Urchins go, the Edible variety is your bog-standard, no frills model. It's a bit like a morbidly obese starfish. We've seen some chubby starfish in our time, but the Edible Sea Urchin is so chubby that the arms are completely gone and all that's left is a roughly spherical body.
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A naked Sea Urchin
The shell is called the test. It can be divided into five parts because it's an echinoderm, and five-fold symmetry is just one of those weird echinoderm things. Each of those parts can be further divided into two rows of so-called ambularcal plates and two rows of inter-ambulacral plates.
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If you're accustomed to the stubby, little tube feet of starfish, it can be pretty weird seeing Sea Urchin feet waving around in the water like Medusa having an angry hair day. You needn't worry, though. They don't bite and they're full of nothing more dangerous than water. Sea Urchins have a little pore at the top of their shell which they use to filter seawater into the system so they can pump those feet up.
Edible Sea Urchins can be found from the coast all the way down to depths of 1,200 m (3,900 ft). This versatility is due in part to the fact that they can eat pretty much anything that can't get away. Of course, most things can get away; a lot of Sea Urchins are so sluggish that even sea slugs leave them in their wake. Luckily, in the rocky areas that Edible Sea Urchins live in, there are many encrusting creatures that live directly attached to the rock. Sea Urchins can scour the place clean of deliciously stationary algae, barnacles, bryozoans and tunicates.
You need some tough mouth parts if you're going to scrub rocks with them. You also need some tough gloves if your going to pick up a Sea Urchin and turn it upside down. If you do that, you'll see a very round mouth conveniently situated in the centre of the Sea Urchin's underside. You'll also see yet more evidence of the echinodermic passion for the number five. Inside that mouth are exactly five, calcium carbonate teeth. They're arranged in a circle, like some kind of five-piece pincer, or maybe two and half tweezers.
Oh, and what goes in must come out, right? Well, a Sea Urchin's anus is situated in what must surely be the worse possible place. It's in the middle, like the mouth, but it's in the middle of the top side. It takes pride of place at the very apex of the Sea Urchin's body so that when it uses the bathroom, the stuff kind of pours out all over them. Yikes! This cannonball gets worse and worse!
Video: Fish Species
The teeth aren't the only things that go snip. Being such slow-moving animals, Sea Urchins need to be careful not to get mistaken for a rock by the very encrusting creatures they eat. To that end, they're covered in tiny pincers-on-stalks called pedicellariae. With these things they can nip those algae in the bud and barnacles and bryozoans in the butt.
And then there are the spines. Sea Urchins tend of have rather a lot of spines. The Edible one is covered all over in short secondary spines, while a collection of larger primary spines are found on every second or third ambulatory area. They're not necessarily razor sharp but they are, shall we say, dissuasive. They can also be waggled. Yes, I said waggled. It's not the word Carl Linnaeus would use, but Sea Urchins have quite a lot of control over those spines, getting them to point this way and that. It's not like you can sneak up on a Sea Urchin, but still.
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A male releasing sperm. Look how excited he is!
A large female can spawn up to 20 million eggs! The tiny larvae drift about as plankton for some 2 months, during which time almost all of them get eaten. They'll never be as edible as that again, and the survivors soon settle down to adult life.
Sea Urchins really value their gonads. Can you guess how many of them they have? Yup, it's five. Each with their own pipe leading up to a hole in the test. Unfortunately, humans value those gonads, too. They consider them ever so... esculentus.
Wow. And I thought Sea Urchins would eat pretty much anything!