Sunday, 22 January 2017

Edible Sea Urchin

Image: gordon.milligan
Echinus esculentus
It's the hedgehog of the sea!

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.'

Image: François Xavier TESTU
Next time you visit your past-life regression hypnotherapist, ask her to take you back to 1758. The Seven Years Wars is really hotting up; the French have just settled in Buffalo Creek; before the year is out, a light will appear in the sky and be stunned to find itself henceforth owned by some guy called Edmond Halley; and a man in his early 50's will see the publication of the 10th edition of his book (deep breath), Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis.

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.

Image: Bengt Littorin
That brings us to Echinus esculentus, the Edible Sea Urchin. And yes, esculentus means things like edible, nourishing and delicious. The name is something of a kick in the teeth. It's true, but the whole point of all those spines is to hide that fact. And then here comes Mr. Linnaeus not only drawing attention to it, but practically starting an advertising campaign extolling the virtues of Echinus meat. I bet hedgehogs were pretty annoyed, too.

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.

Image: Daniel P. B. Smith
A naked Sea Urchin
Anyone hoping to make a meal of a Sea Urchin, especially one that was clearly advertised as edible, would likely be disappointed at first. It doesn't look much like a meal. It looks more like an innovation in anti-personnel cannonball technology, being nothing more than a hard, calcium carbonate shell covered in spines. If you think that it serves as an effective test of a predator's ingenuity and nut-cracking skills then I have to say, you're exactly right...

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.

Image: Natural England
The word 'ambulacral' comes from the same Latin word from which we get 'amble,' which means to stroll at a leisurely pace, and 'ambulance,' which is something that does quite the opposite. Sea Urchins are very much on the 'leisurely pace' end of the scale. The ambulacral plates are full of little holes to let the tube feet through, enabling the Sea Urchin to very slowly crawl around.

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.


Video: fcmovie

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.

Image: gordon.milligan
The visible teeth are just the tip of an intricate structure called Aristotle's lantern. It's absolutely made for chomping, capable of being raised up and down so that the Sea Urchin can open wide and really dig those teeth in. It can also move side to side to tear chunks out of their food.

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.

Image: Bernard Picton
A male releasing sperm. Look how excited he is!
If all goes according to plan, then our Edible Sea Urchin will grow up big and strong. Or at least round and spiky. Now they get down to the business of making new, tiny Edible Sea Urchins. And boy are they tiny! When it comes to mating, Sea Urchins aren't the least bit vigorous. They simply pump their sperm or their eggs into the sea and let them get on with it.

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!
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