Up close to a venomous fire urchin

Rowan Hooper, news editor

(Image: David Fleetham/naturepl.com)

I CAN'T be the only person who sees this beast - rudely captured as it expels its guts through its mouth - and thinks of a dramatic scene at the end of the Watchmen graphic novel. In it, a giant alien writhes in its death throes in New York City. OK, perhaps I am the only one. In fact, this beauty is the blue-spotted sea urchin (Astropyga radiata), also known as the fire urchin for its painful sting. It was shot by photographer David Fleetham off the coast of Maui, Hawaii.

We see the animal from above, with the five "arms" displaying the radial symmetry common to echinoderms - familiar seaside animals such as starfish and sea cucumbers. Urchins don't use these arms to walk, however. For that, they rely on hundreds of tiny tube feet, which grip the substrate beneath them and pull them along. That substrate coud be the seabed, or, as is common in this species, the back of a crab.

"A good analogy," says biologist Henry Astley of Brown University in Providence, Rhode Island, "is to imagine a giant, building-sized ball - the urchin - with hundreds of people underneath - the tube feet."

Be careful if you get that close: the venom in the constantly waving spines is only the primary defence mechanism. At the bases of the spines, which are articulated by ball-and-socket joints, are tiny, venom-filled pincers. So if you are impaled on the spines, there is more poison to endure.

Plenty of other animals take advantage of this formidable defence - and the urchin is often seen harbouring commensal shrimp, crabs and cardinalfish among its spines. It also forms large aggregations, banding together with other urchins and moving in a giant spiky mass.

"It doesn't make much difference whether the tube feet are each moving independently, like a crowd, or all moving together in step, like marching," says Astley, "there are so many of them that any irregularities are smoothed out."

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