Understand the science of this seasonal treasure
Uncovering the science of seasonal treasures.
SEA URCHIN
A confession: I have a soft spot for the odd creature that inhabited the chalky case illustrated on the right. This is the shell, or test, of a green sea urchin, one of Britain’s most abundant species. In life it contained a coil of gut, a five-toothed feeding apparatus known resplendently as Aristotle’s lantern, and a radiating system of nerves and hydraulic canals. Oh – and gonads. In fact at this time of year, urchins are mostly gonad.
The test bears the five-way symmetry that hallmarks most echinoderms, the spiny-skinned group that also includes starfish and sea cucumbers. Look closer and you’ll see double rows of pores where the hydraulic tube feet protruded, each one ending in a tiny sucker that helps attach the urchin to the seafloor, passes food to the mouth and arranges adornments of gravel, shell and weed as camouflage. Between the pores are white bumps, or tubercles, the ‘ball’ part of ball-and-socket joints where individual spines were attached. Each spine would have been able to swivel, in defence or to wedge the body in a crevice.
If you find a live urchin and look closer still (a hand lens will help, or use your binoculars backwards), you’ll see a third kind of appendage. These are globiferous pedicellariae, as delicate as their name is clumsy. Each tiny pincer-like structure, mounted on a slender stalk, can deliver a minute jab of toxin. You might feel these stings when handling a live urchin, but generally our skin is too thick to be troubled by them.
The test is made up of hundreds of plates, which grow slowly from the edges. Empty tests often break along the joints; if so, take comfort knowing that the former occupant was probably healthy. Loose joints indicate active growth was taking place, while the plates of a starving urchin become welded together.
YOU MIGHT FEEL THESE STINGS, BUT GENERALLY OUR SKIN IS TOO THICK TO BE TROUBLED BY THEM”