What are sea urchins?

A confession: I have a soft spot for the sea urchin, an echinoderm in the class Echinoidea. 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.

Dr Amy-Jane Beer6

What are sea urchin spines made of?

As with many structural biological materials in marine environments – those of coral reefs and mollusc shells, for example – sea urchin spines are composed of calcium carbonate. In its basic state, this is a rather brittle mineral – think chalk – but evolution has come up with a variety of ways to strengthen it.

When found in nacre (mother of pearl), it is laid down as multiple layers of thin sheets, an arrangement that prevents fractures from spreading.

When found in sea urchin spines, microscopic blocks of the mineral are cemented together.

This design is inspiring human engineers to develop super-tough, concrete-like materials that, in theory at least, could support structures eight kilometres high – ten times the height of the world’s tallest building.

Stuart Blackman

What do sea urchins eat?

Sea urchins eat algae and marine mollusks

Main image © Frédéric Ducarme, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons