Brittle starOphiuroidea

A sea star crawls. A brittle star runs.

They look related because they are, but the machinery is different. In a brittle star the disk is the whole animal, holding the gut and the gonads, and the five arms attached to it are pure locomotion: a stack of ossicles joined like vertebrae, ball and socket, with muscle blocks between them. The tube feet along the arms have no suckers and do almost none of the walking. The animal shoves with two arms, pulls with the others, and covers ground faster than anything else in the phylum, which is worth waiting for if you have only ever seen a sea star move.

The name is honest. The arms break at preset fracture planes, and an animal handled roughly comes apart in your fingers, so they are best watched rather than picked up.

Most of them spend daylight jammed under a rock or buried in a sponge with a couple of arm tips out, tasting the water. At night they emerge. On the 🌊 Red Sea reef flats one species lines up along the wave wash at low tide and sweeps the surface film for whatever floats in on it, arms hooked over the edge in the foam. The basket stars are the other extreme: a tight ball wedged in a gorgonian all day, unrolling after dark into a branching mesh of thousands of tips held across the current like a net.

Then there is the skeleton, which appears to be part of the sensory system. Every plate is a single crystal of calcite, and in some species the plates covering the arms carry thousands of microscopic domes sitting directly over nerve bundles. Researchers still argue about how much focusing those domes actually do, but the behavior is not in doubt: an animal with no eyes anywhere on it will pick out a dark shape and move into cover.