StarfishAsteroidea

A sea star eats by turning its stomach inside out.

It has no jaws and no teeth, so it takes the stomach to the food instead. The mouth is in the middle of the underside; the stomach pushes out through it, spreads over whatever is being eaten, digests it where it lies and is then hauled back in with the meal already dissolved. Against a clam this is a siege. Hundreds of tube feet grip both shells and pull in relays, no single foot working long enough to tire, until the muscle holding the shell shut gives up a fraction of a millimeter, which is all the room the stomach needs.

Losing an arm costs one less than you would think. Most species regrow a lost arm from the stump, and in some, a detached arm regrows everything else instead: for a while the animal is one long arm with four stubby ones behind it, a stage divers call a comet. A few species skip the accident entirely and simply split themselves in half to reproduce.

Everything in that toolkit is also what makes the crown-of-thorns dangerous. It is a sea star that has specialized in living coral, and it feeds the ordinary way, laying its stomach over a colony overnight and moving on at dawn, leaving a clean white patch of bare skeleton. One of them on a reef is a normal animal, part of the system. The problem is that their numbers periodically explode, and an aggregation of them can strip a reef faster than the coral can rebuild. The long venomous spines mean nothing much eats them.

The rest of the family here is quieter. Cushion stars swollen into pentagonal loaves with the arms nearly gone, sand stars that bury themselves and hunt buried molluscs, small red and orange species picking over the reef surface for the film of algae and debris on the rock.