Squat lobstersGalatheidae

Find a feather star on a night dive, look into the middle of it, and there is usually something living in there with a pair of arms longer than the rest of it. It is a squat lobster, which is not a lobster at all: it belongs with the hermit crabs and the porcelain crabs, and the tail it is named for is not something it swims with: it is folded flat forward under the body and carried there, out of the way, which is why the animal looks like a crab that has been stretched.

The first pair of legs is most of the animal. The claws run several times the length of the body and are held out in front like a pair of dividers, used for reach, for sorting through debris and for arguments with other squat lobsters rather than for killing anything. At the other end of the row the fifth pair has shrunk to almost nothing and is kept tucked inside the shell, where its only job is combing silt off the gills.

Because nothing here is a hunter. Squat lobsters feed by straining: fine bristles on the mouthparts and legs comb particles out of the current and wipe them into the mouth. The species you meet on this coast has taken the obvious shortcut and moved onto an animal that is already fishing. A feather star spends the night with its arms open across the current, trapping plankton and walking it down grooves toward its mouth, and the squat lobster sits among those arms, matched to their color and banding so closely that individuals on differently marked hosts look like different species, taking its share off the conveyor.

The reef is a sideline for this group. Squat lobsters are mostly a deep-water animal, and on cold-water coral and seamount rock they pack the bottom in numbers, with new species still coming up unnamed. One deep-sea relative, the yeti crab of hydrothermal vents, farms bacteria on the hairs of its arms and eats them. Here you get one common species, and you find it by looking at crinoid arms rather than at the reef.