Snapping shrimp

A stout shrimp, mostly under 5 cm, with one claw enormously enlarged and the other small and ordinary. Colour is no guide: species in this genus run from plain translucent grey through banded orange and white to solid maroon, and several may share one stretch of sand. The enlarged asymmetric claw separates an alpheid from every other shrimp on a muck site, and in Alpheus the eyes are roofed over by the carapace and cannot be seen from above, which distinguishes the genus from the sponge-dwelling Synalpheus, whose eyes are exposed. Beyond that the entry is pitched at the genus deliberately. Separating the species needs the proportions of the claw, the teeth on the rostrum and often the animal in the hand, so a photograph of a shrimp at a burrow entrance is normally identifiable no further than Alpheus.

Many species here live with a goby. The shrimp digs and continually repairs a burrow in sand, silt or rubble, hauling spoil to the entrance and pushing it clear in loads, while the goby lies at the mouth of the hole and watches; the shrimp keeps one antenna resting on the fish and reads its movements, and a flick of the goby’s tail sends both underground. Pairings are not random — particular shrimp species are found with particular gobies — but the shrimp is almost always the harder half to identify. It eats detritus, algae and small animals turned up in the digging. Other species in the genus live in rubble, sponges and coral heads with no partner at all, and those are heard far more often than seen.

The snap is made by the large claw, which is cocked open and released so fast that the water jet leaving it drops below its vapour pressure and forms a cavitation bubble; the collapse of that bubble makes the sound, and produces a brief flash of light too faint to see. It is used to stun prey and to settle disputes with other shrimps. The continuous crackle audible on almost any tropical dive is these animals, several per square metre of rubble, and it is loud enough to interfere with sonar.