Commensal Sponge Shrimp

A sponge is a pump, and this shrimp has moved into it.

The animal filters continuously, drawing water in through thousands of tiny pores and driving it out through the large openings on top, and a shrimp sitting in that internal cavity is inside a permanent current of clean, oxygenated, food-carrying water it does not have to generate. The walls are structural. The entrances are too small for anything that eats shrimp. The rent is nothing.

It is such a good arrangement that sponges on a reef are full of tenants: shrimp, brittle stars, worms, small fish, sometimes hundreds of individuals in one sponge, and in one branch of the snapping shrimp the arrangement went further than anywhere else in the sea and produced true colonies with a single breeding queen, like an insect hive.

This species is a small translucent shrimp with fine banding, usually several to a sponge, and it has been recorded from shallow reef down past four hundred meters, which the sponge makes possible: inside one, the outside world matters much less.