Snails move at a surprising speed underwater, much faster than their terrestrial counterparts. This shell was moving much to fast even for a marine snail. A hermit crab, of course. The shell was familiar, I had photographed a snail wearing the same shell a few mere minutes earlier. The hermit crab was using a basket snail shell.
The pimpled basket snail is quite common, finding a couple of them in the course of a night dive is no real surprise. Finding and photographing the same type of shell, with two different occupants is an odd serendipity.
In the course of identifying the hermit crab I was surprised to learn it was an anemone crab. These usually host an anemone or three on their shells. This crab had none. A new shell? No time yet to recruit the usual anemone? It was fairly small for the species, perhaps a young example.