By Mary K. Wicksten - Texas A&M University
September 22, 2015
Hermit crabs are unusual in having a soft posterior region, the abdomen, hidden within a snail shell or other covering. A hermit crab must change shells throughout its life as it grows: that is, unless it is a deepwater hermit crab, family Parapaguridae.
Members of this family have a very strange association with a sea anemone, family Hormathiidae. The crab starts out inhabiting a shell. The sea anemone settles on the shell and then overgrows it, eventually dissolving the shell and forming a covering that expands as the crab grows.
This arrangement is useful for both the crab and the anemone. Experiments on shallow-water crabs indicate that the stinging tentacles of the anemone repel octopods. The anemone, in turn, gets free transportation.