A stranger in his own home

Bilaterality is an important evolutionary principle, but hermit crabs seem to have abandoned the concept, at least partially. Hermit crabs are asymmetric with a curved, soft abdomen that fits nicely into the empty shells they inhabit. This asymmetry extends even to the neuromuscular system, although the first-stage free-swimming larvae, known as zoea, are symmetrical. Hermit crabs cannot excrete their own shells, so adults usually seek the abandoned shells of gastropods to inhabit. Although the asymmetric soft abdominal parts need protection, this muscular system has evolved to act as a hydrostatic support for each new shell. The abdomen has become decalcified to fit a range of shells, and the abdominal musculature can support and “grasp” the shell in its spirals. As the availability of inhabitable shells may be limited, a housing crisis often leads to pugilism among crabs seeking to acquire a new shell. Fights are fierce and may lead to the death of a crab over a new shell. Once a new shell has been selected, the crab will rather quickly exchange its old home for a …