The Urban Pantheist

I have always assumed that the following experience was basically universal to anyone who grew up on a coast: While playing at low tide on the rocky coast, you notice that some of the periwinkles are creeping along on jointed white feet instead of a mucusy mantle. Now I know that I am wrong on several counts: periwinkles are mostly a European animal, occurring by human accident along the North Atlantic beaches. The little crabs inside those periwinkle shells are native to the North Atlantic beaches, but only recently started using the new resource of the alien snail shell.

Hermit crabs have a broad range--they are crustaceans more closely related to lobsters than other animals we call crabs. They lack calcium armor on their abdomens, an adaptation which exchanges bodily resources for a weird sequence of behaviors wherein the animal borrows or steals the empty shell of a snail to protect itself. In the case of our little native hermit crabs Pagurus longicarpus, the introduction of the alien periwinkle could have been an ecological catastrophe--the periwinkles outcompeted native snails, depriving the crabs of a key part of their life cycle. Fortunately for the crabs, they were able to adapt to the new resource, and while the shape of the newly available shell was not ideal, they were able to make use of it. The result is that most P. longicarpus hermit crabs found on New England shores live in the shells of the European Littorina littorea snails.