hermit crabs do the twist

After this post about when hermit crabs become, well, hermits and hide their soft abdomens in shells, Sherry asked:

Do hermit crab abdomens always spiral in the same direction?

I'm not gonna lie: I was stumped. The logical person in me thought: not all snail shells spiral in the same direction, so not all hermit crab abdomens spiral in the same direction, right? Shells can spiral to the right (dextral) or to the left (sinistral), and, just to really ruin your day blow your mind, some gastropods will display BOTH spiral directions within the same species!

Sinistral (on the left) and dextral (on the right) shells from

Amphidromus perversus snails (aka super cute land snails)

Then I thought, that may be so, but that doesn't mean that hermit crabs will necessarily inhabit every type of shell ever produced by nature, RIGHT? What I found is that, while hermit crabs prefer dextral gastropod shells, they can and will inhabit sinistral shells. But this seems to be a last-option-for-housing kind of situation, as the occurrence of crabs in the left-spiraling shells is so rare.

to the left, to the left!
everything about me in a shell to the left!

So are the hermit crabs that dwell in sinistral shells curling to the left? NO! Larval hermit crabs have symmetrical abdomens, but as they grow and develop, the abdomen begins to curl. It TURNS out (ha!) that the abdomen curls in a spiral towards the right for hermit crabs! (I'm afraid to say that this is true for all hermit crabs, but so far my quick literature review hasn't told me otherwise.) 

this hermit crab is doing its Tilda Swinton impersonation

I hope that cleared it up for you, Sherry! (Because the shell is clear...) Hey-oh!

Want some light reading?
Chapple, W. D. 1977. Central and peripheral origins of asymmetry in the abdominal motor system of the hermit crab. Annals of the New York Academy of Sciences 299: 43 - 58.

Kosuge, T., and M. Imafuku. 1997. Records of hermit crabs that live in sinistral shells. Crustaceana 70: 380 - 384.