Some snails are "lefthanded" in the sense that the shell spirals the opposite direction to what they normally do. I've recently learnt that these snails are extremely rare. What is more, a "lefthanded" snail can only mate with another lefthanded snail making them even rarer.
On a recent holiday I saw some hermit crabs and it got me thinking, as they only have one big claw with which they guard the entrance, do you have lefthanded hermit crabs and would they require "lefthanded" snail shells?