Why hermit crabs migrate?

Yes, out of the water to a giant orgy on the beach. No kidding, they start in the morning, and storm the nearest beach, usually a full moon in August/September.

Once the giant mass is assembled, they shed their shells and fornicate through the moonlit night. As the sun comes up, it resembles a scene from New Orleans, everyone is scrambling to pick up a new shell and heads back out to sea.

Great Video of one of these migrations

http://vimeo.com/48466065