Hermit crab migration

On Tuesday I wrote about a crustacean with an unusual larval form. The larvae occur in mid-water while the adults occur in the deep sea. Habitat shifts associated with changes in life history stage are not uncommon and some of them are even more dramatic than changing depth. Take the hermit crabs in the Virgin Islands. The adults are terrestrial, but the larvae develop in the sea. The adults must migrate to the waters edge in order to reproduce and release their eggs into the water.

Hermit Crab Migration from Steve Simonsen on Vimeo.

Hermit crabs are not true crabs. True crabs are in the crustacean infraorder Brachyura, while hermit crabs are in the infraorder Anomura. There are Brachyurans that perform similar mass migrations to release their eggs into the sea. One of the best known is the migration of the Christmas Island crab.

If the eggs and larvae survive out in the ocean, tiny juveniles climb back onto land and migrate into the forests where they grow into adults. In most years, only a very small number will make it back to land. In some rare years, the numbers that arrive on the shore are mind-boggling.