http://vimeo.com/48466065#
The above is a video making the rounds on the internets showing a massive mating event in Concordia, St Johns, US Virgin Islands.  Despite being land crabs, they still require seawater to mate and lay eggs, so these huge events take place at least once a year and usually last several days to weeks.  A similar process occurs every November on Christmas Island, where the Red Crabs blanket the entire island twice as they first cross the island to mate, and then their newborns cross back over the island two weeks later.  If you grew up in the USA, you may have had one of these cool animals as a pet, so seeing them in such numbers is an incredible sight!  Thanks to Pam Gaffin for recording this great video.