Migrating Christmas Island red crabs congregate on a beach in the Australian territory of Christmas Island (map) in an undated picture.
A new study solves a longstanding mystery surrounding the crabs: how the the normally sedentary species has the stamina to "undergo one of the most arduous migrations on Earth," in the words of study co-author Lucy Turner.
(See an interactive red crab and exclusive video of the red crab migration from the Great Migrations TV miniseries premiering on the National Geographic Channel in November.)
When the wet season blows into Christmas Island (map) each year, millions of Christmas Island red crabs hike for several days, from a high rain forest plateau down to Indian Ocean beaches, where the crabs mate in burrows.
"It's an amazing feat—going from not being able to exercise for more than ten minutes to walking for several miles," said Turner, a biologist at the University of Bristol in the United Kingdom.
But by sampling circulatory fluid—the equivalent of blood—from migrating crabs, Turner and colleagues discovered that a surge in the crustacean hyperglycemic hormone works with glucose, an energy-producing sugar, to fuel the epic trek.
(See "World's Longest Migration Found—Two Times Longer Than Thought.")
The crab's endocrine system also regulates the ability of the crabs to access glucose from their energy stores, Turner added.
Study published in the September issue of the Journal of Experimental Biology.