The Amazing Christmas Island Red Crab
Every year an amazing event happens on this small island, owned by Australia, which is 220 miles away from the nearest land mass. Christmas Island's geographic isolation and history of limited human disturbance has brought about a high level of species not found elsewhere in the world. Among these species is the Christmas Island Red Crab. Millions of these crabs simultaneously embark on a five kilometer journey to their ocean breeding grounds. Scientists from the University of Bristol and Bangor University believe they have unlocked the mystery to this incredible feat.
The project was led by Professor Steve Morris of the University of Bristol and Professor Simon Webster of Bangor University. They conclude that it is hormones that make the journey possible for the Christmas Island Red Crab. Specifically, it is the Crustacean Hyperglycaemic Hormone that enables them to efficiently use the stored energy in their muscles.
This added energy is essential for this small terrestrial crab to travel five kilometers over land from their home on the high rainforest plateau to their spawning grounds in the ocean. The island is not heavily populated, so there is not much danger from humans, however, the path does cross over several roadways. The estimated 120 million crabs literally blanket the landscape as they travel. They are prompted to commence their journey with the arrival of the monsoon season in November or December.
What is amazing is that during their non-migrating period, the crabs are relatively inactive, staying in their burrows on the forest floor. They only emerge for a short period in the early morning to feed. To go from such a hypoactive state to an incredibly hyperactive state requires some internal tinkering to get the metabolism moving.
Surprisingly, the hyperglycaemic hormone levels were lower during the migrating season than during the inactive dry season. To figure out the puzzle, the scientists gave the crabs glucose (sugars). According to Professor Webster, during the migrating season, the glucose prevented the release of the exercise-dependent hormone. This put the crabs in a negative feedback loop.
Therefore, during migration, the glucose stored in the crabs’ glycogen stores is sufficient to fuel the long journey. This also allows the ability to release the hyperglycaemic hormone which unlocks that stored energy. Then, during the inactive dry season, the hormone shuts down, and the glycogen stores are built up again. Isn't evolution amazing?
Link to published article in the Journal of Experimental Biology: http://jeb.biologists.org/cgi/content/full/213/17/3062