Hormone responsible for Christmas Island Red Crab's dramatic migration identified

One of the most spectacular migrations on earth is that of the Christmas Island red crab, Gecarcoidea natalis. Every year at the start of the monsoon season, in late November or early December, tens of millions of the crabs (which are endemic to this island) simultaneously undergo a breeding migration, travelling several kilometres through the rain forest of Christmas Island, a tiny island in the Indian Ocean nearby Java, to eventually reach the sea where they mate and spawn.

Major funding from the Natural Environment Research Council enabled Bangor and Bristol Universities to conduct a three year research project investigating the mechanics of this dramatic migration which represents a drastic 'lifestyle change' for the crabs.

Now reaching a conclusion, the project has yielded fascinating insights into the role of a hormone in the crab's annual migration.

Scientists at Bangor University selected the red crab in order to study a hormone about which little is known, and which they suspected was responsible for the dramatic switch in the crab's behaviour.

"The migration is extremely energetically demanding, since the crabs must walk several kilometres over a few days. During the non-migratory period, the crabs are relatively inactive and stay in their burrows on the floor of the rain forest, only emerging for a brief period at dawn, to feed. The behaviour change reflects a fundamental change in the metabolic status of the animal," explains endocrinologist, Professor Simon Webster, of Bangor University.

The scientists have proved that the hormone, crustacean hyperglycaemic hormone, is central to the migration. It has now been established that it is of critical importance in enabling the crabs to make the most efficient use of their stored energy in the muscles (glycogen) and its conversion to glucose to fuel the migration.

Work by Professor Simon Webster and researcher Mrinalini at the University's School of Biological Sciences involved firstly developing very sensitive immunochemical methods for measuring the vanishingly small levels (just a few hundreds of thousand hormone molecules) of hyperglycaemic hormone in the blood of the crabs. This method was 100 times more sensitive than previous methods and required removing the equivalent of droplets of blood from the crabs for testing.

Professor Webster explains: "We then travelled to Christmas Island to work in the rainforest, in both dry and wet seasons, taking blood samples from crabs during day and night-time, whilst getting eaten alive by mosquitoes, and monitoring the crabs' behaviour in their natural environment.

"Somewhat surprisingly we found that hyperglycaemic hormone levels were lower in actively migrating crabs than those which were inactive during the dry season. This puzzling paradox was resolved when we persuaded crabs to run and walk after injecting them with glucose. During the dry season, forced activity resulted in a tremendous release of hormone, within two minutes, irrespective of whether glucose had been injected. However in the wet season, injection of glucose completely prevented exercise dependent hormone release- showing that they were controlled by a negative feedback loop. Glucose levels were clearly regulating hormone release at this time. This makes sense since it ensures that during migration, glucose is only released from glycogen stores when glucose levels are low, thus eking out the crabs' precious reserves of glycogen, to ensure that they can complete the migration."

"Our experiments were also important to invertebrate neuroendocrinologists since they showed the tremendous dynamism of hormone release processes -- hyperglycaemic hormone is rapidly degraded in the blood in just a few minutes, thus release of hormone must occur at precisely controlled times, otherwise it would be useless as a chemical messenger!"
Photo and video credits:Mrinalini.
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Publication date: 17 August 2010