Giant Robber Crabs Tracked Using GPS

Researchers have tracked the world's largest living arthropod on Christmas Island, south of Indonesia, using the global positioning system.

Arthropods are invertebrate animals that have an external skeleton. They include insects, crustaceans, spiders, wasps and arachnids. At least 55 huge robber crabs were tracked between 2008 and 2011 using the GPS technology in order to monitor their behavior and lengthy migration on the island.

Researchers recorded more than 1,500 crab days of activity. Some individual crabs were tracked for as long as three months. Robber crabs are the largest land-living arthropods and have a lifespan of up to 60 years. These crabs can weigh around nine pounds.

The research team found that the robber crabs have a leg span of up to 3.3 feet (1 meter). The crabs were found to take shelter in holes in dead wood, between tree roots, and hide in rock crevices during daytime, suggesting that the animals are active mostly during the night.

Based on observations, researchers noticed that the crabs are not sessile animals (fixed in one place) and are mostly in motion. They cover long-distances between the coast and the inland rainforest. Previous studies have suggested that female crabs alone migrated to the coast for breeding after copulating in inland forest.

However, the GPS tracking system has revealed that the males also move towards the coast during wet season and stay there for a relatively short period of time. The males might migrate possibly for mating, drinking saltwater and foraging for food, the researchers wrote in the report. The coastal movement was  common among males that were tracked during the wet season than during the dry season. 

Experts also noticed long-distance homing behavior in land crabs. For their study, researchers carried the crabs in a bag up to 0.6 miles from their home. When the crabs were released along the migratory route, they were able to find their way back home, showing evidence that the crabs have the ability to return home over large distances. However, when the crabs were released outside their migratory path, they became lost and never returned home, a report in OurAmazingPlanet said.

The findings of the study, "Giant Robber Crabs Monitored from Space: GPS-Based Telemetric Studies on Christmas Island (Indian Ocean)," are published in the journal PLOS ONE.