Notes on the Foraging Strategies of the Giant Robber Crab (Anomala) on Christmas Island: Evidence for Active Predation on Red Crabs (Brachyura).

Christmas Island, Indian Ocean, currently supports one of the largest populations of the giant terrestrial robber crab, (Crustacea, Anomala, Coenobitidae), the largest land living arthropod. Robber crabs are considered opportunistic omnivores feeding on a diversity of plants as well as animal derived foods. Previous reports indicate that the is primarily an omnivorous scavenger; however, there is some anecdotal evidence suggesting that the species may also hunt actively. Between 2010 and 2012 during three field trips to Christmas Island, we observed and documented active predatory behavior of on the endemic Christmas Island red crab, (Brachyura, Gecarcinidae). Our observations suggest that does actively hunt, and exhibits at least two distinct predatory strategies. Large robber crabs are able to actively overwhelm and kill red crabs, while smaller individuals are likely to provoke red crabs to autotomize limbs on which to feed. These findings may indicate a much tighter predator-prey relationship between robber crabs and red crabs than considered before.