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

Abstract

Jakob Krieger, Michelle M. Drew, Bill S. Hansson, and Steffen Harzsch (2016) Christmas Island, Indian Ocean, currently supports one of the largest populations of the giant terrestrial robber crab, Birgus latro (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 B. latro 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 B. latro on the endemic Christmas Island red crab, Gecarcoidea natalis (Brachyura, Gecarcinidae). Our observations suggest that B. latro 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.