Insect-like olfactory adaptations in the terrestrial giant robber crab.

The robber crab (Birgus latro), also known as the coconut crab, is the world's largest land-living arthropod, with a weight reaching 4 kg and a length of over half a meter. Apart from the marine larval stage, this crab is fully terrestrial, and will actually drown if submerged in water. A transition from sea to land raises dramatically new demands on the sensory equipment of an animal. In olfaction, the stimulus changes from hydrophilic molecules in aqueous solution to mainly hydrophobic in the gaseous phase. The olfactory system of land crabs thus represents an excellent opportunity for investigating the effects of the transition from sea to land. Have land crabs come to the same solutions as other terrestrial animals, or is their olfactory sense characterized by unique innovations? Here, we show that the robber crab has evolved an olfactory sense with a high degree of resemblance to the insect system. The similarities extend to physiological, behavioral, and morphological characters. The insect nose of the robber crab is a striking example of convergent evolution and nicely illustrates how similar selection pressures result in similar adaptation.