Scents of converging

The tropical robber crab of the Indian and Pacific Oceans represents a remarkable example of evolution. It has become the largest land-living arthropod weighing up to 4 kg and spanning half a metre in diameter. And after a marine larval stage it spends its time on land, and will drown if submerged under water. Sea to land transitions are demanding of many aspects of life, and in a new study reported in this issue of Current Biology, a team of researchers has looked at the species' olfactory abilities. Sensing smells on land requires quite different capabilities than detecting them under water. The robber crab is widely believed to detect food through smell and the researchers found that this was indeed the case, with baits drawing crabs rapidly to them over a large area after sunset. The researchers then looked in detail at the mechanisms used by the crabs to detect smells and found that they were very similar to those also used by insects. It appears evolution has led to convergent solutions for the same problems in quite different organisms in this case of food detection. The researchers also looked at the sensitivity of crabs to different smells. They found them highly sensitive to a chemical typically produced by decaying corpses — the remains of another species of crab is a favoured food source. But their renowned ability to feed occasionally on coconuts scored less well. The chemical we associate with coconuts elicited little excitement in the animals. Weighty: The robber or coconut crab is the world's largest land-based arthropod. Although it prefers carrion, it can tackle other food as large as the coconut, and relies largely on olfaction to find it. New research suggests the crabs have developed an olfaction system similar to insects to serve their purpose. (Nineteenth-century engraving courtesy of Richard Vogt.)