Pilfering crab has insect's nose

The robber crab climbs trees to pinch coconuts Image: Current Biology

A land crab re-invented key features of the insect nose over millions of years - a striking example of convergent evolution, Current Biology reports.

An animal's sense of smell needs to operate under very different conditions in air compared with water.

The crab has achieved this in the same way as the ancestors of insects did.

The robber crab, which is descended from marine crabs, had to develop a new way of smelling things when it moved out of the sea and on to land.

Insects evolved some 438-408 million years ago, from an ancestor that also crawled out of water into an air-filled terrestrial environment.

Aggressive opportunist

Convergent evolution describes the situation when animals that are distantly related - like the robber crab and an insect - can evolve similar adaptations in response to natural selection.

They are strong, aggressive opportunists

Marcus Stensmyr, of the Swedish University of Agricultural Science, and colleagues observe that the crab's insect nose "nicely illustrates how similar selection pressures result in similar adaptation".

Robber crabs (Birgus latro) are the world's largest land-dwelling arthropods, reaching a length of more than half a metre and a weight of 4kg.

Found on islands from the Indian Ocean to the central Pacific, these aggressive, nocturnal scavengers are known for their ability to climb tall palm trees in search of coconuts - which they later crack open with their massive claws.

The crabs are fully adapted to life on land and will actually drown if submerged in water.

Long-distance attraction

The team studied robber crabs from Christmas Island in Australia. Islanders describe the crabs as "highly inquisitive" and say they will steal any smelly objects that are left unattended.

The smelling organs of robber crabs are called aesthetascs. These are mounted on paired antennae, or antennules.

In robber crabs, these smelling organs share many features with insect olfactory organs called sensilia - including a short, blunt shape. The crab's aesthetascs are very different to those of marine crabs.

The researchers laid bait for the crustaceans

The researchers also recorded nerve activity in the crab's antennules in response to different chemical odours, carbon dioxide and water.

The activity was nearly identical to that in insect olfactory nerves as measured using the same technique.

Despite what islanders might say, the crabs do not respond indiscriminately to smelly objects. They were strongly attracted to the chemical odour of decaying meat, just like carrion-loving insects such as blowflies.

By placing baits around the island containing familiar foods such as dead red crabs and coconut flesh, as well as empty control baits, the scientists were able to confirm the crabs' famous ability detect odours at long distances.