Crabs know their friends and foes

Researchers, led by The Vision Centre in Australia, have found that despite their simple compound eyes, crabs quickly learn to recognise if an approaching creature is a threat, a mate or or a harmless passer-by according to its direction of approach, `Journal of Experimental Biology` reported.

"Fiddler crabs have extremely poor sight, with no depth perception and no ability to see in detail. In a situation where every `blob` that moves in the environment can be a threat, they have to strike a balance between succumbing to paranoia - and ending up as bird feed. Crabs achieve this through a process called habituation where they learn from repeated events to differentiate threats from harmless objects. Humans too use habituation - for instance we learn to ignore the sound from an air conditioner once we grow accustomed to it. We found that crabs have a very selective and finely tuned habituation response – instead of relying solely on the physical appearance of an object, they associate the object with its past behaviour in their living environment, such as its direction of approach," said Chloe Raderschall, who led the research, said.

In the study, the researchers used dummy predators to approach groups of fiddler crabs from two different compass directions. They did two dozen runs of a dummy approaching from direction A without attacking the crabs, and within five runs, the crabs started to ignore it. When they switched to another dummy coming from direction B, the crabs were scared witless and headed straight to their burrows.

When the researchers switched back to direction A, they found the crabs did not attempt to escape, indicating that they clearly distinguish between the dummies approaching from the two directions.

"As both dummies were identical and there was no difference in the timing of their movements, we conclude that the crabs used the direction of approach to determine whether an approaching object was a threat or not," Raderschall said.

She explains that this finding confirms that crabs have an extremely specific habituation response.