When Jelle Atema first arrived in Woods Hole in the 1970s, he had never seen a lobster before. Intrigued by underwater chemical communication, he needed to find a water dweller with a good nose. Once he began studying American lobsters, he never looked back; the crustaceans turned out to be the ideal guide to the mysterious underwater world of odours wafting along on ocean currents', Atema says. Atema and his colleagues soon discovered that lobsters recognise individuals by their smell, but the pathway responsible for this discrimination remained elusive. Now, Atema and Meg Johnson report that chemoreceptors called aesthetascs allow American lobsters to tell individuals apart (p. 2865).

In the murky underwater world, vision can be pretty limited. So it's hardly surprising that male lobsters sniff out other males using the smell of their urine. A lobster needs to know who he's just bumped into, because his behaviour depends on his place in the dominance hierarchy. Put two male lobsters in a tank, and they'll battle it out until a winner emerges. The defeated lobster remembers who clobbered him; as soon as he catches a whiff of the victorious male, he's smart enough to back off and lick his wounds. But place a different victorious male in front of the loser and he'll eagerly pick another fight; lobsters recognise which specific individuals have thrashed them, not just any male that's just won a fight.

Atema and Johnson set out to discover exactly which chemosensory pathway allows a lobster to recognise individual winners. They knew exactly where to look: Atema and his team had already found that lobsters need their first pair of antennae, the antennules, to recognise individuals. The antennules are covered in minute odour-sensitive hairs. Atema suspected that aesthetascs, the most abundant of these chemosensory hairs, enable a puny lobster to remember previous encounters with superior fighters. From other studies, he knew that lobsters perform complex olfactory tasks without their odour-sensitive aesthetascs. So what are the aesthetascs used for then? Surely these numerous sensory hairs have a unique function', Atema reasoned.

To find out if aesthetascs allow lobsters to recognise others, Atema and Johnson decided to see if lobsters still recognise a stronger opponent after their aesthetascs have been shaved off. They set up boxing matches' between pairs of male lobsters, recorded fight duration and scored aggression on a scale ranging from ignoring the competitor to all-out battle with scissoring claws. Then they carefully shaved off the lobsters' aesthetascs and reunited the pairs. Atema and Johnson expected that the lobsters wouldn't be able to recognise each other without their aesthetascs. Sure enough, the defeated lobsters didn't recognise the bully that had just beaten them; the pairs fought just as bitterly in their second round as they had in their first round. Lobsters clearly need their aesthetascs to recognise others. Since aesthetascs are directly linked to lobsters' olfactory lobes, this suggests that lobsters need an olfactory brain to recognise individual odour codes,'Atema says, `which implies that this difficult discrimination task is sufficiently important to develop the brain capacity for it.' Pleased that this piece of the puzzle is finally in place, Atema now faces a new challenge:to figure out exactly what the aesthetascs respond to in lobsters' urine.

Johnson, M. E. and Atema, J. (
2005
). The olfactory pathway for individual recognition in the American lobster Homarus americanus.
J. Exp. Biol.
208
,
2865
-2872.