Just as we brush our teeth after a meal, spiny lobsters carefully clean their antennae after eating by pulling them through little brush-like pads on their mouthparts. Lobsters begin grooming when their antennae detect monosodium glutamate, the notorious flavour enhancer in Chinese food. To investigate the neural underpinnings of grooming behaviour, Manfred Schmidt of Georgia State University took a closer look at the sensory hairs on the lobsters' antennae to find out which of the crustaceans' neural pathways is fired up by glutamate (p. 233).
Schmidt explains that lobsters only groom their small first pair of antennae, which they move swiftly up and down to sense small molecules like glutamate in the water. The first antennae are covered with a variety of microscopic hairs containing sensory neurons that connect to two distinct pathways in the lobster brain. The sensory neurons in aesthetascs, the most abundant of the sensory hairs, are linked to the crustaceans' olfactory pathway. Associated with the aesthetascs are a much smaller number of shorter hairs called asymmetric setae. The sensory neurons in these hairs are linked to the lateral antennular neuropil (LAN), a part of the crustaceans' brain containing motor neurons that control movement of the first antennae. A recent study came to the conclusion that aesthetascs are responsible for grooming,implying that the olfactory pathway mediates grooming. But Schmidt suspected that the non-olfactory LAN pathway was a much more likely candidate to mediate grooming than the olfactory pathway. `Lobsters need to move their first pair of antennae to their mouthparts to groom them' explains Schmidt, `Since the LAN pathway connects directly to the first antennae motor neurons, the animals have a direct link to first antennae movement through the LAN that they don't have with the olfactory pathway. It is a much simpler explanation, and simpler explanations are usually right.'
To find out whether the olfactory or the LAN pathway mediates grooming,Schmidt decided to see if the lobsters still groomed after he removed their aesthetascs and asymmetric setae in turn. If aesthetascs are responsible for grooming, lobsters with amputated aesthetascs should not groom. Schmidt carefully shaved off lobsters' aesthetascs, added glutamate to the lobsters'aquaria to trigger grooming and waited to see what would happen. The lobsters still happily groomed their antennae, showing that they didn't need aesthetascs for normal grooming. But were asymmetric setae responsible for grooming? Schmidt shaved off the lobsters' asymmetric setae, and admits that he was nervous waiting for the lobsters to recover. But he didn't need to worry - when he wafted glutamate past their antennae, the lobsters didn't groom at all. Schmidt was delighted with this clear indication that the asymmetric setae were necessary for grooming. `It was amazing, the animals just sat there doing nothing' Schmidt recalls, `I didn't expect to find such a clear lack of response.'
Unravelling crustacean behaviour patterns that are elicited by chemical stimuli is challenging because often several neural pathways are involved.`Grooming is unique because it is mediated by a single neural pathway,'Schmidt concludes, adding: `It's satisfying to be able to tie this behaviour down to the non-olfactory LAN pathway.'