Even if you're armed with a fierce pair of pincers, life can be risky. There's always going to be a larger crab that might decide to dine on you. So, crabs tend to give the odour produced by injured crabs a wide berth, treating it as a warning that a larger predator may be lurking nearby. ‘The environment is filled with threats,’ says Marc Weissburg from the Georgia Institute of Technology, USA, although it is also full of the scents of enticing tasty treats. ‘Unless you are immune to eating, then you are always going to have to confront the conflict between aversive and attractive odours’, Weissburg says. Intrigued by how crabs successfully negotiate this conflict, Weissburg and a team of undergraduate student researchers set out to discover which course blue crabs (Callinectes sapidus) steer when presented with a whiff of lunch spiced with danger (p. 4175).
Collecting crabs from the ocean off Savannah, Georgia, Weissburg returned with them to the lab where he and Kimberly Berkenkamp started to test their reactions to plumes of different odours in a flow tank. Sure enough, when the animals were downstream of luscious shrimp aroma they practically galloped toward the origin. However, when the water smelled of injured crab, the animals became evasive and some even buried themselves to avoid the stench.
But, how would they react when the two odours were flowing in parallel in close proximity? This time, the crabs were much more cautious. Although they continued to pursue the attractive scent, the crabs actively avoided the side of the tunnel that smelled of injured crab. Discriminating between the plumes and following the attractive odour to its source, the crabs were able to successfully home in on the food despite the close proximity of the warning signal.
However, when the duo disrupted the flow by introducing a cylinder into the water, to generate turbulence and mix the plumes, Weissburg says, ‘The animals were no longer willing to track to the attractive source and they reacted as if there was only an aversive source’. Thoroughly mixing the two odours had altered the crab's perception of the tasty prawn aroma sufficiently for it to no longer appear attractive. Weissburg decided to find out which of their many olfactory organs the crabs use to distinguish between the attractive and aversive odours.
‘In blue crabs, the control over orientation toward an attractive substance is split between the antennules, which are the small structures between the eyes on the head, and chemosensors on the tips of the walking legs’, explains Weissburg. So, together with Lauren Atkins and Danielle Mankin, he tested the animals' reactions to the merged plume when one or other of the ‘nostrils’ were effectively plugged.
Desensitizing the scent receptors on a crab's legs by immersing them in fresh water, Weissburg then released the animal into the blended plume: the animal tried to take cover. However, after gently removing the scent receptors from another crabs' antennules and positioning it in the plume mixture, the animal bounded toward the source of the attractive aroma of food as if it was uncontaminated. The crab was no longer repelled by the aversive odour. ‘We had knocked out the input [from the antennules] that suppresses navigation. The behaviour of the animals to track the attractive cue in the presence of the aversive cue was rescued’, explains Weissburg.