Most crabs defend themselves with a pair of mighty claws when threatened, but Neohelice granulata crabs scamper for the nearest burrow. However, when Tomas Luppi, from the University of Mar del Plata, Argentina, told his colleague Daniel Tomsic, from the University of Buenos Aires, about an isolated population of these crabs that appeared to be relatively unbothered by predatory birds, Tomsic realised that he may have a chance to challenge a fundamental biological question. Might it be possible that a riskier lifestyle had directly altered the strength of the crab's visual response to danger, and could Tomsic track those changes to individual neurons in the nervous system? In short, could the team show a direct link between the selection pressure of predation on the crabs from the population that was at greater risk and modifications in their nervous systems?
First, Tomsic, Luppi, Fiorella Magani and Jesus Nuñez had to coordinate collecting the crabs from the two locations and returning them to the lab for testing. ‘One beach is 400 km from Buenos Aires and the other is farther south, but we had to collect them and bring them to the lab on the same day’, says Tomsic, who was determined to ensure that the crabs’ experiences were essentially identical. Then Magani and Tomsic placed individual crabs in a tracking device and recorded their reactions to a threatening piece of card moving in a similar way to an attacking gull. The crabs from the high-risk population struggled harder to escape the threat than crabs from the population on the safe beach.
However, when the duo measured the crabs’ reactions to a mild electric shock, both populations reacted to the same extent, and when moving images were projected on the enclosure surrounding the crabs, all of the animals moved in time with the images as they tried to hold their view of the world steady. So the differences in the crabs’ reactions to the simulated gull visual threat appeared to be a result of the reduced risk of predation at the southern beach. And when Luppi and Nuñez returned to the beaches to monitor the antics of the bird population, they could clearly see that the crab population from the northern beach was far more menaced by the gulls than the southern population.
But would the differences in the crabs’ responses to a visual threat correlate with an increase in activity in the motion-sensitive nerves that trigger a response to danger? Gently inserting a microelectrode into the large neurons that trigger an escape response in a crab's brain, Magani and Tomsic recorded the signals in the neuron as they replayed the simulated gull attack; they were delighted to see much stronger signals generated in the neurons of the crabs that live under constant threat than in the crabs from the more peaceful beach. ‘In the low-risk population it [the neural response] was approximately half of the spikes (on average) than we recorded from the animals in the high-risk population’, says Tomsic.
The increased risk of predation had directly affected the nerves that triggered the animal's escape response, and Tomsic is eager to discover whether the two populations are genetically different, or whether the crabs that live in the riskier situation simply learn to react more strongly to visual threats in order to survive.
