It's an undignified way to go: ending your days as another animal's dinner. Rather than risk being found by or trying to escape from a crayfish predator,pond snails (Lymnaea stagnalis) take evasive action. They respond to a crayfish's smell by crawling above the waterline to escape their shell-crushing claws. But since most anti-predator responses have been tested in wild-caught snails, researchers didn't know if the snails' predator response is innate or learned. Using snails that have been bred as a lab colony since the 1950s, Ken Lukowiak and his colleagues at the University of Calgary, Canada, investigated whether snails that have never encountered a crayfish still have anti-predator responses().
The team exposed snails to pond water, pond water that had contained crayfish, and boiled crayfish water. Testing two of the snails' vigilance behaviours after exposure, they found that snails exposed to crayfish water righted themselves more quickly when they were placed on their backs. These snails also took longer to emerge from their shells and start exploring their environment. When a shadow passes over a snail's breathing tube, indicating danger, they withdraw the tube into their shell until it is safe to emerge again. The snails exposed to crayfish water were more likely to pull in their breathing tube when exposed to a shadow, indicating that crayfish water made the snails more vigilant to the presence of a predator than pond water or boiled crayfish water.
Because snails escaping crayfish are known to slither above the water's surface, the team wanted to know if exposure to crayfish water changed their breathing behaviour, since snails near the surface rely less on respiring through the skin and more on breathing through their breathing tube. They found that crayfish water caused snails to open their breathing tube more frequently, meaning that they breathed for longer, consistent with how they would behave when a real predator is about.
Having shown the behavioural effects of crayfish water, the team then moved on to the physiological effects. They found that crayfish water didn't increase the heart rate in the snails; `we expected it to go up', Lukowiak says, `it's possible that the snails don't want to expend energy keeping the heart rate up'. Although there was no difference in the overall oxygen consumption in snails exposed to pond water and crayfish water, in the first 8 min of the experiment the crayfish water snails consumed oxygen more slowly than the others in pond water. This suggests that the snails go into an energy saving mode when a predator is nearby.
Finally the team focussed on the nervous system, to see if there was a neuronal anti-predator response. They recorded the electrical activity of a neuron called RPeD1 using microelectrodes. This neuron initiates the rhythmic activity that drives breathing behaviour and is also `absolutely necessary for memory formation', says Lukowiak. In crayfish-water exposed snails,spontaneous firing activity, the number of spike bursts and number of spikes in each burst all went down, showing that predator detection has a direct influence on the neuron. Although the team still have to work out the link between the neuron's activity and the behavioural and physiological changes they saw, `the most important implication is that the snails have maintained a knowledge of who's the predator', Lukowiak says, `there is a genetic memory'.
