Fear can do all sorts of things to memory. A traumatic experience can disrupt memory formation, while a brush with a predator can make a memory stick for days, weeks and even years for others. Ken Lukowiak from the University of Calgary is fascinated by the mechanics of memory formation, but rather than untangle the complex networks that hold our own memories, Lukowiak has focused on the circuit that regulates how Lymnaea stagnalis pond snails remember to breathe. Lukowiak explains that Dutch pond snails' memories improve significantly after a fearful encounter: a sniff of their predator,the crayfish, makes a trained snail remember not to breathe air for days rather than just hours. But how would the snails' memories respond to a predator that they had never experienced before? Would their memories improve after smelling a predator from another part of the world, such as the tiger salamander that preys on Canadian pond snails? Lukowiak and his student Michael Orr decided to test Canadian and Dutch snails' abilities to form memories after a sniff of their own, and each others', predators(p. 2237).
So how do snails' memories work? Lukowiak explains that the snails usually breathe through their skins. However, when oxygen levels are low they supplement their oxygen supply by breathing air through their breathing tubes(pneumostomes); `It's like they are yawning,' he says. Lukowiak uses this simple behaviour to test the snails' memories. He explains that the snails can learn to keep their pneumostomes closed when the water's oxygen is low(hypoxic). By gently tapping the molluscs whenever they open their pneumostomes, Lukowiak trains the snails to keep their pneumostomes closed,even when they are out of breath. Returning the snail to hypoxic water several hours later, Lukowiak tests whether the snail remembers to keep its pneumostome closed, or whether the mollusc has lost the memory and pops its pneumostome up for a gulp of air.
Collecting wild Dutch snails from their native polders and wild Canadian snails from an isolated pond on his neighbour's farm, Lukowiak teamed up with Karla Hittel to test the snails' memories. They gave the snails a half hour long sniff of their own predator's `scent' (Dutch snails were placed in water taken from a crayfish's tank, while the Canadian snails were placed in water from a salamander's tank) before training the molluscs to keep their pneumostomes closed in hypoxic water. After training, the duo tested the snail's memories, transferring them to hypoxic water 3, 24 and 72 h later to see if the molluscs remembered to keep their pneumostomes closed and find out whether the smell of their own predator had improved the length of their memories.
It had. Both snails remembered to keep their pneumostomes closed 24 h after training, while snails that had not sniffed predator water only retained the memory for 3 h. And when Michael Orr tested neural activity in the key nerve cell that stores the memory, known as RPeD1, there was no activity. The memory had inactivated the neuron, just as he expected.
So what happened when the team switched the smell of fear to a predator that neither snail had previously encountered? Resting Dutch snails in salamander water and Canadian snails in crayfish water before training, the team tested the snails' memories 3, 24 and 72 h later. This time the snails had completely forgotten to keep their pneumostomes closed by 24 h, and the RPeD1 memory cell had recovered its activity, firing whenever the snail opened its pneumostome. The snails' memories were only improved by the scent of their own predator.