Postcode-dependent memory

Posted on July 25, 2012

Experiments on fish and birds suggest that different populations of the same vertebrate species often vary in terms of memory capacity.  Do populations of invertebrate species also vary in this way, and if so, why?  To find out, researchers compared the learning and memory capacities of four populations of the pond snail Lymnaea stagnalis in Somerset, U.K.  Two of the study sites were in large, stable drainage canals containing predatory fish, and the other two sites were in small drainage ditches with wider environmental fluctuations and invertebrate predators only.  In the lab, the oxygen content of the water was lowered to induce snails to switch to air-based respiration.  Then snails were trained to reduce their rate of air-breathing by prodding their breathing hole every time they opened it.  Learning and memory were assessed by measuring  the breathing rate at different times after training.  Snails from all tested populations showed intermediate-term memory (assessed one hour after training), but only one population showed evidence of long-term memory (assessed  24 hours after training).  Since some populations showing memory differences were located less than a kilometre apart, it seems that memory function can vary at a microgeographic scale, but the observed population differences weren’t clearly related to environmental variability or predation pressure and more research is needed on the causes of the observed site differences.  In parallel experiments where snails were trained in water taken from tanks that housed predators (crayfish and tench), all four snail populations showed long-term memory, which showed that  interpopulation differences can be overridden in the presence of predators.  The protective significance of this  response may be related, at least partly, to the fact that a snail’s vulnerable body parts are exposed to predators when its breathing hole is open. 

Reference:  Dalesman, S., Rundle, S.D. & Lukowiak, K.  2011.  Microgeographical variability in long-term memory formation in the pond snail, Lymnaea stagnalis.  Animal Behaviour 82, 311-319.