The Malacologist | The Malacological Society of London The Malacological Society of London The Malacologist

Volume 48

The evolution of phenotypic plasticity in freshwater snails

Jason Hoverman, University of Pittsburgh

In May 2002, I set out to test the central tenet of evolutionary theory about phenotypic plasticity. Briefly stated the theory predicts that plastic phenotypes evolve in variable environments whereas non-plastic phenotypes evolve in constant environments. Freshwater snails (Helisoma trivolvis) and their predators (i.e. water bugs, Belostoma flumineum) provided an excellent opportunity to empirically test this evolutionary theory. Using outdoor mesocosms, I reared snails in three treatments: 1) constant no-predator, 2) constant predator, and 3) variable predator (predator added and removed every three months). For each of the outdoor mesocosms, I have conducted annual assessments of the populations' trait changes to track the course of evolution. The funding provided by the Malacological Society of London Centenary Research Grant has provided the resources necessary to continue my annual assessments of trait evolution for the past two summers. The 2005 assessment demonstrated that the snail populations exposed to constant no-predator environments were less responsive to water bugs and tended to constantly produce poorly defended phenotypes (i.e. narrow shells). However, there was no indication that snail populations exposed to constant water bug environments were becoming canalized for well defended phenotypes (i.e. wider shells). This past September I conducted the final assessment of the snail populations. Over the next several months, I will complete the measurements on the snails and data analysis. Although the data from the past assessments is encouraging, it would be premature to reach conclusions about the experiment without the results from the final assessment.

In summary, I would like to thank the Malacological Society of London for their support on this long-term project.