Multivariate selection and the range limit of Alderia willowi: effects of salinity, interspecific competition and trait covariance.

For estuarine animals, range limits may be set by complex interactions between abiotic factors, such as geographical gradients in salinity and temperature, and interspecific competition. Further, multivariate selection can inhibit adaptation to any one stressor. Thus, range limits may occur in different places than would be predicted by considering one environmental factor or trait in isolation. The northern range limit of the sea slug Alderia willowi may be set by low salinity following winter rains, and/or competition with its sister species A. modesta. We studied multivariate selection on A. willowi based on short-term fitness in laboratory experiments. Slugs will be stressed by combinations of low salinity and presence of A. modesta, and egg production measured for 1-2 weeks. Analyses of multivariate selection considered the relationship of number of cerata (appendages that circulate body fluid), rate of ceratal beating, and body size on fitness. Preliminary results indicate that lower salinities significantly suppressed initial egg production, but slugs recovered to control levels after a one-week period of acclimation. There was no interaction between body size and fitness in preliminary experiments. Results of ongoing research will provide insight into the basis for northern range limits of estuarine animals along the U.S. west coast.