Small spatial-scale differentiation among populations of an introduced colonial invertebrate.

Introduced species frequently undergo rapid evolutionary change in ecologically important traits that may strongly influence their performance in their introduced range. But for many introduced species, and particularly those in marine systems, the spatial and temporal dimensions of this evolutionary change are largely unknown. In this study, I conducted a common garden transplant experiment using replicate clones of an introduced tunicate, Botrylloides sp., collected from three sites in the southern Gulf of Maine to test for differential growth and survival in response to local thermal regimes. This species has a restricted larval development period, which may permit significant population differentiation over relatively small spatial scales. I found dramatic among-site differences in survival rates: tunicates collected from both the transplant site and from a distant northern site had high survival, while those collected from a southern site experienced 100% mortality. The most likely cause of this differential mortality was an anomalous upwelling event that produced some of the coldest mid-summer temperatures in the last 10 years based on NOAA weather buoy data. I also found significant differences in growth rates among sites as well as among clones within a site in comparison with growth-rate differences within clones. An estimate of broad-sense heritability based on these data was consistent with a genetic basis for among-site differences in growth rates, which were strongly predictive of subsequent mortality. I conclude that this species has undergone significant evolutionary change that has resulted in population differentiation with respect to temperature tolerances among sites separated by <60 km.