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Abstract: Invasive ascidians are a growing concern for ecologists and natural resource managers, yet few studies have documented their short- and long-term temporal patterns of abundance. This study focuses on the invasion of the Gulf of Maine by the colonial ascidians Botryllus schlosseri, Botrylloides violaceus, Diplosoma listerianum and Didemnum sp. A. We examined the time of arrival and potential vectors for these four invasive ascidians using survey data (collected from 1969 onwards) and literature documentation. We also compared the dominance and seasonal patterns of abundance of these species using data from two identical panel studies; one conducted from 1979 to 1980, the other from 2003 to 2005. Didemnum and Botrylloides were most likely first introduced into the Damariscotta River, Maine in the early 1970''s through oyster aquaculture while Botryllus and Diplosoma were probably transported by commercial and recreational vessels. The overall abundance of colonial ascidians has increased since 1979 and 1980. Botryllus was the only invasive colonial ascidian present during the 1979 to 1980 study and accounted for an average of 6.16% cover on panels. From 2003 to 2005, the more recently arrived colonial ascidians Botrylloides and Didemnum accounted for 7.38% and 2.08% cover respectively, while Botryllus covered only 1.16%. Our results reveal a shift in seasonal abundance between 1979 to 1980 and 2003 to 2004. In 1979 and 1980, colonial ascidians had the highest percent cover in fall and winter while in 2003 and 2005 they had highest percent cover in summer and fall. Seasonal patterns of space occupation by colonial ascidians were correlated with seasonal changes in seawater temperature. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]