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Variation in temperature tolerance in a widely invasive bryozoan species complex (Watersipora spp.)
Marine habitats are under increasing threat of invasion by exotic species, the successful establishment and spread of which are poorly understood. Cryptic yet genetically distinct bryozoans in the genus Watersipora have invaded bays throughout California and show specific geographical patterns of invasion, with W. “new species” and W. subtorquata found more frequently in northern and southern California, respectively. This pattern suggests that successful invasions may depend on water temperature. To test whether these two species have different temperature tolerances, our lab collected colonies of W. “new species” and W. subtorquata from bays along the California coast and induced them to release larvae at the Telonicher Marine Lab. Newly settled larvae were then subjected to two treatments representing the average summertime water temperatures in northern (11 ºC) and southern (18 ºC) California. In the course of three common garden experiments, I found that warm water negatively impacts the growth and fecundity of W. “new species”. Watersipora subtorquata also outgrew “new species” in the warm temperature treatment, and produced more brooded larvae relative to “new species”. However, the growth of W. subtorquata was greater in cold water than in warm. Lastly, colonies grown at different temperatures differed in shape; warm water colonies had an irregular, multi-lobed morphology compared to colonies in cold water, which were generally regular or circular in shape. These results suggest major differences in temperature tolerance among closely related bryozoans, which helps to explain the current pattern of invasion and may help to predict future successful invasions.