Reaping the benefits of an anthropogenic contaminant: the evolution of copper tolerance in a marine invader

Abstract

The global spread of fouling invasive species continues despite the use of antifouling biocides on vessel hulls. Furthermore, previous evidence suggests that non-indigenous species (NIS) introduced via hull fouling may have been selected for metal tolerance. Consequently, this process of selection may have then enhanced their introduction and establishment success in metal-polluted environments. This thesis examines the basis of copper tolerance in the non-indigenous bryozoan Watersipora subtorquata, and whether this successful NIS has evolved to benefit from a common contaminant in the marine environment. Using a series of laboratory-based experiments I found that tolerance to copper in W. subtorquata is a heritable genetic trait that varies on an individual basis. While there was no difference in tolerance between sites within an estuary for larval W. subtorquata, there was a significant interaction between parental colony and copper, with large variation in the response of colony offspring within sites. Larval size differed significantly both between sites and between colonies and was positively correlated with tolerance. In adult W. subtorquata colonies there was individual variation in tolerance to a gradient of copper concentrations and in the ability to recover from exposure to copper. Growth during and after exposure appear to be separate traits, while a trade-off between growth in control and copper environments during exposure suggests a cost to tolerance. The genetic variation present within this species indicates that further evolutionary change is possible under strong directional selection. The relationship between W. subtorquata and copper was further examined in field-based experiments. Copper appears to act as a settlement cue for W. subtorquata larvae by significantly increasing recruitment, although short term fitness costs were associated. Exposure to copper during initial settlement decreased recruit size and resulted in greater post-settlement mortality. In contrast, long term exposure to copper post-settlement significantly increased growth and reproductive fitness. The negative consequences of initial exposure during recruitment appear to be outweighed by the benefits of long term exposure which significantly increased overall fitness. In summary I have found that tolerance to copper is a heritable trait in W. subtorquata. Moreover, in an entirely novel finding, it appears that W. subtorquata has evolved to benefit directly from copper pollution, as well as indirectly through reduced competition from less tolerant species.