Gape-limited predators as agents of selection on the defensive morphology of an invasive invertebrate.

Invasive species have widespread and pronounced effects on ecosystems and adaptive evolution of invaders is often considered responsible for their success. Despite the potential importance of adaptation to invasion, we still have limited knowledge of the agents of natural selection on invasive species. Bythotrephes longimanus, a cladoceran zooplankton, invaded multiple Canadian Shield lakes over the past several decades. Bythotrephes have a conspicuous caudal process (tail spine) that provides a morphological defense against fish predation. We measured viability selection on the longest component of the Bythotrephes spine, the distal spine segment, through a comparison of the lengths of first and second instar Bythotrephes collected from lakes differing in the dominance of gape-limited predation (GLP) and nongape-limited predation (NGLP) by fish. We found that natural selection varied by predator gape-limitation, with strong selection (selection intensity: 0.20-0.79) for increased distal spine length in lakes dominated by GLP, and no significant selection in lakes dominated by NGLP. Further, distal spine length was 17% longer in lakes dominated by GLP, suggesting the possibility of local adaptation. As all study lakes were invaded less than 20 years prior to our collections, our results suggest rapid divergence in defensive morphology in response to selection from fish predators.