Lack of substrate specificity contributes to invasion success and persistence of Membranipora membranacea in the northwest Atlantic

Selective settlement by planktonic larvae plays a significant role in determining the distribution and abundance of many species of marine invertebrates. For non-indigenous species, larval settlement behavior can determine invasive potential by influencing initial invasion success, secondary spread, and persistence of species outside their native environments. Membranipora membranacea is an ecologically significant invasive bryozoan in the northwest Atlantic, where settlers are most abundant on some but not all species of kelp. Whether the increased abundance of M. membranacea on select kelp species is the result of larval settlement preference remains unknown. In this study, we examine selective settlement by larvae of M. membranacea by (1) quantifying settlers in mixed kelp beds and determining whether larvae settle preferentially with respect to kelp species, (2) conducting laboratory settlement preference experiments using the most abundant kelp substrates in Nova Scotia, and (3) examining whether the presence of kelp beds provides a settlement cue for larvae by quantifying settlement of M. membranacea on plates deployed within and outside of kelp beds. Contrary to settlement behavior described for native populations, our results suggest that larvae of M. membranacea in invaded habitats do not exhibit preference for settling on particular kelp species or within kelp beds. Instead, larvae settle on substrates extending furthest above the primary substratum. Lack of substrate specificity suggests that M. membranacea will continue to persist in the northwest Atlantic despite significant declines in regional kelp abundance. Our results emphasize the importance of early life-history strategies in contributing to invasion success.