Seasonal patterns of population structure in a colonial marine invertebrate (Bugula stolonifera, Bryozoa)

For sessile invertebrates, the degree to which dispersal mechanisms transport individuals away from their natal grounds can have significant ecological implications. Even though the larvae of the marine bryozoan Bugula stolonifera have limited dispersal potential, high levels of genetic mixing have been found within their conspecific aggregations. In this study, we investigated whether this high mixing within aggregations of B. stolonifera also resulted in high mixing between aggregations. Adult colonies were collected from five sites within and one site outside of Eel Pond, Woods Hole, Massachusetts, in August 2009 and genotyped at 10 microsatellite loci. Significant genotypic differentiation was found between most sites, suggesting limited connectivity across sites, even those separated by only 100 m. This investigation was extended to determine if low levels of genetic mixing throughout the reproductive season could result in increased homogeneity between sites. Four of the five sites in Eel Pond were sampled early, mid-, and late in the reproductive season in 2010, and again in early 2011. Inter- and intra-annual genotypic differentiation was then assessed within and between sites. Results from these analyses document that low levels of mixing could result in increased homogeneity between some aggregations, but that barriers to genetic exchange prevented mixing between most sites. Further, results from inter-annual comparisons within sites suggest that any potential homogeneity achieved throughout the reproductive season will likely be lost by the beginning of the next reproductive season due to the annual cycle of colony die-back and regrowth experienced by B. stolonifera colonies in Eel Pond.