Comparative population genetics and demographic history of two polychaete species suggest that coastal lagoon populations evolve under alternate regimes of gene flow

Here, we compare the population genetic structure and the demographic history of two polychaete species along one bay and eight coastal lagoons distributed over ~ 200 km of the Southwest Atlantic to understand the evolution of discontinuous and confined inland ocean-connected waters populations. A total of 515 sequences of COI and 16S were obtained for Laeonereis culveri and Capitella nonatoi. Levels of genetic diversity and population genetic structure were higher for C. nonatoi than L. culveri, possibly reflecting the differences in life-history strategies. Furthermore, the genetic diversity of both species was, in general, smaller in more confined populations. Populations of both species showed signs of recent demographic expansion, although it was more pronounced in C. nonatoi. As the population size of both species may reach high densities in highly eutrophic environments, these expansions are probably associated to the coastal lagoon formation, which favors the process of organic matter accumulation and the water oxygenation reduction. The general pattern of haplotype distribution revealed high levels of haplotype sharing among populations, mainly in L. culveri. On the other hand, the observed number of exclusive haplotypes indicates that the genetic exchange among populations is not as high as it may seem. This result suggests that coastal lagoons populations evolve under alternate regimes of soft (physiological barriers) and strong (physiological and physical barriers) gene flow restriction.