Fine-scale genetic structure of the periwinkle Echinolittorina lineolata (Gastropoda: Littorinidae): the interplay between space and time

Some marine species show homogeneous populations across most of their distributional range, but also population genetic structure at a smaller geographic scale. This intriguing scenario can be caused by different spatial and temporal factors, such as local oceanographic features, larval aggregation during dispersal, variable reproductive success and even natural selection. In this study, we evaluated small-scale population genetic structure in Echinolittorina lineolata, a gastropod with homogeneous populations at a large geographic scale in Brazil. Three concatenated mitochondrial DNA markers were used to infer genetic diversity and assess population-genetic differentiation between two locations. Repeated sampling of juveniles over time allowed us to test for constancy of genetic differentiation across four generations. Our diversity analysis revealed 103 haplotypes with low nucleotide diversity. The majority of the haplotypes were unique, but four were in high frequency and commonly found in individuals from both locations. There was only one location-specific haplotype with high frequency. When considering all samples, we detected spatial population differentiation within the dispersal range of E. lineolata. However, spatial differentiation was present in the first two generations sampled, while the following two showed genetic homogeneity between locations. In addition, there was genetic differentiation among individuals sampled from distinct rocks inside each location. When comparing generations within locations, different results were observed: in one location the first two generations were genetically similar, but differed from the following two generations; in the other location there was no genetic differentiation among all four generations. We hypothesize that E. lineolata is influenced by variance in reproductive success and resulting kinship among recruits. However, other factors could also cause transient genetic structure. We suggest that these curious genetic patterns can be better understood by combining the genetic information with in-depth knowledge of the species' biology and local oceanographic features.