Unexpected Limited Gene Flow and Possible Local Phenotypic Adaptation in the Native Crab Cyrtograpsus angulatus Along the Southwest Atlantic Coast

Cyrtograpsus angulatus (Decapoda: Varunidae), a widely distributed native crab in the southwestern Atlantic Ocean, is commonly found in estuarine and marine waters. An extended pelagic larval phase suggests high dispersal potential and the possibility of extensive gene flow between populations. We also expect a phylogeographic break in the Río de la Plata, which may act as a barrier to larval dispersal. Here, we integrate geometric morphometrics and molecular analysis of the mitochondrial gene to characterize the variation, expecting local variation to be associated with observed differences in habitat type among sampling sites. We found a high genetic differentiation mainly among the southernmost populations, and a moderate genetic differentiation between distant populations such as Montevideo and Puerto Deseado. Estuarine populations, for which larval retention is possible, exhibit higher differentiation than open-sea populations. In addition, C. angulatus has significant morphological differentiation, possibly due to local adaptation. This study shows how life history traits and environmental conditions could contribute significantly to marine species’ differentiation and potential speciation.