Wide Distribution but Low Differentiation: Genetic Diversity of Globose Shore Crab Cyclograpsus integer (Brachyura: Varunidae) along the Western Atlantic Ocean.

AbstractDecapod crustaceans with wide distributions have shown different patterns of population structure in certain species, which clarified processes related to population dynamics and intraspecific variation. The aim of the current study is to assess phylogeographic patterns of the intertidal crab Cyclograpsus integer H. Milne Edwards, 1837, as well as its wide distribution in the western Atlantic Ocean, from the US to Brazil. We hypothesize that the genetic structure of C. integer is influenced by the interaction of different geographic barriers. We analyzed partial sequences of the mitochondrial gene cytochrome c oxidase subunit I deriving from specimens collected along the western Atlantic distribution of C. integer. The phylogenetic reconstruction, haplotype networks, demographic history, divergence time, and discrete phylogeography of C. integer do not confirm our hypothesis. The wide distribution can be explained by the potential of C. integer larvae to disperse through northern Brazilian and Brazilian currents, which helps maintain gene flow across natural barriers. In addition, C. integer population size has increased in the Atlantic Ocean over the past 0.01-0.10 million years, with emphasis on two large separate groups and on the north-to-south spatiotemporal diffusion of its populations. While there is some evidence of genetic differentiation between the tropical northwestern Atlantic and the tropical/warm-temperate southwestern Atlantic regions, the overall genetic structure is low. The Amazon-Orinoco Plume appears to have limited influence as a barrier, further emphasizing the species' capacity for larval dispersal and genetic homogenization across its range. These findings highlight the importance of oceanographic features in shaping genetic patterns of widely distributed marine organisms and underscore the value of C. integer as a model for studying genetic connectivity in marine ecosystems.