A large-scale panmictic population: phylogeography of Onchidella floridana (Gastropoda: Onchidiidae) across the Brazilian coastline

Molluscs are among the most abundant inhabitants of rocky shores and represent the second-largest animal phylum in species richness. Within this group, the genus Onchidella occurs in eulittoral zones of sandy, muddy, and rocky shores, as well as estuarine environments. Onchidella floridana Dall, 1885, which lacks a planktonic larval phase, has been reported from the Gulf of Mexico, the Caribbean Sea, and the western Atlantic. Although the absence of larvae is often associated with restricted dispersal, genetic structure in rocky-shore molluscs can also be shaped by alternative dispersal mechanisms, oceanographic features, local adaptation, and life-history strategies. To evaluate whether the absence of a planktonic larval stage necessarily promotes strong genetic differentiation, we investigated the phylogeographic patterns of the direct-developing O. floridana along the Brazilian coast using two mitochondrial markers—Cytochrome c oxidase subunit I (COI) and 16S rRNA. Analyses across 16 localities revealed a panmictic population (F _ST = 0), high genetic diversity (16S rRNA average θπ = 2.2; COI average θπ = 4.3), and shared haplotypes across ~ 3,600 km. Demographic analyses indicated gradual increases in effective population size over time, with a more pronounced rise in the recent past. These findings suggest that larval developmental mode alone does not reliably predict genetic diversity or gene flow. By showing that O. floridana maintains connectivity across broad spatial scales even in periods encompassing climatic fluctuations such as the Last Glacial Maximum, this study contributes to understanding how life-history traits influence genetic structure in marine systems.