Inner‐Shelf Geomorphology and Hydrodynamics as Biogeographic Barriers to Long‐Distance Dispersal of the Interstitial Annelid Saccocirrus (Saccocirridae)

ABSTRACT Aim This study aimed to investigate the population structure and demographic history of Saccocirrus pussicus Marcus, 1948, an interstitial annelid that inhabits coarse sandy beaches in Brazil. We tested the hypothesis that the limited space between sediment grains constrains dispersal, resulting in restricted gene flow, despite the presence of a larval stage. We also tested whether oceanic conditions could influence connectivity and the distribution of genetic diversity. Location Samples were collected from coastal sites in Brazil, USA, Cuba, Panama and Italy. Taxon Saccocirrus pussicus Marcus, 1948 (Annelida, Saccocirridae). Methods We analysed fragments of the mitochondrial 16S rRNA and nuclear 28S rRNA genes from 190 Saccocirrus specimens to assess genetic diversity, lineage structure and connectivity patterns. Phylogenetic analyses, gene flow estimates and larval dispersal simulations were performed to infer population structure and migration dynamics. Results Between six and nine evolutionary lineages were identified across all sampled locations. Along the Brazilian coast, seven localities exhibited a high number of shared haplotypes and elevated migration probabilities among distant populations, suggesting extensive admixture over approximately 2000 km. These results refute the initial hypothesis of limited dispersal. Gene flow estimates and larval dispersal simulations indicate that migration of S. pussicus occurs from south to north along the Brazilian coast. Nonetheless, there is evidence of isolation between the northernmost and southernmost localities, likely due to the presence of a biogeographic barrier whose nature remains unclear. Main Conclusions Small‐bodied interstitial organisms with planktonic larvae appear to be influenced by coastal transport processes previously recognized only in biogeographic frameworks for larger organisms. Littoral drift and surface circulation can act both as barriers and as facilitators of dispersal. These insights underscore the need for further comparative studies across taxa to clarify the mechanisms driving population connectivity in interstitial habitats at macrogeographic scales.