Hidden but not Forgotten: Molecular Diversity and Species Delimitation of Proseriata (Platyhelminthes: Rhabditophora) across the Brazilian Coastline, with Focus on Genus <i>Kata</i>.

The Proseriata (Platyhelminthes) order represents one of the most abundant meiofauna groups in swash zones. Predicting the diversity and distribution of these organisms is challenging due to the presence of cryptic species. For this reason, molecular data have been increasingly integrated into phylogenetic and population analyses to support reliable species identification. Here, we aimed to assess proseriates diversity along the Brazilian coastline and explore connectivity patterns within sandy beaches. To achieve this, we used partial sequences of the 28S rRNA gene from 32 Proseriata specimens collected in Brazil and Florida (USA). We then performed species delimitation and phylogeographic analyses, focusing on the <i>Kata</i> genus. The four delimitation methods were consistent, with slight variation in the number of evolutionary lineages, ranging from seven to eight. Molecular identification revealed species from the genera <i>Nematoplana</i>, <i>Meidiama</i>, <i>Parotoplana</i>, <i>Kata</i>, and <i>Archotoplana</i> present in the samples. Here, two species of <i>Kata</i> were identified in the Brazilian samples, corresponding to <i>K. leroda</i> Marcus, 1950, and a yet unidentified species. For both, the analyses revealed that despite the lack of a larval stage, there might be some gene flow across distant locations due to shared haplotypes among populations separated by ~900 km. Here, we provided the first population genetics study of Proseriata along the Brazilian coast. Our findings offer significant contributions to the molecular taxonomy field and our understanding of the evolutionary dynamics within this relatively understudied meiofauna group.