A Revised Checklist and Identification Key for Acotylean Flatworms (Rhabditophora: Polycladida: Acotylea) from the Caribbean Coast of Colombia

The Order Polycladida comprises a diverse yet understudied group of free-living flatworms, traditionally divided into Cotylea and Acotylea based on the presence or absence of a ventral adhesive organ known as a cotyl. Species identification, particularly within Acotylea, is often challenging due to the need for histological examination of reproductive structures and the scarcity of molecular data. The Tropical Western Atlantic, especially the Caribbean Sea, hosts high polyclad diversity but remains poorly surveyed. This study updates the checklist of Acotylea from the Colombian Caribbean, combining new collections from Santa Marta with the literature records. Field work yielded 22 acotylean species from 14 genera and 10 families, with DNA sequences (28S rDNA and/or COI mtDNA) obtained for 20 species. 11 species are new records for Colombia, and five for the Caribbean Sea: Latocestus brasiliensis Hyman, 1953, Notocomplana martae (Marcus, 1948), Interplana evelinae (Marcus, 1952), Triadomma curvum Marcus, 1949, and Adenoplana evelinae Marcus, 1950. In total, 29 species are now documented from the region. We provide photographs of live specimens, whole mounts, and histological sections; DNA barcodes for most species; and the first dichotomous key for Colombian Caribbean acotyleans, based primarily on external traits, providing a practical tool to support further taxonomic, ecological, and biodiversity research.