Two conflicting morphological approaches to polyclad systematics highlight the relevance of molecular data for resolving the interrelationships of Polycladida. In the present study, phylogenetic trees were reconstructed based on a short alignment of the 28S rDNA marker gene with 118 polyclad terminals (24 new) including 100 different polyclad species from 44 genera and 22 families, as well as on a combined dataset using 18S and 28S rDNA genes with 27 polyclad terminals (19 new) covering 26 different polyclad species. In both approaches, Theamatidae and Cestoplanidae were included, two families that have previously been shown to switch from Acotylea to Cotylea. Three different alignment methods were used, both with and without alignment curation by Gblocks, and all alignments were subjected to Bayesian inference and maximum likelihood tree calculations. Over all trees of the combined dataset, an extended majority-rule consensus tree had weak support for Theamatidae and Cestoplanidae as acotyleans, and also the cotylean generaBoninia,ChromyellaandPericelisappeared as acotyleans. With the most inclusive short 28S dataset, on the other hand, there is good support for the aforementioned taxa as cotyleans. Especially with the short 28S matrix, taxon sampling, outgroup selection, alignment method and curation, as well as model choice were all decisive for tree topology. Well-supported parts of the phylogeny over all trees include Pseudocerotoidea, Prosthiostomoidea, Stylochoidea, Leptoplanoidea and Cryptoceloidea, the latter three with new definitions. Unstable positions in the tree were found not only for Theamatidae, Cestoplanidae,Boninia,ChromyellaandPericelis, but also forAnonymus,ChromoplanaandCycloporus.

The main focus of this study is the biodiversity of the order Polycladida in the Canary Islands, archipelago belonging to Macaronesia and to the Mediterranean basin hot spot region. Polycladida is a cosmopolitan order with some species distributed worldwide; but it also is comprised of endemic species. Here, 19 polyclad species are revised and determined, ten of which were previously recorded for the Canary Islands (De Vera et al. 2009). A new genus and species Multisepta fengari n. gen., n. sp., of the suborder Acotylea and five new species belonging to the suborder Cotylea including Anonymus ruber n. sp., Enchiridium magec n. sp., Eurylepta guayota n. sp., Acanthozoon aranfaibo n. sp. and Pseudoceros mororum n. sp., are presented and described. In addition, two well-known species, Pseudobiceros wirtzi (Madeira and Cape Verde) and Pericelis cata (Caribbean coasts), are new records for the Canary Islands and for the hot spot region of the Mediterranean basin. Using all available information, including from the morphological study presented here, a key for the species for the Canary Islands has been generated.

In the present work was carried out in the intertidal zone of Las Baulas de Guanacaste National Marine Park (PNMB) located on the Pacific coast of Costa Rica. The main objective was to contribute to knowledge about the invertebrate diversity of the park, one of the richest bioregions on the planet, about which little is known. This study assesses the Order Polycladida Lang, 1884, a cornerstone of this ecosystem and one of the most cosmopolitan and plastic invertebrate taxa in the animal kingdom. In total, 57 individuals were collected in the rocky intertidal zone of Carbón and Langosta beaches. Nine different species were identified, of which four are new for Costa Rica: Semonia bauliensis n. sp.; Cryptostylochus sesei n. sp.; Paraplanocera angeli n. sp., Prostheceraeus fitae n. sp.; and five new records: Paraplanocera oligoglena (Schmarda, 1859); Marcusia ernesti Hyman, 1953; Enchiridium magec Cuadrado, Moro & Noreña, 2017; Pseudobiceros bajae (Hyman, 1953); and the genus Boninia spp.

The increase in the use of molecular methodologies in systematics has driven the necessity for a comprehensive understanding of the limitations of different genetic markers. Not every marker is optimal for all species, which has led to multiple approaches in the study of the taxonomy and phylogeny of polyclad flatworms. The present study evaluates base-substitution rates of nuclear ribosomal (18S rDNA and 28S rDNA), mitochondrial ribosomal (16S rDNA), and protein-codifying (cytb, cox1) markers for this taxonomic group, with the main objective of assessing the robustness of these different markers for phylogenetic studies. Mutation rates and Ti/Tv ratios of the other markers were assessed for the first time. We estimated substitution rates and found cytb to be the most variable, while 18S rDNA was the least variable among them. On the other hand, the transition to transversion (Ti/Tv) ratio of the different genes revealed differences between the markers, with a higher number of transitions in the nuclear gene 28S and a higher number of transversions in the mitochondrial genes. Lastly, we identified that the third codon position of the studied protein-codifying genes was highly variable and that this position was saturated in the cox1 marker but not in cytb. We conclude that it is important to assess the markers employed for different phylogenetic levels for future studies, particularly in the order Polycladida. We encourage the use of mitochondrial genes cytb and 16S for phylogenetic studies at suborder, superfamily, and family levels and species delimitation in polyclads, in addition to the well-known 28S and cox1.

The order Polycladida (Platyhelminthes) in Mexico has historically received limited attention from researchers, primarily due to challenges associated with its low detectability and the scarcity of specialists. This study addresses part of the gap by conducting a comprehensive assessment of polyclad diversity in the southern Gulf of Mexico. Our investigation revealed a total of 27 distinct species, belonging to 17 genera and 12 families, within the suborders Cotylea and Acotylea. Our findings include the identification of 17 species previously undocumented in the Gulf of Mexico. This represents a significant expansion of the region’s known polyclad biodiversity. By revising the polyclad records in the Gulf of Mexico, the known species count has increased from 31 to 50. Furthermore, our research unveiled the presence of two new species, Stylochoplana sisalensissp. nov. and Emprosthopharynx harteisp. nov., also marking the first time a species of the genus Emprosthopharynx has been reported for the Atlantic coast of the Americas.

The evolutionary history of ancient marine lineages is often obscured by geological change, long-term isolation, and morphological conservatism. Here, we investigate whether population genetic patterns can reveal deep-time evolutionary processes by integrating haplotype structure and differentiation, molecular dating, and ancestral-range reconstruction in two early-diverging genera of Polycladida, Boninia and Pericelis. Time-calibrated phylogenies and ancestral-range reconstructions place the origins of both genera in tropical marine regions during the Miocene, with the most recent common ancestor of Boninia dated to 8.3 Ma and that of Pericelis to 14–17 Ma. Despite this shared origin, they followed contrasting evolutionary trajectories. Boninia diversified primarily through vicariant fragmentation, followed by founder-event dispersal and regional radiations, whereas Pericelis preserves evidence of a formerly widespread tropical distribution through reciprocal relict species linking the Atlantic and Indo-Pacific, followed by independent regional diversification. Both genera exhibit pronounced geographic genetic structure, indicating long-term evolutionary persistence rather than recent cosmopolitan dispersal. These results demonstrate the value of integrating regional genetic diversity, molecular dating, and ancestral-range reconstruction to resolve the historical biogeography of early-diverging marine lineages.