The systematics and distribution of the order Polycladida within the Macaronesian archipelagos are analysed. New species (Marcusia alba sp. nov., Prostheceraeus crisostomum sp. nov., Parviplana sodade sp. nov., Euplana claridade sp. nov., Stylochus salis sp. nov. and Distylochus fundae sp. nov.), new variety (Pseudoceros rawlinsonae var. galaxy), new records and records of shared species among different archipelagos are studied to compare the marine flatworm biodiversity of each island. The complex of archipelagos known as Macaronesia (including Madeira, Selvagens Islands, Canary Islands, Azores and Cape Verde) share a volcanic origin and European political influence. The five archipelagos are located along the eastern coast of the Atlantic Ocean and are subject to similar trade winds, streams (like the Gulf Stream) and cold currents. The term Macaronesia has suffered several changes throughout the years and it still is a topic of discussion in present times. The new delimitation of Macaronesia is mainly based on systematic studies on the invertebrate fauna of the islands. The resulting analyses shed new light on the differences and similarities among these archipelagos. In addition, molecular analyses employing 28S nuclear gene sequences are compared to verify relationships among anatomically similar species of marine polyclads.

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.

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 genera Boninia, Chromyella and Pericelis appeared 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, Chromyella and Pericelis, but also for Anonymus, Chromoplana and Cycloporus.

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.

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