The polyclads (Platyhelminthes: Polycladida) from the Tropical Eastern Pacific: Commented checklist and description of a new species Author Ramos-Sánchez, Mariela E9D05B3F-CF64-48E3-B5ED-8D24E47F9D2A Doctorado en Ciencias Biológico Agropecuarias, Área Ciencias Biológico Agropecuaria y Pesquera, Ciudad de la cultura “ Amado Nervo ”, Tepic, Nayarit, 63000, Mexico. & Laboratorio de Sistemática de Invertebrados Marinos (LABSIM), Universidad del Mar, campus Puerto Ángel, Ciudad Universitaria, Puerto Ángel, Oaxaca, 70902, Mexico. rmariela220@gmail.com text European Journal of Taxonomy 2024 2024-10-17 962 1 1 67 https://europeanjournaloftaxonomy.eu/index.php/ejt/article/download/2683/12419 journal article 10.5852/ejt.2024.962.2683 2118-9773 13947260 40171C5B-90EB-4641-91FF-EA8CA2C60D23 Prostheceraeus panamensis Woodworth,1894 Prostheceraeus panamensis Woodworth, 1894: 51 , figs 3–4 ( type locality: Gulf of Panama ). Distribution Gulf of Panama . Habitat Benthic (reef coast). Remarks Woodworth (1894) described P. panamensis based on poorly preserved organism, providing only information on its coloration pattern and a diagram of the anterior region of the body (arrangement of cerebral and tentacular eyes). Faubel (1984b) assigned it as a species incertae sedis. However, based on the anterior morphology of the body represented in the diagram of the species and the coloration pattern, it is considered that this species could belong to the superfamily Pseudocerotoidea Faubel, 1984 , for which reason, the present study recommends the collection of topotypic material and a redescription of the species. Historical review After an exhaustive literature search, 51 works detailing and/or documenting the presence of polyclads in the TEP were reviewed. This analysis spanned a period of 173 years (1851–2024), revealing intermittent study of polyclads over time ( Figs 4–5 ). Pioneers in this field were LeConte (1851) and Schmarda (1859) , who described the first seven species in the Isthmus of Panama (3) and Paita, Peru (4), respectively. Woodworth (1894) contributed two new species and a record in the Gulf of California, the central coast of Oaxaca, and the Isthmus of Panama . Plehn (1896) continued this legacy with the description of two new species and a record for Paita, Peru , and one in the Galapagos Archipelago, followed by Bock (1913) , who contributed two new species, one in the Galapagos Archipelago and another in San José, Panama . After a lapse of 12 years, Bock (1925) described a species for Panama . Seventeen years later, Hyman (1939a , 1939b ) marked a milestone by initiating the systematic study of polyclads in the TEP, describing two insular species, one on Clipperton Island (1) and another in the Galapagos Archipelago (1); her work evolved over time, describing 20 new species and making three additional records between the Gulf of California and the Galapagos Archipelago ( Hyman 1953a , 1953b ). These pioneering contributions laid the groundwork for subsequent research, such as that of Cheng & Lewin (1975) , who recorded the first pelagic polyclad in La Paz, Baja California Sur , Mexico , and Sopott-Ehlers & Schmidt (1975) , who described eight new worm species for the Galapagos Archipelago. Five years later, Brusca (1980) expanded the record with the discovery of two new polyclad species in the Gulf of California. In parallel, Pineda-López (1981) left his mark by documenting an insular species in Nayarit , Mexico . This finding marked the beginning of a series of significant discoveries in the region, as Salgado & López (1981), Marcus & Harry (1982) , and Pineda-López & González-Bulnes (1984) contributed to the description and records of three species on the coasts of Nayarit , Baja California , and Jalisco , Mexico , respectively. Fourteen years passed until Gamboa-Contreras & Tapia-García (1998) recorded a hitherto undescribed species of Pseudoceros in the Gulf of Tehuantepec, Mexico . Another five years later, in 2003, Newman & Cannon surprised the scientific community by documenting two previously unknown species, one belonging to the genus Pericelis and another to the genus Pseudobiceros , in the Galapagos Archipelago. The momentum of discoveries continued, as Brusca (2005) expanded his contribution by recording five additional species for the Gulf of California. The discovery of polyclads evolved with Faubel et al. (2007) , who enriched knowledge by describing a new species in Jalisco , Mexico . Fig. 4. State of knowledge of the polyclads and frequency of described and recorded species by decade in the Tropical Eastern Pacific. In the last 12 years, a notable advancement in understanding the biodiversity of polyclads has been documented. Ramos-Sánchez et al. (2019 , 2020 , 2021 ), Lee et al. (2021) , and Soutullo et al. (2021) have played a crucial role in this progress by detailing 13 new species and contributing six additional records on the coasts of Oaxaca , Mexico , as well as in the regions of Costa Rica and the Panama Canal. This body of research has not only significantly expanded our understanding of the presence and variety of polyclads in this region but has also solidified the ongoing importance of research in this field. Numeralia by countries, states, and distribution The polyclad fauna of TEP is composed of 82 species, belonging to two suborders, Acotylea (50 species) and Cotylea (24 species), and eight taxa incertae sedis. These species are distributed across 22 families (17 of Acotylea and five of Cotylea ) and 53 genera (41 of Acotylea , 10 of Cotylea , two genera incertae sedis). Of the 82 species, 65 have valid taxonomic status, eight are catalogued as incertae sedis, six are characterized to genus level, two are undescribed species but are classified as close (confer) to the nominal species, and this study includes the description of a new species for the coast of Oaxaca , Mexico . Within Acotylea , the families with the highest species richness are Leptoplanidae (nine species), Planoceridae , and Stylochoplanidae (seven each), and Euplanidae (six), and the most species-rich genera are Leptoplana (five species) and Paraplanocera (four). In the suborder Cotylea , the families with the highest species richness are Prosthiostomidae and Pseudocerotidae (seven species each), while the most species-rich genera are Pericelis and Prosthiostomum (four species each one). Fig. 5. State of knowledge of the polyclads described and recorded in the Tropical Eastern Pacific; species accumulation curve by decade. Regarding the distribution of these species, 53 of them have their type locality and single record in the TEP. On the other hand, 21 species exhibit a disjunct distribution, covering two localities, while six have a wide distribution in the region, including three or more localities. It is important to note that Enchiridium magec , Planocera pellucida , Paraplanocera oligoglena and Pseudobiceros splendidus show an interoceanic and pantropical distribution, which raises doubts about their records in the TEP. The species of polyclads recorded from TEP are not distributed homogeneously. The Pacific coast of Mexico hosts 45 species (described and recorded). Within Mexico , Baja California Sur (17 species) and Oaxaca (16 species) are the states with the highest number of species recorded, followed by Sonora (14), Baja California (9), Nayarit , Jalisco , and Colima (two species each), and Sinaloa , Michoacán , and Guerrero (one species each). After Mexico , Ecuador is the next country in species richness, with 18 species, 17 of which are from Galapagos Archipelago and one from continental Ecuador (Salinas). Costa Rica and Panama each harbor 10 species, recorded from Guanacaste (northwest of Costa Rica ) and the Gulf of Panama . Peru has seven species recorded from Paita, and one species was recorded from Clipperton Island . In Mexico, 31 species have their type localities distributed across 10 of the 11 states that make up the Mexican Pacific coast. Notably, Oaxaca stands out as the type locality of 13 of the 16 species of polyclads present in this state. Likewise, the Galapagos Archipelago is a type locality of 16 of the 18 species distributed in that region. Panama has been designated as a type locality for 8 of the 10 species distributed in the country. Paita, Peru , stands out as the type locality of 6 of the 7 recorded species. Finally, Guanacaste , Costa Rica , has been identified as the type locality for 5 of the 10 species present in that area ( Fig. 6 ). Numeralia by habitats and bathymetry The polyclads that inhabit the TEP are distributed in the pelagic zone and in the benthic zone, where they are found under rocks, between coral rock interspaces, or in association with other invertebrates or seaweed. The largest number of species (65) have been recorded in benthic habitats; of these, 57 are free-living species found under rocks or on dead corals; seven species are interstitial, three have been categorized as symbionts (without specifying the symbiotic relationship), two species as commensals, two ectocomensal, and one as epibiont of barnacles, gastropods or seaweed. Additionally, two species are pelagic and two species are considered part of the fouling. Only six species have been recorded occupying two or more environments (benthic or pelagic) or substrates (rock, other invertebrates, or seaweed) ( Table 2 ). Bathymetric analysis revealed variability in the vertical distribution of these flatworms. In terms of bathymetry, of the 64 species distributed in the benthos, 13 species have a greater vertical distribution range, of which Prosthiostomum parvicelis reaches a depth of 18 meters, while Enchiridium magec 20 meters, Stylochus mistus 25 meters, Cryptocelis insularis 36 meters, and Stylochus ( Stylochus ) atentaculatus 51 meters. The greatest distribution depths have been recorded for Koinostylochus burchami and Armatoplana panamensis , found at 70 and 73 meters respectively.