PERUANOCOTYLE PELAGICA N. SP. (MONOGENEA: MONOCOTYLIDAE), PARASITE OF THE PACIFIC COWNOSE RAY RHINOPTERA STEINDACHNERI EVERMANN AND JENKINS, 1891 (BATOIDEA: RHINOPTERIDAE) FROM THE SOUTHERN MEXICAN PACIFIC OCEAN.

Peruanocotyle pelagica n. sp. is described based on specimens collected from the wall of the pharyngeal cavity of the Pacific cownose ray Rhinoptera steindachneri offshore Oaxaca and Guerrero, Mexico. The new species is distinguished from Peruanocotyle chisholmae by its anchors, which include a slender guard that curves towards the tip of the blade and which lack an accessory piece, morphological differences of the seminal vesicle, the lack of a male copulatory organ accessory piece and a greater number of spines, and an unsclerotized vagina. Molecular data of Peruanocotyle pelagica were generated to place the phylogenetic position of the genus within Monocotylidae.

Marine flatworms (Platyhelminthes: Polycladida) found in empty barnacle shells, including a new species, from southern Mexican Pacific.

The association of some species of polyclads of the suborder Acotylea with barnacles has been recorded worldwide. In contrast, no species of the suborder Cotylea has been recorded present in these crustaceans. The aim of this work is to describe the species of polyclads found in the empty barnacle shells on the coasts of Oaxaca, southern Mexican Pacific. Thirty polyclad specimens in the barnacles' empty shells were collected, corresponding to two species. A new species of acotylean, Stylochus mistus sp. nov., was found in Paraconcavus pacificus; and the cotylean Prosthiostomum cf. parvicelis in Megabalanus peninsularis. Both species of polyclads are considered to use the shells of Megabalanus and Paraconcavus as an occasional shelter site; however, it is not ruled out that these species feed on these barnacles, as polyclads normally prey on the communities they inhabit. This work records for the first time the presence of a cotylean polyclad in barnacles, as well as the first record of the polyclad genera Stylochus and Prosthiostomum from the southern Mexican Pacific.

Five new species of cotylean flatworms (Platyhelminthes: Polycladida: Cotylea) from Oaxaca, southern Mexican Pacific.

The flatworms of the suborder Cotylea live in diverse marine substrates such as rocks, seaweed and sand or in association with corals. In the Mexican Pacific, particularly on the coast of Oaxaca, knowledge about cotyleans is scarce, with only one record of the genus Pseudoceros. The aim of this work was to describe the cotylean species present in the area. A total of 48 specimens were collected, fixed, mounted in whole mounts or histological sagittal and frontal histological sections of the reproductive systems. These were examined for taxonomic identification, determining five new species of four genera: Boninia oaxaquensis sp. nov., Pericelis sigmeri sp. nov., Pericelis nazahui sp. nov., Pseudoceros bicuti sp. nov. and Thysanozoon estacahuitensis sp. nov. The present study establishes the first record of the genus Boninia and the family Boniniidae for the Tropical Eastern Pacific. It also confirms the presence of the genus Pseudoceros on the coast of Oaxaca, and break the disjunct distribution of this genus (from Gulf of California to Gulf of Tehuantepec), as well as the genus Pericelis (from Canada to Galapagos Islands) and Thysanozoon (from Chile to Gulf of California).

New genus, new species and new records of marine acotylean flatworms (Platyhelminthes: Polycladida: Acotylea) from Oaxaca, southern Mexican Pacific.

The acotylean polyclads are characterized by the absence of a ventral sucker located posteriorly to the female gonopore. These flatworms are mostly marine, benthic or pelagic, except by two species, one freshwater Limnostylochus borneensis and other semi-terrestrial Myoramyxa pardalota. Oaxaca is a state located in the southern Mexican Pacific, where only three acotyleans species have been formally recorded until now. After sampling littoral to sublittoral zones, a total of 34 specimens were collected and fixed in frozen formalin. Some specimens were stained with Mayer´s carmalum and hematoxylin and cleared in methyl salicylate for whole mounts; the portion containing the reproductive structures of other specimens were dissected and histologically processed in sagittal and frontal sections. Four species were identified as belonging to four genera and three families. The new genus Bisacculosuteri gen. nov. is established, close to the genus Itannia, but lacking double ventral suckers placed on both sides of the female gonopore. Two species are new to science: Bisacculosuteri marcelae sp. nov., characterized by a highly branched central pharynx, prominent seminal vesicle, a penis papilla with stylet, paired strongly muscularized uterine sac and uterine vesicles; and Paraplanocera oligoglenoides sp. nov., with translucent body, light brown pigmentation and black dots distributed in the pharyngeal and reproductive regions, and an oval cirrus sac with intramuscular surrounding hollow spaces and armored with spines and conspicuous teeth. The third species is Euplanoida cf. pacificola that could not be determined as the nominal species, due to ambiguity in its description. The fourth species, Bivesiculoplana lamothei, was previously described from the region and now is recorded in new localities and hosts from Oaxaca.