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We explicitly tested for spatial changes in Pocillopora damicornis-associated invertebrates across several spatial scales in the southern Mexican Pacific. Sorting of invertebrates from 40 coral heads along 882 km of the coast yielded 325 taxa, 283% more than any other Pocillopora spp. coral host study to date, but estimators signals that richness might be 17-39% larger than the current number. Permutation, ordination, and regression analysis indicate that the composition and abundance of invertebrates vary in response to the spatial distance among coral heads: high similarity and variation occur among coral heads within localities (<500 m), probably related to faunal homogenization, but progressively modest reduction in similarity and variation as spatial distance increases suggesting a weak role for environmental sorting across southern Mexican Pacific coral reefs. Future studies should explicitly explore spatial, environmental, and historical biogeography processes that regulate and maintain community structure and biodiversity on eastern Pacific reefs.

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Environmental temperature is one of the most significant factors influencing the developmental rate and survival of embryos and larvae of many marine animals, including polychaetes. The aim of this study was to experimentally evaluate the effect of temperature increase on the embryonic development and larval survival of Spirobranchus incrassatus and S. cf. corniculatus. Adult worms of both species were collected from the western margin of the Gulf of Tehuantepec, Mexico. Embryos and larvae were obtained from these worms and exposed to four temperature treatments (28, 30, 32 and 34°C). The optimal temperature for embryonic development of S. incrassatus and S. cf. corniculatus was 30 and 28°C, respectively. For both species, the maximum critical temperature was 32°C and the lethal temperature was 34°C. The embryonic stages of S. cf. corniculatus were most negatively affected by elevated temperatures. Larval survivorship of S. incrassatus and S. cf. corniculatus was higher at 30°C and 28°C (76.2±2.8%; 72.6±4.2%) and lower at 34°C (28.9±2.6%; 26.3±2.9%), respectively. These results suggest that S. cf. corniculatus has the lowest thermal tolerance. Both Spirobranchus species already live near their upper limit of thermal tolerance in the study region (30°C). In the near future, under a global warming scenario, the distribution of both species could be modified, causing a loss of biodiversity, changes in the trophic chain, and alterations in the water column, such as excess organic matter.

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Temporal variation of the helminth infracommunity structure in the Gafftopsail pompano Trachinotus rhodopus was studied during bi-monthly revisions of samples collected offshore from Puerto Ángel, Oaxaca (Mexican Pacific) in 2018. In total, 110 specimens of T. rhodopus were subjected to a parasitic review. Helminths found were identified to the lowest possible taxonomic level (six species and three genera) by means of morphological and molecular data. Attributes of the helminth infracommunities are described through statistical analyses, showing stability in terms of their richness throughout the year. However, variations were found in helminth abundance related to the seasonality of samplings, which may be associated with the life cycles of the parasites, the host species' gregarious behavior, the availability of intermediate hosts, and/or the diet of T. rhodopus.

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Pomatostegus Schmarda, 1861 is a serpulid genus with three recognized species, mostly from tropical waters. Pomatostegus stellatus (Abildgaard, 1789), was described from the Caribbean Sea, and has been widely recorded in the Pacific and Indian oceans; P. kroyeri Mörch, 1863 was described from Puntarenas, Costa Rica; however, most records from the Tropical Eastern Pacific were referred to P. stellatus. In this work, the two species are redescribed using morphological characters. The main differences are in the shape of the Spirobranchus-type collar chaetae, opercular plate shape and the number of "free" circlets of spines without accompanying of opercular plates. Comments about characters of P. actinoceras Mörch, 1863, described from Philippines, and an identification key for the three species, are included.

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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.

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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).

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Introduction: The sipunculans are a group of marine invertebrates that have been little studied in the tropical eastern Pacific (TEP). Antillesoma antillarum is a species belonging to the monospecific family Antillesomatidae, considered widely distributed in tropical and subtropical localities across the globe. Objective: The main objective of this work was to examine the morphological and molecular differences between specimens from both coasts of tropical America to clarify the taxonomy of this species. Methods: We examined the morphology with material from the Mexican Caribbean and southern Mexican Pacific. To perform molecular analyses, two sequences of the COI molecular marker were obtained from specimens collected in Panteón Beach, Oaxaca, southern Mexican Pacific, and compared with four sequences identified as A. antillarum in GenBank, all of them from different localities. A phylogenetic reconstruction was performed with the maximum likelihood method and genetic distances were calculated with the Kimura 2P model and compared to reference values. Results: The phylogenetic analysis revealed three different lineages of Antillesoma that are well supported by bootstrap values: Antillesoma antillarum sensu stricto from the Caribbean Sea and Florida; a sister group to the one represented by our samples from the Mexican Pacific; and a third group from Thailand. Conclusion: Based on morphological traits and molecular data, Antillesoma mexicanum sp. nov. is described from the Mexican Pacific, differing from A. antillarum in the trunk papillae, color patterns and, additionally, the specimens from the Caribbean attain significantly bigger trunk sizes than the ones Pacific.

Introducción: Los sipúnculos son un grupo de gusanos marinos sin segmentación poco estudiados en el Pacífico oriental tropical (POT). Antillesoma antillarum es una especie perteneciente a la familia monoespecífica Antillesomatidae la cual se consideraba que se distribuía ampliamente en distintas localidades tropicales y subtropicales del mundo. Objetivo: El objetivo principal del trabajo fue examinar las diferencias morfológicas y moleculares entre ejemplares de ambas costas de América. Métodos: Para el análisis morfológico se revisó material del Caribe mexicano y del Pacífico sur de México. Para los análisis moleculares se obtuvieron secuencias del marcador mitocondrial COI de ejemplares de A. mexicanum sp. nov. de la playa de Panteón en Puerto Ángel, Oaxaca del Pacífico sur de México; también se incluyeron cuatro secuencias de GenBank de A. antillarum de diferentes localidades para la comparación filogenética con el método de Máxima Verosimilitud. Se calcularon las distancias genéticas con el modelo Kimura 2P y fueron comparadas con valores de referencia. Resultados: El análisis filogenético evidenció tres linajes diferentes: Antillesoma antillarum sensu stricto del Mar Caribe y Florida, el grupo hermano representado por nuestra recolecta en el Pacífico mexicano y un tercer grupo de Tailandia. Conclusión: Basados en datos morfológicos y moleculares, Antillesoma mexicanum sp. nov. fue descrita para el Pacífico mexicano, que difiere de A. antillarum en las papilas del tronco, el patrón de coloración y, adicionalmente, los ejemplares del Caribe fueron significativamente más grandes que los del Pacífico mexicano.

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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.

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The family Chrysopetalidae has been poorly studied from the Pacific coast of Mexico compared to other families. Specifically from the southern Mexican Pacific, only two species of the family have been recorded in previous studies, Chrysopetalum occidentale and Paleanotus chrysolepis. In this study 311 specimens were revised, and nine species, belonging to six genera were found. Eight of these species are new records from the southern Mexican Pacific coast: Arichlidon watsonae n. sp., Bhawania cf. goodei, Chrysopetalum elegantoides, C. maculata, Hyalopale sp., Paleaequor psamathe, Paleanotus bellis and P. purpurea. A new species is also described, Arichlidon watsonae n. sp., which is characterized by the curved tips blades of the falcigers and their small and ovoid palps. Additionally, the genera Arichlidon and Hyalopale are recorded by first time from the Tropical Eastern Pacific.

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Sabellids and serpulids are two well represented families in the polychaete fauna of the Tropical Eastern Pacific, with 31 and 34 species respectively; however, most records come from the Gulf of California or the western coast of Baja California Peninsula. Only a few records are from localities in the large expanse of the central and southern Mexican Pacific. Thus, sabellids and serpulids were collected from several shallow water habitats along the coast of Mexican Pacific, such as coastal lagoons, coral reefs, rocky shores and from man-made structures as marinas, piers and ships of several harbors; additionally, specimens from national collections were revised. More than 8,400 specimens of sabellids and serpulids from the states of Baja California, Baja California Sur, Sonora, Sinaloa, Michoacán, Guerrero, Oaxaca and Chiapas, and some specimens from Panamá and Perú were examined. In the present work we record new localities of four sabellids and 24 serpulids. One sabellid, Branchiomma bairdi, is an exotic/invasive species in Oaxaca, Sinaloa and Baja California Sur, while four species of serpulids are exotic and/or cryptogenic species: Ficopomatus uschakovi, Hydroides dirampha, H. elegans and H. sanctaecrucis. Additionally, the geographical range has been extended for five species: the sabellids Pseudobranchiomma punctata from Oahu, Hawaii to La Paz Bay, and Parasabella pallida from California to Puerto Escondido, Baja California Sur; and for three serpulids, Hydroides inermis from the Galápagos Islands to Agua Blanca, Oaxaca, H. gairacensis from Panamá to Puerto Ángel, Oaxaca, and H. panamensis from Panamá to Huatulco, Oaxaca and Faro de Bucerías, Michoacán. Hydroides cf. amri, previously recorded as H. brachyacantha from Oahu, Hawaii, is more similar to H. amri from Australia. The number of sabellids recorded for the Tropical Eastern Pacific increased to 33, the serpulid species to 35.

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