The marine microturbellarian diversity is scarcely known from the Pacific coastline of South America. The coastal margin of Peru is highly influenced by cold-water currents (Humboldt Current System). These cold-water currents play important roles in shaping the marine species diversity in this area. However, there are no previous records of marine microturbellarians for Peru. Here, we describe a new species of Rhabdocoela and report on five other species of marine microturbellarians for this country. Carcharodorhynchus giaae n. sp. (Kalyptorhynchia, Schizorhynchidae), can be identified based on the configuration of the proboscis lips and the male and female genital systems. The male copulatory organ lacks a cirrus, but has a funnel-like, curved stylet with three pairs of spines along it, which distinguishes this species from its congeners. The female system has a vagina with a sclerotized, heavily coiled structure — a feature not mentioned in any known species of the same genus. In addition, we describe noteworthy features for another rhabdocoel, Trigonostomum galapagoense (Dalytyphloplanida, Trigonostomidae). Finally, we provide new records for three rhabdocoel species (Alcha evelinae, Itaipusa divae, Utelga heinckei), one species of Fecampiida (Urastoma cyprinae), and an unidentified species of Prolecithophora.

Marine microturbellarians are an assemblage of meiofaunal flatworms abundant in sediments and on seaweeds around the world. The diversity and distribution of these animals in Japan are poorly understood. Here, we provide an overview of all recorded species in Japan and characterize two new species of the rhabdocoel genus Reinhardorhynchus based on morphological features and a molecular phylogeny inferred from 18S and 28S rDNA sequences. Reinhardorhynchus ryukyuensissp. nov. can be distinguished from other species in the genus by the lack of an armed cirrus and by the presence of two larger opposing hooks and five smaller interconnected hooks in its male copulatory organ. Reinhardorhynchus sagamianussp. nov. differs from its congeners because its male copulatory organ combines a bipartite cirrus armed with a belt of overlapping scale-like spines, an unarmed accessory cirrus, and two large distal accessory hooks. Our molecular phylogenetic analyses show that R. ryukyuensissp. nov. and R. sagamianussp. nov. form a clade with all the other species of Reinhardorhynchus for which DNA sequence data are available. Within this clade, R. sagamianussp. nov. is in a clade that also includes R. riegeri and R. anamariae. The discovery of these new species highlights the importance of uncovering and documenting the hidden biodiversity along Japan’s coastal margin.

Species of Macrostomum are present in almost all regions of the world. These flatworms occur in different kinds of environments, from freshwater to marine ecosystems. Although Peru is considered one of the megadiverse countries in South America, little is known about freshwater microturbellarians in this country. Here we report the presence of M. rostratum Papi, 1951 for the first time in Peru, in a coastal brackish water wetland, and that of M. quiritium Beklemischev, 1951 and M. tuba Graff, 1882 in an inland portion of a river near the city of Lima. All the species were studied alive. Comments on the habitat and associated fauna are given. Detailed information on the copulatory system is provided for M. rostratum; the male system has a bilobed seminal vesicle with thick walls and full of spindle-shaped sperm cells, and a stylet in the form of a solid curved inverted cone with a subterminal opening located on the concave side of the stylet. These findings led us to suggest that M. rostratum may exhibit a hypodermic mating syndrome. The three species of Macrostomum reported here are widespread and, although the diagnostic characters of the species are present, some morphological differences in the copulatory stylet suggest they are cryptic species.

Polyclads are marine free-living Platyhelminthes, generally disregarded in marine environmental assessments. Here, Phrikoceros inca (Cotylea, Pseudocerotidae), identified on the basis of external features such as its ear-like, held-erected pseudotentacles, number and position of pseudotentacular eyes, circular-like cerebral eyespot, ruffled-shaped body and coloration patterns, was recorded for Peru for the first time. This finding, which occurred in Pucusana beach, near Lima, on the central coast of Peru, extends the distribution of P. inca ~1700 km north, thus becoming the northernmost record of this polyclad species. Also, the formally described species of polyclads from Peru are reviewed and listed.