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.

Free-living turbellarians mostly live in marine and freshwater environments, but they have been little considered in ecological studies. The coastal plain in southern Brazil harbors a diverse fauna and flora, but only a few studies have related environmental factors to the abundance, richness, and distribution of turbellarians. Hence, we analyzed the structure of turbellarian communities in floating vegetation in three differently sized limnic environments. We aimed to determine differences in abundance, density, and richness of turbellarians among these habitats in relation to environmental factors. We found 1,225 turbellarians (34 species) distributed among 6 taxa. The most abundant was Tricladida with 49.4% relative abundance; the remaining (Catenulida, Dalytyphloplanida, Macrostomorpha, Kalyptorhynchia, and Prorhynchida) were less abundant. We found no significant differences among population parameters and environments. Small shallow lakes are a potentially rich environment, while water channels are the richest environment. Larger shallow lakes are not very diverse for turbellarians. Similarities regarding turbellarian communities among close limnic systems could be explained by the connectivity of bodies of water that presents similar aquatic macrophytes and hydrological subsystems, while larger shallow lakes have a different community because they have their own contributing basin and could be influenced by other factors.

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Within Dalyelliidae Graff, 1905, Gieysztoria Ruebush & Hayes, 1939 is the most widely distributed and speciose genus, members of which live in marine, brackish, or freshwater habitats (Van Steenkiste et al., 2012). Gieysztoria is composed by ~97 free-living species (Tyler et al. 2016), and species identification is mainly made on the male copulatory system, which has an armed penis (stylet) with different configurations of spines (Noreña et al., 2016). The stylet configuration of Gieysztoria species is traditionally split into two groups: Aequales (spines of similar shape and size) and Inaequales (spines of different shape and size) (Luther, 1955). In the neotropics, specimens of Gieysztoria belong to both the Aequales and Inaequales group   and are well represented (Damborenea et al., 2005; Noreña et al., 2003). According to Braccini et al. (2016, 2017), there are 16 species of Gieysztoria in Brazil, mainly distributed in the southern region. South Brazil harbours a mosaic of wetlands that are considered hot spots of biodiversity, including the Taim Ecological Station (ESEC Taim) protected area that was designated as a Ramsar site (Ramsar, 2017). In the present study, Gieysztoria falx Brusa, Damborenea & Noreña, 2003 is registered in the ESEC Taim, located in Rio Grande do Sul. It represents the first record of G. falx in Brazil. Illustrations and comments on stylet configuration are given.

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.