First report of Gieysztoria falx Brusa, Damborenea & Noreña, 2003 (Platyhelminthes: Rhabdocoela) in southern Brazil.

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.