New findings on Rhynchonema Cobb, 1920 (Nematoda: Monhysterida): description of three new species from Brazil and illustrated key to the genus.

Three new species of the genus Rhynchonema Cobb, 1920 are described from sediment samples collected in the Potiguar Basin, Brazil. Rhynchonema brasiliensissp. nov., is characterized by a circular amphideal fovea, located on an amphideal plate (wider cuticle annules which accommodate the entire fovea), short spicules, almost without curvature, gubernaculum with elongated dorsal apophysis and three precloacal supplements. Females are similar to males and have a vagina with visible sclerotization. Rhynchonema nordestinumsp. nov., has an oval amphideal fovea, robust and slightly curved spicules, and a gubernaculum with a dorsal apophysis. Rhynchonema parvumsp. nov., has a small and circular amphideal fovea, the smallest within the genus, thin and curved spicules, gubernaculum with a poorly developed dorsal apophysis. Rhynchonema species were separated into groups. These groupings were designated based on the morphological and morphometric characteristics considered most relevant for specific identification and they are presented as an illustrated key. The presence of an amphideal plate, the shape and size of the fovea, as well as the morphology of the males' copulatory structures are examples of the characteristics used to establish groups of species. The diagnostic characteristics of all species were considered and used in the preparation of a comparative table.