Four new monorchiids from marine teleost fishes of Moreton Bay and the Great Barrier Reef, Australia, including the proposal of a new genus.

We report four new species of monorchiids infecting teleost fishes from Australian waters. Two new species of Paralasiotocus Wee, Cutmore, Pérez-del-Olmo & Cribb, 2020, Pa. abstrusus n. sp. and Pa. tectus n. sp., are described from haemulids of the Great Barrier Reef. The two species are morphologically cryptic and occur in sympatry but differ significantly in cox1 mtDNA and ITS2 rDNA sequence data. Paralasiotocus tectus n. sp. is found only in Plectorhinchus albovittatus (Rüppell) whereas Pa. abstrusus n. sp. infects Pl. albovittatus, Plectorhinchus flavomaculatus (Cuvier) and Plectorhinchus lineatus (Linnaeus). The two species differ from all known species of Paralasiotocus in the possession of a clear gap in the spines of the terminal organ. A new species is described from a mullid, Parupeneus spilurus (Bleeker), from off Heron Island and Moreton Bay. The new species is morphologically broadly consistent with the concept of Paralasiotocus in the possession of an unspined genital atrium, bipartite terminal organ, and lobed ovary. However, it possesses a highly lobed cirrus and is phylogenetically widely separated from the two species of Paralasiotocus characterized here, and thus we propose Lobucirruatus infloresco n. g., n. sp. Proctotrema prominens n. sp., is described from Pl. albovittatus. It is differentiated from all other species of Proctotrema in the combination of a prominent metraterm, slightly fusiform body, slightly funnel-shaped oral sucker, elongate cirrus-sac, unlobed ovary, and caeca that terminate in the post-testicular region.