Monorchiids of silverbiddies (Gerreidae) from Queensland, Australia, including two new genera and species.

Five species of monorchiids are known from fishes of the family Gerreidae, of which one is from Australian waters. Here, we report it and two new monorchiids from three species of Gerres Quoy & Gaimard, 1824 from off Lizard Island, northern Great Barrier Reef, and Moreton Bay in south-eastern Queensland: Gerres oyena (Forsskål), Gerres oblongus Cuvier and Gerres subfasciatus Cuvier. One of the new species, found only in G. oblongus at Lizard Island, conforms most closely to the concept of Proctotrema Odhner, 1911. However, it differs from species of Proctotrema in oral sucker shape and location of intestinal bifurcation and termination. It is phylogenetically distinct from two sequenced species of Proctotrema; thus, we propose Obscuromonorchis ranae n. g., n. sp. The second new species infects all three gerreids, occurs at both Lizard Island and Moreton Bay, and is morphologically most similar to the concept of Monorchicestrahelmins Yamaguti, 1971. However, the combination of the length of the caeca, size of the testis and post-testicular region, and the form of spination in the genital atrium presents a clear genus-level distinction that warrants proposal of a new genus. There are no molecular data for the three recognised species of Monorchicestrahelmins. We propose Argenticola shuyinae n. g., n. sp. for this species. New specimens of Gerricola queenslandensis Wee, Cutmore & Cribb, 2021 were collected from off Lizard Island and Moreton Bay. The three species form a well-supported clade but with internal branch lengths and topology consistent with genus-level differentiation.