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.

The description and delineation of trematode species is a major ongoing task. Across the field there has been, and currently still is, great variation in the standard of this work and in the sophistication of the proposal of taxonomic hypotheses. Although most species are relatively unambiguously distinct from their congeners, many are either morphologically very similar, including the major and rapidly growing component of cryptic species, or are highly variable morphologically despite little to no molecular variation for standard DNA markers. Here we review challenges in species delineation in the context provided to us by the historical literature, and the use of morphological, geographical, host, and molecular data. We observe that there are potential challenges associated with all these information sources. As a result, we encourage careful proposal of taxonomic hypotheses with consideration for underlying species concepts and frank acknowledgement of weaknesses or conflict in the data. It seems clear that there is no single source of data that provides a wholly reliable answer to our taxonomic challenges but that nuanced consideration of information from multiple sources (the ‘integrated approach’) provides the best possibility of developing hypotheses that will stand the test of time.

The techniques employed to collect and store trematodes vary between research groups, and although these differences are sometimes necessitated by distinctions in the hosts examined, they are more commonly an artefact of instruction. As a general rule, we tend to follow what we were taught rather than explore new techniques. A major reason for this is that there are few technique papers in the published literature. Inspired by a collaborative workshop at the Trematodes 2024 symposium, we outline our techniques and processes for collecting adult trematodes from fishes and discuss the improvements we have made over 40 years of dissections of 20,000+ individual marine fishes. We present these techniques for two reasons: first, to encourage unified methods across the globe, with an aim to produce optimally comparable specimens across temporal periods, across geographic localities, and between research groups; and second, as a resource for inexperienced researchers. We stress the importance of understanding differences in host biology and the expected trematode fauna, which ultimately enables organised and productive dissections. We outline our dissection method for each key organ separately, discuss handling, fixation, and storage methods to generate the most uniform and comparable samples, and explore ethical considerations, issues of accurate host identification, and the importance and potential of clear record keeping.