The Transversotrematidae is a small family of trematodes (comprising just 27 species in four genera) that infect a diverse range of teleost fishes belonging to 25 families. The family is predominantly marine and has been reported widely in the tropical Indo-west Pacific. Due to their unique infection location (under the scales of their hosts), transversotrematids are seldom found in routine fish dissections, and the richness of species in the tropical Indo-west Pacific is largely a result of focussed studies in Australian waters, especially on the Great Barrier Reef (Manter, 1970, Cribb et al., 1992, Hunter and Cribb, 2010, Hunter and Cribb, 2012, Hunter et al., 2010, Hunter et al., 2012, Cutmore et al., 2016).

Almost all marine species in this family belong to a single genus, Transversotrema Witenberg, 1944, and unlike most marine trematode genera, almost all Transversotrema species are already represented by genetic data. Of the 22 marine Transversotrema spp., all but two are represented by ITS2 rDNA data. ITS2 data are highly conserved for transversotrematids, with presently recognised species differing at as little as a single base position. Similarly low levels of variation in ITS2 data have been found during recent studies of tropical Indo-west Pacific Aporocotylidae (Cutmore et al., 2021), Bivesiculidae (Cribb et al., 2022), Enenteridae (Huston et al., 2019, Huston et al., 2022), Gorgocephalidae (Huston et al., 2021), and Lepocreadiidae (Bray et al., 2018b, Bray et al., 2022). These studies found that the low levels of variability in ribosomal data make it impossible to reliably distinguish inter-specific differences from geographical variability or to identify cryptic species. For all these studies, the incorporation of mitochondrial data was essential to improving delineation of species and understanding of their geographical distributions.

Here we use mitochondrial data to explore richness, host specificity and geographical distribution within Transversotrema, generating novel cytochrome c oxidase subunit 1 (cox1) mtDNA data for specimens from fishes of Australia, French Polynesia, Japan and Palau. These new data demonstrate a range of distinction within the genus. Some species are identifiable based on morphological data alone, some on host range, some on geographical location, some on a combination of these three data sources, and some are entirely cryptic (same locality, same host and identical morphology) and only identifiable by mitochondrial sequence data.