Conclusions
This study has revealed that carp gudgeons are even more speciose than previously thought, adding several additional candidates to the existing six sexual species plus their various hemiclonal relatives (Unmack et al., 2019; Thacker et al., 2022a; Thacker et al., 2022b). Even ignoring the complication of sympatric hemiclones, many river basins contain at least three or more sexual species, with the geographically-extensive MDB notably harboring six sexual taxa (plus multiple hemiclones). Moreover, the observed partial mismatch between geographic and phylogenetic patterns, plus the presence of a natural hybrid zone in the lower Murray and in at least two Darling tributaries add yet more layers of complication. Given such complexity, future taxonomic and field identification efforts will be particularly challenging, and ideally require the involvement of a molecular identification technology (with SNPs providing the gold standard of unequivocal identification all sexual and unisexual forms) as part of a coordinated accumulation of companion morphological exemplars of each morphotype at each site surveyed. Our own research group has already adopted this strategy where resources permit.
The dynamic boom and bust nature of many Australian freshwater ecosystems highlights the need for monitoring spatial genetic patterns for all resident species over time, particularly after major climate events such as have impacted eastern Australia over the past decade (Hughes, Schmidt & Finn, 2009; Lintermans, 2013; Legge et al., 2022). Our study provides future researchers with a framework to pursue such an endeavor for the Hypseleotris of eastern Australia. As a major component of the biodiversity and ecology of these ecosystems, carp gudgeons also offer great potential for environmental monitoring, provided researchers can identify individuals to their correct taxon. In the past carp gudgeons have often been lumped into one composite ‘taxon’ (Hypseleotris spp.) when included in ecological surveys (Lintermans, 2007), a custom that precludes any genuine assessment of whether Hypseleotris alpha diversity has declined or shifted (e.g. hemiclone ratio/presence) at such sites. Finally, this study further underlines the point that active conservation and management practices for freshwater fishes, including both the intended (i.e., wrong genetic lineage used) and unintended (i.e., where carp gudgeons or other non-target species unknowingly contaminate the hatchery release event) consequences of fish stocking programs (Lintermans, 2004), need to be mindful of the existence of both undescribed candidate species, and deep phylogeographic structure within all species, to avoid undertaking or facilitating translocations or mixing of distinct genetic lineages.