Introduction
Phylogeographic assessments of freshwater fishes have commonly revealed cryptic species (e.g., Baumsteiger, Kinziger & Aquilar, 2012; Adams et al., 2014; Pinacho-Pinacho et al., 2018), thereby improving knowledge of species richness and diversity in aquatic ecosystems globally (Seehausen & Wagner, 2014). In Australia, for example, the number of recognised (but not necessarily described) species increased by 39 between 2002 and 2013 (Allen, Midgley & Allen, 2002; Unmack, 2013), with many of the newly-defined species revealed by molecular phylogenetic and phylogeographic evidence (Adams et al. 2023). Furthermore, phylogeographic studies have indicated that historical, geological, and/or climatic processes can be determinants of contemporary patterns of biodiversity and distribution across riverine landscapes (e.g., Buckley et al., 2021; Shelley et al., 2020; Waters, Burridge & Craw, 2020). Consequently, the phylogeographic assessment of freshwater fishes provides foundational diversity and biogeographic knowledge upon which valid ecological and environmental management studies are dependent (Page et al., 2017).
In this study, we present a molecular phylogeographic assessment of the western carp gudgeon (Eleotridae: Hypseleotris klunzingeri ) throughout its entire Australian range. This small-bodied freshwater fish (approximate maximum size is 60 mm TL) is one of the most abundant and widely-distributed species in eastern Australia (Unmack, 2000; Pusey, Kennard & Arthington, 2004). Here it occurs as a disjunct northern population in the Burdekin Basin in central-northern Queensland, then in coastal drainages from just north of the Fitzroy Basin to the Clarence Basin, as three disjunct southern populations in the Macleay, Hunter and Shoalhaven basins in central New South Wales, inland throughout the Murray-Darling Basin (MDB), and finally west to the Bulloo River and Cooper Creek in the Lake Eyre Basin (Fig. 1; Table1). Hypseleotris klunzingeri is often found in sympatry with other carp gudgeons, all belonging to a species complex comprising a suite of other sexual and hemi-clonal congeners, but is reproductively isolated from them (Bertozzi, Adams & Walker, 2000; Schmidt et al., 2011; Unmack et al., 2019).
As reviewed by Pusey et al. (2004), H. klunzingeri occurs in a wide range of lotic and lentic habitats, with aquatic plants and leaf-litter beds, and slow-flowing water (i.e. <0.2 m/s) the preferred (but not exclusively used) habitat features. The species is primarily benthic, with a diet dominated by microcrustaceans and macroinvertebrates (e.g., chironomids, ephemeropterans and trichotperans). Hypseleotris klunzingeri has a wide range of water quality tolerances but is only known from upper estuarine areas downstream of tidal barrages that prevent upstream movement, indicating that estuarine habitats are not naturally used by the species. Spawning can occur most of the year (excluding winter), but peaks in spring and early summer in response to increasing water temperature and daylight length, and possibly flow events, with flow events reported to catalyse the onset of mass migration of H. klunzingeri (Pusey et al., 2004).
As with many other Australian freshwater fishes, the taxonomy and distribution of H. klunzingeri has historically been equivocal, having first been mistaken by Klunzinger (1872, 1880) as H. cyprinoides (which does not occur in Australia), then described asCarrassiops klunzingeri by Ogilby (1898), to be later reassigned to the genus Hypseleotris . Its common name of western carp gudgeon relates to the early thought that the taxon primarily occurred west of the Great Dividing Range compared with the more easterlyH. galii (e.g., Anderson, Lake & Mackay, 1971), whereas today we know that representatives of both lineages naturally occur both east and west of the Great Dividing Range (Thacker, Geiger & Unmack, 2022a). While the current nomenclature suggests a robust taxonomic status, matrilineal phylogeographic data presented by Thacker et al. (2007) indicated several divergent lineages within H. klunzingeri that to date have not been closely re-assessed. Furthermore, their phylogeographic analyses suggest the low elevation divide between the coastal Burnett River and the Condamine River in the northern MDB is the pathway by which H. klunzingeri was exchanged between these basins (see also Unmack, 2013).
The purpose of this study is to present a thorough molecular phylogeographic assessment of H. klunzingeri , using comprehensive sampling and an expanded suite of molecular datasets. Regarding the latter, our choice of two independent sets of co-dominant nuclear markers (single nucleotide polymorphisms [SNPs] and allozyme loci), plus matrilineal sequence data (the mtDNA gene Cytochrome b[cytb ]), ensures that this study is ideally placed to help resolve the above-described taxonomic uncertainty and re-evaluate biogeographic patterns in this prominent Australian freshwater fish.