Broad patterns within and between candidate species
Taxon KN has an unusual distribution and genetic pattern. The southern portion of the Fitzroy River Basin is quite different to the northern portion, a pattern not replicated in any species examined so far. The species is unknown from coastal basins between the Fitzroy and Burdekin Basins except for the small Herbert Creek catchment (tissues were not available for this study). This absence appears to be real, with moderately-intensive sampling only finding its congener H. bucephala at many sites. Within the Burdekin Basin, KN has a narrow distribution, being found primarily in Lake Dalyrmple and in the Burdekin River upstream of the dam until the vicinity of Charters Towers. It is absent from the arid Belyando River, the main southern tributary to Lake Dalyrymple and containing habitats that are otherwise commonly inhabited by H. klunzingeri . Again, H. bucephalais present at many sites across the entire Burdekin Basin. Either KN was once widespread in coastal basins north to the Burdekin Basin, or it crossed over the drainage divide between the Fitzroy and Burdekin basins, such as via the Rugby portal (Georges et al. 2018) with subsequent extirpation of intervening populations. It is also not fully possible to rule out that KN have been translocated to the Burdekin Basin from the northern Fitzroy Basin as fish stocking contaminants.
The separation between taxa KN and KE corresponds to the Boyne River and Baffle Creek catchment boundary. This separation is also found between two rainbowfish species (Melanotaenia splendida and M. duboulayi ) as well as two hardyhead species (Craterocephalus fulvus and C. stercusmuscarum ), plus is represented by a major disjunction in Hypseleotris acropinna . It also represents the northern most distribution points of two other species (Retropinna semoni and Philypnodon macrostomus ) which do not occur north of Baffle Creek (Unmack 2001). Taxon KE is continuously distributed and common in most streams from Baffle Creek south to the Clarence River. Populations are primarily structured by river basin, with a deeper divergence between Baffle Creek, Burnett River, the Burrum system south to the Maroochy River and populations from the Caboolture River south to the Clarence River. Two populations represented geographic outliers. The first is a clearly introduced population in the Barron Basin (site 1), with a likely origin from the Burrum Basin. The second is the presence of fish in Barkers Creek (site 16, Burnett system) that display both nuclear and matrilinear evidence of admixture between two otherwise distinctive “northern” and “southern” phylogroups. It is unclear from our data if this represents a natural occurrence or a past translocation event.
Complex geographic and genetic patterns are found for H. klunzingeri populations within the MDB. Befitting its extensive geographic coverage and low-relief topography, the basin harbors the pure taxa KS, KE+, and KW+, the latter two showing evidence of historic admixture with KS (as lineages KEm and KWm), plus a relatively-recent and possibly ongoing hybrid zone (KSxKE) between upstream KE+ and its downstream congener KS. Within KS there is no strong pattern of phylogeographic structure. These populations are primarily found in the Murray subcatchment upstream of the Darling River confluence, along with the Macquarie, Gwydir, Namoi and Macintyre subcatchments. Taxon KE+ is restricted to the Condamine-Balonne subcatchment. These KEm lineage individuals have obvious genetic affinities to populations from the Burnett River, a common pattern for those MDB species which are also found in coastal river basins in southeastern Queensland (Unmack 2013). Fish with the KSxKE genetic profile are present in the Darling River south into the lower Murray River, plus in the Castlereagh and Bogan subcatchments. These are likely a result of KEm fish from the Balonne River dispersing further downstream, but also managing to push upstream into nearby tributaries like the Bogan and Castlereagh. Contemporary patterns in the Darling River are likely to vary over time as drought eliminates populations (due to excess water extraction), with recolonisation either coming via floodwaters from either the Balonne (introducing more KEm fish) or Macintyre/Barwon rivers (taxon KS fish), carrying different genotypes into the lower Darling River. The western portion of the Murray-Darling Basin in the Warrego and Paroo subcatchments has admixed populations between KS and KW, represented by KWm. These populations share a long drainage basin boundary with both Bulloo River and Cooper Creek (which contains pure KW). One other fish species has crossed from the Lake Eyre Basin rivers into the Paroo and Warrego subcatchments,Melanotaenia splendida (Lintermans 2007), while the turtleEmydura macquarii has crossed from the Bulloo River into Paroo (Georges et al. 2018). The most likely spot for faunal exchange is via the Bindegolly portal (Georges et al. 2018).
There have been four invasions into east coast river basins of KS from the MDB. There first is located in the upper Maryvale River in the upper Clarence Basin (site 41) which has a mitochondrial haplotype identical to those adjacent in the Border Rivers subscatchment (upper Macintyre River), along with a similar relationship based on SNPs (Figs. S2, S3). The second invasion occurred in the upper Macleay River in Salisbury Waters (site 49), which is adjacent to the Gwydir subcatchment. The fish from Salisbury Waters are most similar to those from the Border Rivers subcatchment in the upper Macintyre River for SNPs, while for mitochondrial DNA from the upper Macintyre and the Gwydir subcatchments were similar. The third transfer occurred with the Hunter Basin. This population has long been considered likely native as they are known to be widespread, although patchy in occurrence, and several other fishes are shared with the Hunter, but not in surrounding coastal basins (e.g.,Craterocephalus amniculus , Mogurnda adspersa ), or they are also present in some additional surrounding basins (e.g., Tandanus tandanus ). Hunter Basin KS (sites 50, 51) had a clear genetic affinity with fish from the Murrumbidgee subcatchment rather than the adjacent Macquarie and Namoi subcatchments. The fourth coastal basin population was found in the Shoalhaven Basin (site 52). This population was genetically closest to fish primarily from the adjacent Murrumbidgee subcatchment.
It is tempting to speculate on whether these four geographic outliers represent native populations or human-mediated introductions. While many such introductions likely remain undocumented, over 50 Australian freshwater fishes are already known or presumed to have been either deliberately or accidentally introduced into catchments outside their native range (Lintermans 2004). If these four coastal KS populations were native, we would expect each to show the greatest genetic similarity to its adjacent MDB population. This is the case for three of these comparisons, with the Hunter Basin being the exception. In addition, we might expect native occurrences to have broader distributions within coastal basins provided they have had a large period of time to disperse. Instead, all but the Hunter appear to only harbor localised populations which have not dispersed far. Introductions could come from nearby populations, thus mimicking a native pattern, or they could be from distant populations, if accidentally introduced along with deliberately-stocked, hatchery-reared sportfish. Unfortunately, each of these basins lacks early historical records, a common situation for Australia’s freshwater fishes. At this stage we consider these four populations are likely introduced, although we acknowledge the evidence is equivocal.
The presence of three out of the four candidate species in the MDB is an unusual distributional pattern. The majority of species of native fish known to occur in the MDB do not share the Basin with widely-distributed and truly-sibling congeners, the exceptions being Craterocephalus fluviatilis (Murray Hardyhead) and C. amniculus (Darling Hardyhead) and several Galaxias species in the mountain galaxiid complex which are closely related (Lintermans 2007; Raadik 2014). Some introgression has been recorded in both fish groups (Adams et al. 2011; Adams et al. 2014). There are also three congeneric species groups present in the MDB which are known to produce hybrids from the generaMaccullochella (Douglas et al. 1995) and Philypnodon(Hammer et al. 2019). In other groups there are examples of introgression such as in the genus Melanotaenia between three species (Unmack et al., unpublished data), along with an admixture zone in the genus Retropinna (Hammer et al. 2007; Unmack et al. 2022). In addition, there is the hemiclonal complex of Hypseleotris carp gudgeons which have hybrid origins (Unmack et al. 2019). Given that even distantly-related fish species are known to readily hybridize (Vespoor & Hammar 1991), the MDB provides considerable opportunities for mixing gene pools from different colonisations and reinvasions of the basin from surrounding river basins over evolutionary time frames across a range of species with different levels of genetic divergence. It is also likely that opportunities for hybridisation have increased as natural habitats in the MDB have been anthropogenically altered or degraded (Scribner, Page & Bartron, 2001; Lintermans, 2007).