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).