There is considerable confusion over the identification of ‘carp gudgeons’ in southeastern Australia. Genetic studies in the early 2000s showed that there were several sexual species of carp gudgeon occurring in coastal and inland drainages in eastern Australia, as well as a range of hemiclonal unisexual hybrids.
Many of the carp gudgeon groups formerly considered as ‘species’ were not formally described (e.g. Lake’s, Murray-Darling, and Midgley’s carp gudgeons), with one of these (Lake’s) now considered to only consist of hemiclonal hybrids. Because many of the hemiclonal and sexual lineages look very similar, for ~20 years carp gudgeons in the MDB have been combined into a generic group and referred to as ‘Hypseleotris spp‘ (i.e. Hypseleotris of undefined species). In late 2022 the taxonomic situation became clearer with five species formally described or redescribed, plus a range of hemiclonal hybrids identified.
In the MDB there are four sexual species present (Western, Bald, Boofhead, and Cryptic carp gudgeons). As well as this there are multiple hemi-clonal unisexual lineages of hybrid origin between the latter three species. In some parts of the MDB up to three sexual species co-occur with three hemiclonal forms, all being captured in the same location. The combining in the scientific literature of all carp gudegons into a single group means that most published ecological information cannot be attributed to a particular species, but for one species that does not form hybrids (Western carp gudgeon) and the most recently discovered species (Bald carp gudgeon) some information can be accurately attributed. Consequently, apart from some information to aid identification, on this website, all but the two of the non-hybrid species (Western and Bald) are treated as a group.
As a group, all carp gudgeons are small and laterally compressed, with a truncate to slightly rounded tail, and small, upturned mouths. Maximum total length varies from 40–70 mm (generally < 60 mm). They have two separate dorsal fins. Fin colouration and size varies between the males of each species, but usually the anal and dorsal fins have a series of coloured bands which can be white, bluish, or orange through red. Western and Cryptic carp gudgeons (and some hybrids) get red and orange colour on their caudal fins. Females of all species have clear fins, but in breeding season have bright yellow, orange or pink coloured bellies. Males of both Bald and Boofhead carp gudgeons (and some hybrid combinations) get very ‘boof-headed’, while Western carp gudgeon has a less blunt head profiles, and the head profile of Cryptic is round to pointed.
Body colouration is yellowish-grey to greenish-brown and the scales of the upper sides and back have darker edges, giving a slightly reticulated appearance.
The following account of biology and habitat is of information gathered before the taxonomy of the group was clarified, but in general is likely to apply to many of the species. This group of species is found in slow-flowing or still waters, normally associated with macrophytes or other aquatic vegetation (which provides cover). Two to three sexual species of carp gudgeon plus three hemiclonal hybrids often occur sympatrically. It is unknown how multiple sympatric carp gudgeon taxa divide habitat, food and other resources between them to maintain their individual populations. Carp gudgeons are generalist carnivores commonly feeding on chironomids, ostracods, copepods and cladocerans, and select habitats that provide optimal feeding opportunities (e.g. abundant biofilms which support preferred prey such as chironomids).
Carp gudgeon species (other than Western carp gudgeon) lay between 200–600 eggs on a hard object near the substrate where the males guard them for 5–8 days until hatching. Larvae are ~3 mm at hatching.
Recent investigations have demonstrated that large numbers of carp gudgeons attempt to move through fishways on the Murrumbidgee and Murray rivers. Whether these attempted movements simply reflect local dispersal movements is unknown. Low light levels (e.g. in road culverts) could be a barrier to carp gudgeon movement. A study in the Condamine and Macintyre rivers in the northern Basin found that carp gudgeons as small as 40 mm (TL) were recorded moving up to 13.7 km upstream over 12 months, with upstream movement of 11 km occurring over a maximum of 20 days and with some individuals moving 2 km in 4 days.
Downstream movements of more than 6 km downstream over 8 months, were also recorded, Peak movement generally occurred in spring, with fish tending to move both upstream and downstream on all flows but more downstream movement occurred on natural (not irrigation) receding flows. Recent research into how the hemiclonal hybrids transmit their genome to offspring shows that either the maternal or paternal genome is selectively eliminated from the germ (reproductive) cells (ova or sperm), so only one genome is transmitted.
As a group, carp gudgeons are widespread and abundant at mid to lower elevations (up to 220–550 m) in the southern and central MDB, and up to 1000 m elevation in the northern MDB. Carp gudgeons comprised >50% of the fish catch in wetlands in SA, almost 5 times the abundance of the second commonest native species (Unspecked hardyhead). In the Sustainable Rivers Audit (2004-13), of the 27,441 carp gudgeons captured, 29% came from the lowlands (<250 m ASL); 40% from the slopes (250–700 m ASL); 18% from uplands (700–1000 m ASL) and 13% from montane sites (> 1000 m ASL). Virtually all the montane records were from the Border Rivers. Carp gudgeons comprised 13% of the total fish catch and were the second-most abundant native taxa caught (behind Bony herring) and overall were fourth behind Bony herring, Eastern gambusia and Carp).
In the Murray Darling Basin Fish Survey 54,783 carp gudgeon have been caught between 2014/15 and 2021/22. They were recorded from 21 MDB river valleys (not the Avoca or the Wimmera) but were recorded from the Wimmera in the SRA.
Carp gudgeons are also found in coastal streams from central NSW to northern Qld. Typically, most carp gudgeon species are common across the Basin.
The distribution map shows all carp gudgeon records (sexual species and unsiexual hemiclones) to demonstrate the widespread range of this group.
The group is widespread and abundant. Predation by alien fish such as Redfin perch in floodplain ponds/billabongs can significantly reduce local abundance. Alienation of floodplains and barriers to fish migration likely alter localised foodwebs localised abundance.
Anderson et al. 1971; Balcombe & Closs 2000, 2004, 2016; Baumgartner 2003; Bertozzi et al. 2000; Dove 1998; Dove et al. 1997; Hutchison et al. 2008; Jones et al. 2017; Larson & Hoese 1996a; Majtánová et al. 2021; McCasker 2009, McCasker et al. 2014; Schmidt et al. 2011, 2013; Smith et al. 2009; Stoffels & Humphries 2003; Thacker et al. 2022; Thacker & Unmack 2005; Unmack 2000, Unmack et al. 2019; Vilizzi & Tarkan 2016.
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