Carp gudgeons are a mysterious species typically found in slow-flowing or still waters that frequently baffle fish enthusiasts and scientists alike. A small and laterally compressed species, the carp gudgeons have caused great confusion over their identification across southeastern Australia. Intially, only one species of carp gudgeon was formally described in the Murray-Darling Basin (MDB) known as the Western carp gudgeon. There were several other informally recognised taxa such as Midley’s, Lake’s and Murray-Darling carp gudgeon.

Western carp gudgeon female. Source: Michael Hammer
Western carp gudgeon male. Source: Neil Armstrong

In the early 2000’s genetic studies showed several species of carp gudgeon occurring in coastal and inland drainages within eastern Australia. Interestingly, the survey additionally found a range of hemiclonal unisexual hybrids. Due to all this confusion, for more than 20 years all carp gudgeons in the MDB were combined into a generic group and referred to as ‘Hypseleotris spp‘ (i.e. Hypseleotris of undefined species).

Hemiclones are naturally or artificially produced individuals that share a single specific genetic halotype (aka a set of DNA variants along a single chromosome that tend to be inherited together). Natural hemiclones are produced via hybridisation between two closely related species.

In biology, unisexual refers to an organism which can only contain either male or female sex cells. This means that in a species with two genders, only one kind of reproductive gonads exists in each animal. In contrast, snails are a hermaphroditic species! This means that they have both sets of reproductive gonads.

The use of hybrid refers to offspring that is a result of two organisms of different varieties or species through sexual reproduction. An example would be the combining of male donkey and a female horse to create a mule.

Now, that’s a lot of crazy words – to put it simply, these little guys have been busy! However, the picture was an incomplete one, with mystery DNA appearing seemingly out of thin air. No one could find any example of the ‘species X’ responsible for leaving its genetic signature in carp gudgeon hybrids.

The missing piece to the taxonomic puzzle came in 2022 with the discovery of the ‘species X’, now known as the Bald carp gudgeon. This mysterious little fish is the most isolated of the gudgeons – only found in two spots in the upper Lachlan (despite their genetic signature being found in thousands of fish). As a result of the Bald carp gudgeon being found, there are two additional new/redescribed carp gudeons in the MDB called the Boofhead carp gudgeon and the Cryptic carp gudgeon.

Cryptic carp gudgeon female. Source: Michael Hammer
Cryptic carp gudgeon male. Source: Michael Hammer

As well as the known species, there are multiple hemi-clonal unisexual lineages of hybrid origin that are incredibly difficult to identify. In some parts of the MDB up to three sexual species co-occur with three hemiclonal forms, all being captured in the same location.

Boofhead carp gudgeon female. Source: Gunther Schmida
Boofhead carp gudgeon male. Source: Gunther Schmida

Unfortunately, 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. Consequently, all but the two of the non-hybrid species (Western and Bald) are treated as a group. Genetic issues such as hybridisation and fragmentation are the newest threats to native fish in the MDB.

Carp gudgeons distribution map. Source: Lintermans, M. 2023, Fishes of the Murray–Darling Basin, Australian River Restoration Centre, Canberra.

For example, the Bald carp gudgeon is listed as critically endangered on the IUCN Red List and is teetering dangerously close to extinction due to genetic and geographical isolation. These threats are still being investigated and we are always discovering more about our native fish.

Featured image: Bald carp gudgeon.

Source: Tarmo A. Raadik

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