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Home > Research & Collections > News > Australia’s Weird River Fishes

Australia’s Weird River Fishes

Fish that strain the species definition

NHMLA Ichthyology Curator Chris Thacker studies fascinating fish that live in the Murray-Darling River in Australia. Photo by Mike Russell via Flickr.


Some species you can recognize at the mere sight of them, like an African lion, emperor penguin, or hammerhead shark. You may even know their scientific names, like Homo sapiens, Tyrannosaurus rex, or E. coli.

But some species look so similar you can only tell the difference if you sequence their DNA. And others make it more complicated still, blurring the very concept of what a species is to begin with. I’d like to introduce you to the fat-headed gudgeon.

This fish is only found in the rivers of Australia — a country famous for its many weird animals, and the fish are no exception. These fishes don’t follow the same set of rules many other species do. Ordinarily, within a species, animals reproduce and make babies of the same species. A lion and a lion have another lion. Octopus + octopus = octopus. But these fish make hybrids, and then the hybrids clone themselves.

a photo of a small grey fish on a white background
This small river fish is called a lake's carp gudgeon. About 4 cm long, it’s one of NHMLA Ichthyology Curator Chris Thacker’s study species. Photo by Daniel Geiger.

It works like this. Two different species of these gudgeons reproduce and form a hybrid. This isn’t terribly unusual — a horse and a donkey can mate, and the hybrid is a mule. But mules are sterile. Two mules can’t make more mules. But in these fish, the hybrids can make more hybrids. And that’s just the start of the strangeness: These fish hybrids are sexual parasites (um, yikes). They mate with a member of one of their parent species (not their actual parents, but a member of one of the original, non-hybrid species), but reject the DNA of their mate, kicking it out after the egg and sperm fuse. The result is a clone — a new individual with the same DNA as its one parent.

Using the mule again as a comparison, it would be like if a mule mated with a horse (or a donkey), but gave birth to a mule with the exact same DNA as itself.

a photo of a river fish on a white background. it has a dark-colored head, and lighter-colored body
NHMLA Ichthyology Curator Chris Thacker will soon name and describe this species of gudgeon fish from Australia. Photo by Daniel Geiger.

This checks all the boxes of parasitism because the hybrid benefits by making copies of itself, and the individual of the parent species suffers because it went to the trouble of attempting to reproduce without any of its DNA making it to the next generation. Parasitism: it’s just rude.

NHMLA Ichthyology Curator Chris Thacker is an expert on gobies, the family of fish these fat-headed gudgeons belong to, and she’s currently unwinding the knot of this strange (or shall we say, fishy?) situation. When this “hybrid clone lineage” of fishes is fully sorted out, Thacker will probably have three new species of fish in need of names.

“Species rules and definitions and naming conventions have to do a lot with assuming that a species is not hybridizing,” said Thacker. “If you found out a corn plant and an oak tree made a baby, is that a species? What is that? How do you name it? That’s what I’m doing right now.”



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