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In Jordan Peele’s incredible 2019 horror film Us, an army of human doppelgängers called The Tethered arise to take the place of the existing human population. It hits at some of our core fears, that we might actually be the monsters, and that some other version of ourselves might usurp our favored place upon the Earth. It’s later revealed that the Tethered are genetic clones created by the government and abandoned.
Human cloning is, as yet, still beyond our reach. But nature has the process locked down. A number of animals including some reptiles, birds, and sharks clone themselves through asexual reproduction known as parthenogenesis. That elite club of clone animals has a new member.
Over the last few decades, the planet has been at the mercy of a ten-legged, many-clawed crustacean ravenously creating a clone army bent on world domination. No, it isn’t an interplanetary interloper or the result of an uncontained government experiment. This is biology gone wrong, or if you happen to be a marbled crayfish, biology gone horribly right.
Today, the freshwater marbled crayfish populates various ecosystems across Asia, Europe, and Africa, and they all trace back to a single genetically identical individual born less than three decades ago. Their precise population numbers are unknown, but there are an estimated 23,000 living in a single small lake in Germany, which measures less than a tenth of a square kilometer, so it stands to reason there are a lot of them.
Their invasive nature and rapid spread across a significant portion of the planet made them an intriguing target for scientific investigation. An international team of scientists completed an analysis of their genome in an effort to uncover their origin and found that they were stranger than we could have dreamed. Their findings were published in the journal Nature Ecology Evolution.
The genome of the marbled crayfish has 3.5 million base pairs — that's more than the human genome — comprised of roughly 21,000 genes from 92 chromosomes. What’s unusual is that instead of the expected two copies of their chromosomes, marbled crayfish have three. Their genetic composition is similar to the Slough crayfish, a close relative, leading scientists to conclude that the first marbled crayfish was born through an unusual reproductive happenstance when two Slough crayfish mated.
What’s more, it appears that the Slough parents hail from different parts of the world, making it unlikely that they met in the wild. Instead, it’s believed they might have been dropped in the same aquarium tank and met in captivity where they would later give birth to their unusual progeny.
That might have been the end of the story, but this new genetic aberration must somehow have escaped the tank — or else one of its own cloned offspring did — and made its way into the wild. It’s often thought that clonal species are at higher risk because they lack the genetic diversity which comes with sexual reproduction, but that hasn’t been a challenge for the marbled crayfish, at least thus far.
Despite its unusual origin and asexual reproductive strategy, it has succeeded in gaining a clawhold all over the world. While it hasn’t yet appeared in the wilds of the United States, some areas are taking preventative action, naming them as prohibited, even in the aquarium trade where they have become popular.
It’s a worthwhile strategy. Once they find their way into an ecosystem, there’s likely no stopping them. A single individual can lay 700 eggs, all copies of itself, and they can survive drought conditions by burrowing into the ground and migrate over land. All the while, they outcompete and reduce the numbers of endemic species.
Let’s just hope the marbled crayfish never sets its sights on humanity. If they do, we may never be able to stop them.