Sometime before 1995, a container of freshwater crayfish from Florida got too hot or too cold en route to a pet shop in Germany. The shock disrupted the development of an egg being carried by one of the females, creating an army of clones that are invading rivers and lakes in continental Europe, Madagascar and Japan.
That, at least, is our best guess of what happened. And it has now been shown that this self-cloning crayfish,聽known as marbled crayfish, is so distinct from the original Florida crayfish that it should be regarded as a whole new species. 鈥淚t appeared immediately,鈥� says Gunter Vogt of the University of Heidelberg. 鈥淣ot over several generations.鈥�
Vogt first proposed this back in 2015, but other biologists argued that the marbled crayfish is not different enough to count as a separate species. Now his team has carried out a larger study that shows that the marbled crayfish is bigger, longer-lived, dramatically more fertile, tolerates a wider range of conditions and already occupies a bigger area than the Florida crayfish.
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The marbled crayfish can produce at least 5 to 10 times as many offspring, Vogt says. This is partly because every adult is female and can produce offspring. It鈥檚 also partly because the marbled crayfish is larger 鈥� up to 11 centimetres long 鈥� and weighs twice as much as the slough crayfish of Florida, Procambarus fallax. But Vogt鈥檚 team has also shown that individual marbled crayfish produce 40 per cent more eggs than slough crayfish of the same size.
Attack of the clones
This extraordinary fecundity helps explain why the species is highly invasive 鈥� a single pet crayfish released into the wild can multiply like crazy. But it鈥檚 very much a monster of our own making 鈥� the main reason it is spreading so fast is because it鈥檚 been spread around the world via the pet trade, says Vogt.聽His聽group has dubbed the new species Procambarus virginalis.
Bizarrely, the asexual clones are also more adaptable than the mother species. They are thriving even in Sweden, where the water temperature drops to near freezing.
Marbled crayfish 鈥� also called marmorkrebs 鈥� arose when an abnormal egg with two copies of the genome instead of the normal one was fertilised by a sperm, meaning each of their cells has three copies of the genome. This would normally be harmful 鈥� none of 15,000 other species in the group of crustaceans to which the slough crayfish belongs are so-called triploids 鈥� but in this one case it seems to have created a kind of super crayfish.
There are a handful of animals where there are both sexual and asexual forms, yet they are still regarded as a single species, such as the water flea Daphnia pulex. But in these cases the sexual and asexual forms live in the same area and occasionally interbreed, Vogt points out.
By contrast, the marbled crayfish are reproductively isolated 鈥� they not only live in different parts of the world, they cannot breed with male slough crayfish.
Several other species of American crayfish are already widespread in Europe, including in the UK. They are wiping out native crayfish, mainly because they carry a disease to which the native crayfish have no resistance. The marbled crayfish has not spread as widely as it鈥檚 a much more recent arrival, but it鈥檚 on the brink of reaching the Rhine and Danube rivers.
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