No one knows exactly when the clones first appeared, but humans only became aware of them in the early 2000s.
It was a German aquarium owner who first brought it to scientists’ attention. In 1995, he had acquired a bag of “Texas crayfish” from an American pet trader, only to find his tank inexplicably filling up with the creatures. They were all, it turns out, clones. Sometime, somewhere, the biological rule that making baby crayfish required a mama crayfish and papa crayfish was no longer inviolate. The eggs of the hobbyist’s all-female crayfish did not need to be fertilized. They simply grew into copies of their “mother”—in a process known as parthenogenesis.
Crayfish specialists were astonished. No one had seen anything like it. But the proof was before their eyes and in 2003, scientists dubbed the creatures marbled crayfish, or Marmorkreb in German.
Scientists quickly realized the marbled crayfish were not just in German aquariums. The self-replicating creatures were out in the wild, and they were aggressive invaders. “Every single one has the ability to reproduce. Every single one could start a new population,” says Zen Faulkes, a crustacean researcher at the University of Texas at Rio Grande Valley who keeps a map of marbled crayfish invasions. You can easily buy marbled crayfish online (though they are now banned in the European Union and some U.S. states). The species has shown up in the wild in Germany, Italy, Slovakia, Sweden, Japan, and Madagascar. “We’re being invaded by an army of clones,” says Faulkes.
For the first time, scientists have now fully sequenced the DNA of the marbled crayfish. In fact, they sequenced not one but 11 crayfish—including those originating from German pet shops as well as wild ones caught in Madagascar. The creatures are indeed clones of each other, all descended from a single crayfish that somehow gained the ability to reproduce on its own. They had remarkably little genetic diversity. At most four letters in their entire DNA sequence differed in a meaningful way.
Another intriguing fact, says Frank Lyko, who led the study, is that marbled crayfish are triploid, meaning they have three sets of chromosomes. Most crayfish—and most other animals—have two sets, one inherited from the mother and the other from the father. It’s unclear, however, whether these three sets of chromosomes are the cause or consequence of its self-cloning ability. Despite having the DNA sequence in hand, “the reason and origin of parthenogenesis is still somewhat mysterious,” says Gerhard Scholtz, a zoologist at the Humboldt University of Berlin who first described the marbled crayfish in 2003.
Lyko is interested in the marbled crayfish because he studies epigenetics—or how genes are turned on and off without changing the underlying genetic code. Normally, he studies this in cancer cells, as he works at the German Cancer Research Center. But the marbled crayfish are an intriguing model system for epigenetics. They are virtually identical genetically, yet they differ in size and pattern. These changes may be epigenetic in nature.
Lyko also collaborated with scientists in Madagascar, where the marbled crayfish is displacing the native crayfish species. Their interest is more ecological. In the past 10 years, they estimate, the marbled crayfish population has expanded its area in Madagascar 100-fold—despite, or perhaps because of, a local appetite for them. Marbled crayfish evolved from a species native to Florida, so they are used to a warm and humid climate. “You’ll find that in Madagascar. Not so much in Germany,” says Lyko.
Yet marbled crayfish are in German lakes, too. Lyko says that a graduate student at his institute had found marbled crayfish in one near her family’s house. They threw some of the crayfish on the grill. Lyko himself is less keen on eating the clone invaders. “I tried other crayfish once,” he says. “I didn’t like them so much to be frank, so I’m not in a rush to eat marbled crayfish.”