Mutant crayfish clones itself

Frank Lyko, a biologist at the German Cancer Research Centre in Heidelberg, studies the six-inch-long marbled crawfish. Finding specimens is easy: Frank can buy the crawfish at pet stores in Germany, or he can head with colleagues to a nearby lake. Wait till dark, switch on head lamps, and wander into the shallows. The marbled crayfish will emerge from hiding and begin swarming around your ankles.

Over the past five years, Frank and his colleagues have sequenced the genomes of marbled crawfish. In a new study, the researchers demonstrate that the marble crawfish, while common, is one of the most remarkable species known to science. Before about 25 years ago, the species simply did not exist. Then one mutation in a single crawfish produced the marbled crawfish in an instant. The mutation made it possible for the creature to clone itself, and now it has spread across much of Europe and gained a toehold on other continents.

A species of its own

The marbled crawfish became popular among German aquarium hobbyists in the late 1990s. The earliest report of the creature comes from a hobbyist who told Frank he bought what were described to him as 'Texas crawfish' in 1995. The hobbyist was struck by the large size of the crawfish and its enormous batches of eggs. One marbled crawfish can produce hundreds of eggs at a time. Soon the hobbyist was giving away the crawfish to his friends. And not long afterwards, the animals, also called marmorkrebs, were showing up in pet stores in Germany and beyond.

As marmorkrebs became more popular, owners grew increasingly puzzled. The fish seemed to be laying eggs without mating. The progeny were all female, and each one grew up ready to reproduce. In 2003, scientists confirmed that the marbled crawfish were indeed making clones of themselves. They sequenced small bits of DNA from the animals, which bore a striking similarity to a group of crawfish species called Procambarus, native to North America and Central America. Ten years later, Frank and his colleagues set out to determine the entire genome of the marbled crawfish. By then, it was no longer just an aquarium oddity.

For nearly two decades, marbled crawfish have been multiplying like Tribbles on a classic Star Trek episode. "People would start out with a single animal, and a year later they would have a couple hundred," Frank said. Many owners apparently drove to nearby lakes and dumped their marmorkrebs. And it turned out that the marbled crawfish didn't need to be pampered to thrive. Marmorkrebs established growing populations in the wild, sometimes walking hundreds of yards to reach new lakes and streams. Feral populations started turning up in the Czech Republic, Hungary, Croatia and Ukraine in Europe, and later in Japan and Madagascar.

Growing population

In December 2017, Frank and his colleagues officially declared the marbled crawfish to be a species of its own, which they named Procambarus virginalis. All the marbled crawfish Frank's team studied were almost genetically identical to one another. Yet, that single genome has allowed the clones to thrive in all manner of habitats - from abandoned coal fields in Germany to rice paddies in Madagascar.

In the study, published in the journal Nature Ecology and Evolution, the researchers show that the marbled crawfish has spread across Madagascar at an astonishing pace. Thanks to the young age of the species, marbled crawfish could shed light on one of the big mysteries about the animal kingdom: why so many animals have sex. Only about 1 in 10,000 species comprise cloning females.

In the long term, however, there are benefits to sex. Sexually reproducing animals may be better at fighting off diseases, for example. If a pathogen evolves a way to attack one clone, its strategy will succeed on every clone. Sexually reproducing species mix their genes together into new combinations, increasing their odds of developing a defence. The marbled crawfish offers scientists a chance to watch this drama play out from the beginning. In its first couple decades, it's doing extremely well. But sooner or later, the marbled crawfish's fortunes may well turn.