By Carl ZimmerThe New York Times
HEIDELBERG, Germany — Frank Lyko, a biologist at the German Cancer Research Center in Heidelberg, studies the 6-inch-long marbled crawfish. Finding specimens is easy: Lyko 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 crawfish will emerge from hiding and begin swarming around ankles.
‘‘It’s extremely impressive,’’ Lyko said. ‘‘Three of us once caught 150 animals within one hour, just with our hands.’’
Over the past five years, Lyko and his colleagues have sequenced the genomes of marbled crawfish. In a study published earlier this month, the researchers demonstrate that the marbled crawfish, though 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. In Madagascar, where it arrived about 2007, it now numbers in the millions and threatens native crawfish.
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 Lyko he bought what were described to him as ‘‘Texas crawfish’’ in 1995.
The hobbyist was struck by their large size and the 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 afterward, the animals, also called marmorkrebs, were showing up in pet stores.
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, Lyko and his colleagues set out to determine the entire genome of the marbled crawfish. By then, it was no longer just an aquarium oddity.
After some aquarium owners dumped their overpopulating crawfish into lakes, marmorkrebs established growing populations in the wild. Feral populations started turning up in lakes and streams in the Czech Republic, Hungary, Croatia and Ukraine in Europe, and later in Japan and Madagascar.
In December, Lyko and his colleagues officially declared the marbled crawfish to be a species of its own, which they named Procambarus virginalis. The scientists aren’t sure where the species began. There are no wild populations of marbled crawfish in the United States, so it’s conceivable that the new species arose in a German aquarium.
All the marbled crawfish that Lyko’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 their new study, published in the journal Nature Ecology and Evolution, the researchers show that the marbled crawfish has spread across Madagascar at an astonishing pace, across an area the size of Indiana in about a decade.
There are a lot of clear advantages to being a clone. Marbled crawfish produce nothing but fertile offspring, allowing their populations to explode. In the long term, however, there are benefits to sex. Sexually reproducing animals are 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 defense.
The marbled crawfish offers scientists a chance to watch this drama play out practically from the beginning. In its first couple decades, it’s doing extremely well. But sooner or later, the marbled crawfish’s fortunes mightl turn.
‘‘Maybe they just survive for 100,000 years,’’ Lyko said. ‘‘That would be a long time for me personally, but in evolution it would just be a blip on the radar.’’