Frank Lyko, a biologist at the German Cancer Research Center, studies the six-inch-long marbled crayfish. Finding specimens is easy: Dr. Lyko can buy the crayfish 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.

“It’s extremely impressive,” said Dr. Lyko. “Three of us once caught 150 animals within one hour, just with our hands.”

Over the past five years, Dr. Lyko and his colleagues have sequenced the genomes of marbled crayfish. In a study published on Monday, the researchers demonstrate that the marble crayfish, while common, is one of the most remarkable species known to science.

Before about 25 years ago, the species simply did not exist. A single drastic mutation in a single crayfish produced the marbled crayfish 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 crayfish.

“We may never have caught the genome of a species so soon after it became a species,” said Zen Faulkes, a biologist at the University of Texas Rio Grande Valley, who was not involved in the new study.

The marbled crayfish became popular among German aquarium hobbyists in the late 1990s. The earliest report of the creature comes from a hobbyist who told Dr. Lyko he bought what were described to him as “Texas crayfish” in 1995.

The hobbyist — whom Dr. Lyko declined to identify — was struck by the large size of the crayfish and its enormous batches of eggs. A single marbled crayfish can produce hundreds of eggs at a time.

Soon the hobbyist was giving away the crayfish to his friends. And not long afterward, so-called marmorkrebs were showing up in pet stores in Germany and beyond.

As marmorkrebs became more popular, owners grew increasingly puzzled. The crayfish 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 crayfish were indeed making clones of themselves. They sequenced small bits of DNA from the animals, which bore a striking similarity to a group of crayfish species called Procambarus, native to North America and Central America.

Ten years later, Dr. Lyko and his colleagues set out to determine the entire genome of the marbled crayfish. By then, it was no longer just an aquarium oddity.

For nearly two decades, marbled crayfish have been multiplying like Tribbles on the legendary “Star Trek” episode. “People would start out with a single animal, and a year later they would have a couple hundred,” said Dr. Lyko.

Many owners apparently drove to nearby lakes and dumped their marmorkrebs. And it turned out that the marbled crayfish 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.

Sequencing the genome of this animal was not easy: No one had sequenced the genome of a crayfish. In fact, no one had ever sequenced any close relative of crayfish.

Dr. Lyko and his colleagues struggled for years to piece together fragments of DNA into a single map of its genome. Once they succeeded, they sequenced the genomes of 15 other specimens, including marbled crayfish living in German lakes and those belonging to other species.

The rich genetic detail gave the scientists a much clearer look at the freakish origins of the marbled crayfish.