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.