The accidental species
In about 1995, a German aquarium hobbyist acquired a bag of what were sold as "Texas crayfish" at a pet fair. The owner soon found these animals reproduced at an astonishing rate, and all of them were female. This was the accidental origin of a new species, now known as the Marmorkrebs or marbled crayfish, Procambarus virginalis. Every Marmorkrebs in existence today is a clone descended from that single mutant female. Genetic analysis has confirmed that the global population is genetically uniform.
The species arose from its close relative, the slough crayfish (Procambarus fallax), which is native to Florida and reproduces sexually. Scientists hypothesize that a genetic accident occurred, possibly during the formation of an egg cell in a P. fallax. This resulted in an egg with two sets of chromosomes instead of one. When fertilized, it created a triploid organism, an animal with three sets of chromosomes instead of the usual two. This triploid state is likely the cause of its unique reproductive ability. The new species was not formally described and named Procambarus virginalis until 2017.
Biology of a clonal invader
The Marmorkrebs is the only known decapod crustacean that reproduces exclusively through apomictic parthenogenesis. This means the all-female population lays unfertilized eggs that develop into genetically identical offspring. A single female can produce hundreds of eggs at a time, allowing for explosive population growth. This reproductive strategy means that just one escaped or released individual can establish a new, thriving population.
Physically, the Marmorkrebs grows up to 13 cm long and gets its name from the marbled or spotted pattern on its carapace. This pattern is often more pronounced in aquarium-kept animals than in their wild counterparts. While genetically identical, individuals can show a range of physical traits and behaviors due to epigenetic modifications, chemical switches that turn genes on or off without changing the DNA sequence itself. Adaptability, despite a lack of genetic diversity, helps them thrive in varied environments from Central Europe to subtropical Madagascar. Because they reproduce clonally and adapt epigenetically, researchers at the German Cancer Research Center study them as a model for how tumors evolve and adapt to their environment.
From pet to global pest
The very traits that made the Marmorkrebs a curiosity in the pet trade—its rapid, all-female reproduction—also is a "perfect invader." Overburdened aquarists released them into local waterways, and populations are now established across Germany and other parts of Europe. They are also found in Japan and, most consequentially, Madagascar.
First observed in Madagascar around 2005, the crayfish has spread dramatically, with its distribution area increasing a hundred-fold by 2017. It poses a severe threat to the island's seven species of native crayfish by outcompeting them for resources. Like other North American crayfish, the Marmorkrebs is also a carrier of the crayfish plague (Aphanomyces astaci), a fungus-like pathogen that is lethal to European and Malagasy crayfish but harmless to the carrier. In Berlin, authorities have hired a commercial fisherman to trap invasive crayfish, including Marmorkrebs and the red swamp crayfish, which are then sold to restaurants as "Berlin lobster."