An all-female mega-population of giant crayfish clones was discovered in the German aquarium trade back in 1995. Called marbled crayfish or Marmorkrebs, they are very popular pets. However, when they escaped or were released into the wild, they quickly became a threat to smaller, native crayfish in freshwater ecosystems in Central Europe and Madagascar.
While marbled crayfish reproduce asexually – that is, without fertilization by males – they are actually descendants of the sexually-reproducing slough crayfish (Procambarus fallax) of Florida and Georgia. Marmorkrebs, however, have three chromosomes instead of the normal two – a condition known as triploidy. Whether or not they’re a separate species is unclear. After all, every marbled crayfish is genetically identical, and they all share a single origin.
Now, based on cross-breeding experiments and parentage analysis, Günter Vogt and colleagues at the University of Heidelberg argue that marbled crayfish are, in fact, their own species and reproductively isolated from Procambarus fallax. They propose calling this new independent asexual species Procambarus virginalis, the virgin form of the genus Procambarus. Their findings are available on bioRxiv.
In experiments, the two species of crayfish copulated readily, but slough crayfish males couldn’t fertilize marbled crayfish eggs. That indicates their reproductive barrier is set at the cellular level, and not the behavioral one. Furthermore, marbled crayfish show significant differences from their mother species – both genetically and epigenetically (which refers to changes in DNA expression without a change in the underlying DNA sequence). These epigenetic differences likely account for the superior size and fecundity of marbled crayfish. At 250 days after hatching, when females of both species reach sexual maturity, marbled crayfish weigh almost twice as much as slough crayfish females on average.
New species typically arise gradually over long periods of time, Science explains, but genetic analyses indicate that speciation in this case was virtually instantaneous – something that happens in plants but rarely in animals. It’s possible that marbled crayfish came about through a heat or cold shock during the sensitive phase of egg development in a single captive slough crayfish female, possibly during transport.