Unmasking the Marmorkreb
by George Thomas
Among all the fish and invertebrates available to us in the hobby, few possess an origin story that is the stuff of urban legend. It was not under a rock in a stream, or a specimen scavenging the shallows of a murky lake where the marmorkreb was first found. It was “discovered” by a customer in a German pet shop, where it was labeled as a “Texas crayfish.” The date, much like the facts surrounding how it arrived in that pet shop, is disputed, but it’s said to have been the early nineties, shortly after the fall of the Berlin Wall.
The customer, believing it to be a simple crayfish from the Lonestar state, was astounded when he spotted several baby crayfish scurrying throughout the fishtank. He returned to the pet shop and asked how a lone crayfish could produce offspring without a mate. This is the first known observation of the marmorkreb, colloquially called the marbled crayfish or the self-cloning lobster.
Marbled crayfish were an instant sensation among hobbyists and scientists alike, becoming an ubiquitous addition to German aquariums throughout the nineties, and an enigma to scientists who spent years simply trying to determine their origin. In February of this year, The New York Times even published an article about marmorkrebs.
Initially, marmorkrebs were considered a close relative, even a mutation of the slough crayfish Procambarus fallax, native to Georgia and Florida. However, no crayfish matching the marmorkreb were ever found inhabiting North American waters. This led to it being the only decapod in the aquarium hobby besides daphnia to have it’s entire genome sequenced! In December 2017, the marmorkreb was finally designated as a species of its own: Procambarus virginalis.
The popularity of marmorkrebs, coupled with their unstoppable parthenogenesis, led to one of the most important rules in fishkeeping to be broken: don’t release your fish and invertebrates into the wild. Here is where another characteristic of this crayfish was realized the hard way — they adapt to virtually every freshwater environment they inhabit. Numerous countries throughout Europe list marmorkrebs as an invasive species, causing the European Union to enact an unconditional ban on owning or selling P. virginalis. The species has even invaded waters in Japan and Madagascar, where it’s feared to out-compete native crayfish populations. Though their true origin may never be known, it is agreed that whatever mutation had occurred, a truly invulnerable creature arose.
My own story with this aquatic anomaly began much like the informal discovery of them in Germany. At one of my local fish stores, I spotted several tiny, white crayfish labeled as “self-cloning lobsters” on sale, three for five dollars.
I quarantined them in a twenty gallon tank, and fed them sinking crab pellets and beef heart. The trio grew up together in relative harmony until they reached two inches in length, when they started to exhibit signs of sparring with each other. Each crayfish was moved to community aquariums, where I observed stunning differences in their personalities.
This is not a shy crayfish. Like some bottom-dwellers, I noticed that increased activity towards the middle and upper levels of my aquariums encouraged the marmorkrebs to come out of hiding, and remain out in the open even as I tended to the aquarium. Two of my marmorkrebs did not show predatory behavior towards fish, however, one would attack any fish that approached the bottom of the tank, going as far as chasing the fish in an attempt to grab it. Take care not to keep these crayfish with slow and tranquil fish, or bottom-dwellers like corydoras.
Marmorkrebs have voracious appetites, which is no surprise considering their nature as scavengers. I feed mine sinking pellets such as those one might find recommended for shrimp, crabs, or omnivores. They readily eat beef heart, blood worm, and in my humble opinion, are one of the most effective snail eaters next to assassin snails. I do not recommend any crayfish for planted tanks, but if you have pest snail problem, they will provide a solution. I often collect pest snails from aquariums and feed them to my crayfish as a part of their daily diet.
Conditions for keeping marmorkrebs are practically effortless. I maintain their temperature at 76 degrees Fahrenheit, but have kept them in a variety of temperatures from 68 up to 82 degrees. So long as room temperature falls within those parameters, a heater isn’t required. As with any aquarium, water quality ought to be clean and healthy as scavengers eat a lot and produce a lot of detritus. However, a simple bubble filter will suffice so long as water changes are consistent. They appreciate ample hiding spaces, especially when they’re molting or when multiple marmorkrebs are housed together. A single marmorkreb will thrive in a minimum tank size of 30″ x 12″ (20 gallon long).
As far as breeding marmorkrebs is concerned, it takes only one to tango. They produce clones of themselves through self-fertilization. I would advise removing the mother crayfish when hatchlings are found. Although the offspring are regarded as genetic reproductions of the mother, they will often develop unique colors that can change for a variety of instances. Mine generally develop marbled blue bodies, whereas other “kreb keepers” have noted instances of purple, red, green, and translucent or opaque carapaces.
Before acquiring this fascinating crayfish, it is imperative to make sure it is legal to keep P. virginalis in your state. Like the European Union, some states are proactively passing laws regarding the possession and release of invertebrates.
Overview: Procambarus virginalis
Maximum adult size: 4″-6″
Minimum tank size: 20 gallons
Temperature: 68-82 degrees