Cohab (cohabitation) in relevance to invertebrates is a term used to describe housing multiple species in the same enclosure together. This is a practice that many keepers are interested in because it has shiny promises of observing many species interacting together in a single setup, as well as reducing the amount of setups required and maximizing the space used. After all, isopods are all pretty similar, right?
Cohabbing is a practice that should only be used in very specific circumstances, and is not recommended for individual cultures. The only instances we recommend cohabbing isopods is when building a bioactive setup because different sizes and kinds of isopods perform different cleaner duties, allowing for different “layers” of cleaner crew and a more efficient system. The animals that also live in bioactive make this possible, because they predate on the isopod populations and keep them in check. Bioactive isn’t always a sure fire way to have successful cohabitation; it’s recommended to keep an additional separate culture of the isopods to re-seed the enclosure as the isopods die off to keep a stable population.
So, what are the dangers of cohabbing? The first thought that comes to mind is that the different species will breed together and hybridize. While isopods are often observed attempting to breed with other species, the act is only mechanical with no genetic success- similar to a dog loving on its favorite oversized duck toy. Not only are the isopods genetically different, preventing fertilization, but their genitalia are incompatible, inhibiting successful copulation. For the identification of separate invertebrate species, entomologists often need to go as far as examining and documenting the penile papillae. Additionally, the attempts of males to breed with females of a different species stresses the female: she may abort her eggs or even die due to stress. This causes the competing population to have a considerable reproductive advantage. So far, no hybridization between different species has been documented. There are a handful of subspecies that can hybridize – the most prominent example is that of Oniscus asellus asellus and Oniscus asellus occidentalis. Both asellus asellus and asellus occidentalis have uniquely presenting penile papillae, and interestingly enough, the offspring of the two subspecies has penile papillae also unique from either parent species!
Isopods of other species may also stress females even without attempting to breed with them. Trichorhina tomentosa, or “dwarf white isopod” is notorious for taking over cultures. Dwarf whites, despite their small size, quickly overwhelm any other species. This is a unique combination of their quick breeding behavior, their preference for burrowing, and ability to swarm food sources. Dwarf whites quickly eat up available protein and other such sources that larger isopods need in order to successfully reproduce. When the dwarves reach swarm conditions (which doesn’t take very long since each dwarf is producing broods within 4 weeks of birth), they stress the females of larger species, causing them to either not carry broods to term or become so stressed they birth very small litters. Trichorhina tomentosa has become the dirge of some keepers’ existences for causing so many colonies to crash. If kept in a mixed culture with other dwarves, even other quickly breeding species, they outbreed and overwhelm them. While the giant isopods such as Porcellio magnificus seem to be particularly prone to succumbing to unwanted roommates, we’ve personally had dwarf whites cause even particularly hardy species such as Porcellionides pruinosus populations to taper off and virtually disappear.
So, obviously dwarf white can’t be housed with other species. What about other isopods? Aggressive species such as Porcellio scaber should be avoided because they are known to actually hunt and predate on other invertebrates. Many people believe that if offered substantial protein and other nutritional sources this will not occur; however space is always a limited resource, so unless the dominant population is continually removed, the lesser species stands no chance. Porcellio scaber are so aggressive they are one of the few species that stands a chance against the dwarf white isopods because they eat them!
Other common Porcellio species are even more aggressive. The species Porcellio dilatatus and Porcellio laevis are insidious in their invasion due to their burrowing behavior; while at the surface they may seem to be coexisting in harmony with their roommates, gradually over time the competing species disappears, and many keepers discover complex burrows throughout the setup filled exclusively with these Porcellio. Due to similar size, breeding rate, and behavior, laevis and dilatatus are actually one of the few species that can be successfully kept together.
So, can any isopods be kept together? Yes! We do not recommend it for anyone intending to be a vendor for these animals as it makes not only harvesting the animals difficult, but identifying the separate species can be difficult because compatible species are so similar. For a fun project or just a pet tank, there are possibilities. Armadillidium vulgare and Armadillidium nasatum are two species that are compatible. Many Armadillidium, in fact, live together quite peacefully but should be monitored closely. For example, rarer species like Armadillidium gestroi reproduce at a much slower rate than vulgare and may be easily overwhelmed.
One species, in particular, stands out as a particularly amicable roommate: Porcellionides pruinosus. This species has been the cause of frustration for many keepers because of its inherent wanderlust; it seems to spontaneously appear in cultures across the room. The addition of pruinosus to cultures does not pose any threat to the intended inhabitants, they’re just annoying to those that want to keep pure cultures. Pruinosus happily exist and thrive in culture and other species, dwarf, Porcellio, and Armadillidium alike continue to thrive without issue.
While keeping many species together at a time may seem like a fun experience, we generally recommend avoiding it. Cohabbing is a practice that should only be done after keeping a few species and getting to know them, and even then only under circumstances with particular goals in mind. Please be careful when proceeding with this practice!