Sexing Isopods

A question Post Pods receives from time to time is ‘Can you guarantee ‘x’ number of males and females in an isopod colony?’ The short answer is no, unfortunately, at Post Pods we don’t sex our isopods before trade or sale. This is typical across the hobby and is not seen as pivotal information as colonies are usually sold in counts of 10 or more which should provide a good distribution of male and females. However, it is possible to purchase smaller colonies especially for the more niche or expensive isopod species and hobbyists run the risk of receiving a poor sex ratio if they are intending for their isopod colonies to continue to breed and flourish.

 

One method to sex isopods is through sexual dimorphism. This is more easily demonstrated in the larger isopod species such as the giant Porcellio. Males may reach 5mm more in length than females and their uropods can be as much as four times in length than a fully grown female. Males are also comparatively thinner (which contributes to their longer body length) and the females are wider, probably to accommodate for the brood pouch and producing manca. Males use their longer uropods as a threat display against predators and in fights with other males, sticking them straight up like a wanna-be scorpion.

 

Of interest, the Philoscia sp. “Thai” display sexual dimorphism not in body structure, but colouration. This is one of few examples of isopods that Males of this species are a dull red with a grey line marking the digestive tract, while females are a dull grey.

 

However, this is far from the most reliable method since most species breed at ½-2/3 adult size, and males of this age have a female appearance as they are not fully sexually mature. Males can only reliably be sexed by viewing their uropods when fully sexually mature, some males are late bloomers and keep a “feminine” appearance until late adulthood, and of course there is variation in uropod length between males. Some species of Porcellio, like P. werneri do not display a visual difference in uropod length, instead the females are only vaguely broader, and species in other genera like Armadillidium and Cubaris have no visual differences at all.

 

So what’s the most reliable way to sex isopods? The same as any other way, looking for the female and male sexual organs.

 

The best way to sex an isopod is to observe the underside near the posterior end and looking for the pleopods, the small plates beneath the tail end. One way to do this is by placing an isopod in a clear or glass container whilst using a stereomicroscope or magnifying glass and a light source (you may need to be prepared to gently hold the isopod in place). Alternatively, if you feel comfortable handling them by hand, by bracing the isopod between the thumb and fore finger, using either the ring finger or the opposing hand to press down the rear end of the isopod to inspect more closely. This difficult process that takes a lot of patience and careful handling, as the isopod may get hurt.

 

 

 

 

Now, the penis of the male won’t be visible, but a hard external appendage called the penile pappilae is. This appendage assists the male in directing his sexual organs into the female for mating. If it is present, it is a male isopod; if there is none, it is a female isopod. Additionally, the pleopods (underside segments on the back portion of the isopod) are angled and thinner in males, in females they are flat and rectangular shaped. This method is an accurate way to sex isopods across all terrestrial species.

 

A photographic illustration assists in explaining the visual differences between male and female isopods as below. Photo credit to Smug Bug.

 

 

There are some isopods which are exclusively female and reproduce through a method call parthenogenesis such as the dwarf species of the Trichoniscus genus. Without a male isopod to reproduce with, the female dwarf Trichoniscus isopod essentially reproduces by cloning themselves.

 

The process of sexing isopods can be difficult at first but with a trained and practiced eye it is possible to sex isopods if the need or curiosity arises.