How to Sex Isopods

A male (left) and female (right) P. magnificus. Male magnificus demonstrate very long uropods.

When keepers get more comfortable handling their isopods, a common question that will emerge is how to sex them. Now, this isn’t really pivotal information because a healthy isopod colony starts out with 10 or more animals, and there should be a good distribution of male and female in larger numbers. However, obtaining more expensive species is more financially plausible with the smaller amounts of 5 or 6, which can be a large concern of getting potentially all of one sex or a poor ratio.

Defensive presentation by a male "black" P. hoffmannseggi

The most popular way that people like to discuss sexing isopods is demonstrated with the giant Porcellio hailing from Spain and surrounding countries. These large isopods will display sexual dimorphism similar to birds; the males are larger and more ornate. 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.

A male Philoscia sp. (top, right) compared to grey females.

An additional note for sexual dimorphism is the interesting chase of Philoscia sp. “Thai”. This is one of few examples of isopods that display sexual dimorphism not in body structure, but coloration. Males of this species are a dull red with a grey line marking the digestive tract, while females are a dull grey. There are a few other species in captivity to display sexual dimorphism, but these are as yet unconfirmed.

Believe it or not, this is actually a male subadult P. expansus that just hasn't grown his prongs yet.

The uropod method of sexing is far from reliable, however. Most species will 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 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.

Female pleopods (left) are large, recangular and flat; while male (right) are angled, thinner, and of course have a penis at the center

So what’s the most reliable way to sex isopods? It’s the same as in mammals: look for the penis. Now, the penis itself 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 animal; if there is none, it is a female animal. Additionally, the pleopods (underside segments on the back portion of the animal) 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.

That's not a leg- it's his penis! (or, penile papillae)

Lots of people are concerned about hurting the isopods while sexing them. We won’t lie: it is a difficult process that takes a lot of patience and careful handling, and the animals may get hurt. The best method is to have a bright light illuminating the area to looks for the male organs; a desk lamp is what we recommend, or a bright headlamp (there are many models available as low as $15). Brace the animal between the thumb and fore finger, using either the ring finger or the opposing hand to press down the rear end of the animal to inspect more closely. If papillae are present, they will bend out slightly from the animal. Another method is to press the animal against a clear, thin surface such as a plastic lid- this way, there isn’t a worry about damage to the animal from handling, and the underside is still visible. The trickiest aspect of this method is to find an object that is transparent enough to see the papillae clearly.

A very gravid female T. tomentosa (dwarf white isopod)

Some isopods are exclusively female and reproduce through a method call parthenogenesis; these obviously require no sexing. Examples include dwarf species of isopods in the Trichoniscus genus such as dwarf white and dwarf purple. Without males, they essentially reproduce by cloning themselves.

From left to right: A very gravid female P. laevis; a not gravid female laevis, and a male laevis

Sexing isopods is difficult at first, but with practice, a trained eye, and a good light becomes a simple task. The most stressful part of this endeavor is handling the isopods without hurting them, so it is a good idea to practice handling the animals prior to attempting to sex them. However, most people sex to sate their curiosity – not out of an inherent need.