How do isopods breed? Reproduction explained

One of the most rewarding aspects, if not the most rewarding, of keeping isopods is seeing a healthy colony establish itself, thrive, and eventually bring forth the next generation. Some species multiply at such a pace that it becomes impossible to keep count, while others take a slower, more measured approach that really tests our patience to the max. Regardless of the species, watching isopods mate, brood, and eventually release their tiny mancae never loses its sense of wonder for me. This article explores the reproductive system of isopods, how the entire process unfolds, and what your role as a keeper is in supporting it.

A quick note before we begin: breeding is essential not only for maintaining colonies, but also for developing new morphs and refining selective traits within the hobby. It forms the backbone of every line, every colour variation, and every pattern we enjoy today. Without consistent and successful reproduction, none of these genetic possibilities could be cultivated. I’ve written more about morph development and selective breeding in a separate article.

Reproduction among invertebrates is wonderfully diverse. Stick and leaf insects flick their eggs to the ground, butterflies glue them neatly to stems and leaves, beetles bury theirs deep within the soil, and mantids encase theirs in hardened, foamy oothecae. Isopods take a very different route. They do not lay external eggs at all. Instead, they retain and protect their developing young inside their bodies, carrying them until the moment of release. This method is far more similar to marine crustaceans than to most terrestrial invertebrates and is one of the key adaptations that isopods have kept from their ancestors when transitioning to life on land.

Male structures:
In male isopods, the reproductive openings are found on the underside of the seventh body segment. These openings appear as two narrow tubes positioned close together, and in some species they can even look partly fused. The male’s pleopods, which are the little flaps underneath the tail area, also play an important role in mating. Although pleopods normally help with breathing and managing moisture, the first two pairs are specially adapted for reproduction. Their tips are long and flexible, working almost like a gentle delivery tool. One forms a thin channel while the other helps push the sperm into the female. This is why the male positions himself at an angle during mating as each of the female’s two openings needs to be reached separately.

Female structures:
Female isopods have two reproductive openings, called oopores, located on the underside of the fifth body segment, one on each side. Each oopore leads to its own ovary, and because they are spaced far apart, the male must inseminate each one individually. This simple-looking arrangement is the foundation of a reproductive system that becomes more fascinating the more you learn about it.

Before mating takes place, you may notice increased activity in the tub. Males search determinedly for receptive females, sensing them through subtle chemical cues. They may chase after them, attempt brief mounts, or tap the edges of the female’s segment plates with their hind legs, a behaviour often overlooked but almost certainly a form of stimulation or communication. Females may signal receptiveness through posture or pheromones.

A behaviour commonly mistaken for mating is mate guarding. A male may hover over or cling to a female for hours, not actually inseminating her, but preventing other males from doing so. This guarding increases his chances of being the successful mate once she is ready. Hence the name guarding!

Actual mating looks different. Instead of aligning parallel like millipedes, the male positions himself at an angle to the female. Once mating concludes, the pair separates and resumes normal activity, leaving the true magic to happen inside the female’s brood pouch.

Following mating, the female fertilises her eggs internally and transfers them into a specialised chamber beneath her first several pairs of legs called the marsupium. The marsupium appears as a series of thin overlapping plates called oostegites, which create a watertight compartment filled with nourishing fluid. Cotyledons within the marsupium help regulate moisture and provide nutrients and gases to the developing embryos. This system is remarkably sophisticated for such small creatures. It is, in essence, a travelling nursery. A self-contained, mobile ecosystem that provides everything the offspring need until they are ready to face the world on their own.

In many species, the brood pouch is also the most reliable way to determine sex. Healthy females approaching or in sexual maturity will show the beginning of oostegite formation, which males lack entirely.

Eggs remain in this sheltered environment throughout their entire development, protected from desiccation, fluctuations in humidity, and predators. The marsupium is one of the adaptations that enabled ancestral isopods to colonise land multiple times independently across their evolutionary history.

Once transferred to the marsupium, the eggs begin developing. The duration varies widely between species. Some complete development in a few weeks, while others require several months. Environmental factors such as temperature, humidity, and nutrition also influence the timeline.

When ready, the eggs hatch inside the pouch into tiny young called mancae. These miniature isopods resemble adults but lack their final pair of walking legs, which will develop through subsequent moults. At first they are almost translucent, soft-bodied, and far paler than adults. Their colours and patterns intensify as they grow.

For the first days after release, mancae often stay huddled beneath or near the mother before beginning to disperse throughout the enclosure. Their appearance is one that always makes me smile. It is proof that the colony is thriving and that my care has provided the stability required for successful reproduction.

Larger females generally produce larger broods. You might think this sounds obvious and in many ways, it is, but it’s worth understanding why this happens. Younger, less mature females often produce smaller broods simply because they haven’t reached their full size yet. Many species continue to grow even after reaching sexual maturity, and this ongoing growth can significantly increase their reproductive capacity over time. Larger bodies provide more internal space for the marsupium and offer more resources for developing offspring, which naturally leads to larger, more productive broods.

Studies also show that in nearly half of the species examined, larger females sometimes produce slightly larger offspring as well, although this varies by species and even by brood. Conversely, there is often a trade-off between brood size and offspring size. Large broods may consist of slightly smaller mancae, while small broods can contain larger young.

Different isopod species vary greatly in how often they breed. The well-known Porcellio laevis “Dairy Cow” is famous for its prolific nature, often producing brood after brood with little delay. Keepers can end up with colonies of 1000s of individuals. In contrast, certain Cubaris species are much slower at breeding. For example, “Amber Panda,” “Siberian Tiger,” “Lemon Blue,” and “White Ducky”. They may take months between broods and are far more sensitive to disturbance.

In the wild, tropical species may reproduce throughout the year, while species from cooler climates produce only one or two broods annually. As keepers, understanding where your species originates from can help you recreate the conditions necessary for reliable breeding and also understand when culture numbers are likely to fluctuate.

Mating does not always happen out of sight. You might lift a piece of cork bark and catch a pair in the act or see a male guarding a female before she is ready to mate. Brooding females often look slightly swollen, and if you look closely enough, you can sometimes spot the faint movements of developing mancae beneath the oostegites.

The moment you discover newly released mancae exploring their world is always memorable. It means you are providing the correct care and environment for your pods! However, at this stage it’s crucial to be extremely careful when checking the tub. The young are incredibly delicate, and a brooding or freshly released female can easily be startled into running off, potentially leaving her offspring scattered or vulnerable.

Isopod reproduction beautifully demonstrates how even the simplest of creatures can display remarkable complexity in their biology. Their marsupial method allows them to safeguard their young in a way few other terrestrial invertebrates can.

For you as a keeper, successful reproduction is the clearest sign of a healthy, balanced colony. Seeing those tiny mancae exploring the enclosure brings a genuine sense of achievement and reassurance that your care is on the right path. Proof that when conditions are right, the results speak for themselves.

That said, breeding isopods isn’t always straightforward. Some species will test your patience, refusing to breed until every environmental factor aligns perfectly. You may fail several times before finding the right balance, especially with rarer or more sensitive species. Failures and setbacks are simply part of the isopod-keeping journey, so although it can be disheartening at times, try not to feel discouraged when things don’t go exactly to plan. Once the colony finally settles in and begins to thrive, it’s worth every moment of waiting.