Isopods breed readily in captivity. That is one of the main reasons people keep them. If your husbandry is correct, breeding happens without any special effort on your part. But understanding how it works helps you troubleshoot when a colony is not growing, and lets you make informed decisions about splitting cultures and managing population.
How isopods reproduce
Isopods are not insects, and their reproduction reflects that. Females carry fertilised eggs in a fluid-filled brood pouch called a marsupium, located on the underside of their body between the legs. The eggs develop inside the marsupium and the young emerge as mancae, which are essentially miniature versions of the adults, just smaller and paler. There is no larval stage and no metamorphosis. The mancae moult and grow, adding size with each moult until they reach maturity.
A gravid female (one carrying eggs) is easy to spot if you look at the ventral side. The marsupium appears as a pale, swollen area between the front pairs of legs. She will tend to stay hidden during brooding, retreating into the substrate or under bark.
Sexing isopods
Males and females can be distinguished in most species, though it takes practice. Males tend to be slightly narrower and sometimes have larger uropods (the two appendages at the rear). In Armadillidium species, males are often a bit smaller and slimmer than females of the same age. In Porcellio species, the differences are more subtle.
The most reliable method is looking at the pleopods on the underside. Males have a pair of modified pleopods (the endopods of the first pair) that form the copulatory apparatus. Under a magnifying glass or jeweller's loupe, these are visible as two elongated structures. But honestly, for most keepers it is easier to just buy a group of 15+ and let statistics sort out the sex ratio.
Conditions that encourage breeding
The same conditions that keep isopods healthy are the conditions that trigger breeding:
- Stable humidity with a proper moisture gradient
- Temperature in the species' preferred range (18-24C for temperate, 22-26C for tropical)
- Adequate calcium supply (females need calcium to form the marsupium and support developing mancae)
- Sufficient food, especially protein around breeding time
- Enough substrate depth for gravid females to retreat and brood
- A sense of security: plenty of hides, leaf litter, bark
If the colony has all of this and still is not breeding, the most common reasons are: the colony is too young (isopods need to reach maturity, which takes a few months depending on species), the sex ratio is heavily skewed, or the temperature is too low. Warmer temperatures within the species' range tend to speed up breeding cycles.
Brood size and frequency
Brood size varies by species. Porcellio scaber females produce around 20-40 mancae per brood. Porcellio laevis can produce more, sometimes 50+. Armadillidium vulgare broods tend to be somewhat smaller, around 15-30. Cubaris species produce small broods, often fewer than 10 mancae at a time, which is one reason they are slower to establish and more expensive.
Under good conditions, a female can produce multiple broods per year. The interval between broods depends on temperature and food availability. Warmer temperatures and good nutrition shorten the interval. Some fast-breeding species like P. laevis can produce a new brood every 4-6 weeks in optimal conditions.
Caring for mancae
Mancae do not need separating from the adults. They are born into the colony and start feeding on the same food as the adults straight away: leaf litter, decaying wood, and any supplemental food you provide. The main risk to mancae is desiccation. They are tiny and lose moisture fast. Make sure the damp side of the enclosure stays damp, and do not let the substrate dry out. Waterlogging is equally dangerous: mancae can drown in standing water or overly saturated substrate.
Calcium should always be available. Growing mancae moult frequently and need calcium for each new exoskeleton. A colony that is low on calcium will lose mancae to failed moults before they reach adulthood.
Population growth patterns
New keepers often worry that the colony is not growing. For the first few months, growth is slow and mostly invisible. The original adults are settling in, the first few broods are being produced, and the mancae are tiny and hidden in the substrate. Then, after 3-6 months, the population starts to accelerate as the first generation of captive-bred individuals reach maturity and start breeding themselves. Growth becomes exponential for a while before levelling off as the enclosure reaches carrying capacity.
If you started with 10-15 individuals and nothing seems to be happening after 2-3 months, check the basics: humidity, calcium, temperature, food. If those are all correct, be patient. Some species just take time. Armadillidium species are generally slower to build up than Porcellio.
Splitting colonies
Once the colony is well established (100+ individuals is a reasonable benchmark), you can split it. Take a portion of the substrate along with the isopods, because the substrate contains mancae and eggs that you cannot see. Move this to a new enclosure with fresh substrate mixed in. Both the original and the new colony will recover and continue growing.
Do not split a colony that is still small. Taking half of a 20-isopod colony gives you two fragile groups of 10 that may both struggle. Wait until the population is genuinely abundant before dividing.
Line breeding and morphs
If you are keeping colour morphs, keep each morph in a separate culture. Mixing morphs produces offspring that revert toward the wild-type colouration over a few generations. The morph colours are typically recessive traits, so breeding a Dalmatian P. scaber with a wild-type P. scaber produces wild-type offspring that carry the Dalmatian gene but do not show it.
Line breeding (breeding within a closed population to maintain a morph) does carry a risk of inbreeding depression over many generations. In practice, isopod colonies are large enough and generation times short enough that this is less of an issue than it would be with vertebrates. But if you notice reduced brood sizes or increased mortality in a long-established morph line, introducing a few unrelated individuals of the same morph can help.
Selling and trading
Once your colonies are producing surplus, selling or trading with other keepers is common in the UK hobby. Online forums, Facebook groups, and reptile expos are the main channels. Price depends on species, morph, and current availability. Common species like wild-type P. scaber go for very little, while rarer morphs and species command higher prices. Selling cultures of 10-15 is standard.