From egg to adult, the life of an isopod is genuinely fascinating - a story of internal brooding, live-born young, repeated moults and a clever reproductive trick or two. Most commonly-kept isopods live around 2-4 years, breeding across several seasons, and watching that cycle play out is one of the real pleasures of keeping a colony. This guide walks through the whole journey: courtship and mating, the marsupium and brooding, the emergence of the young (mancae), and how they grow up to begin the cycle again.
If you'd like the practical side alongside the biology, our guide to breeding isopods covers how to encourage all this in your own enclosure.
A Quick Bit of Background
Isopods are crustaceans, not insects - more closely related to crabs and shrimp. There are over 10,000 known species across marine, freshwater and land habitats, but the ones we keep as pets are nearly all terrestrial woodlice and pill bugs (suborder Oniscidea). Their name comes from the Greek for "equal foot," a nod to the seven pairs of similar-looking legs an adult carries along its thorax.
The single most important thing to know for understanding their life cycle is a structure unique to this group's reproduction: the marsupium. It's what lets isopods raise their young on land without ever returning to water, and it shapes everything about how they breed.
It Starts With Courtship
An isopod's life cycle begins with a male and female and a period of courtship. Breeding often picks up in spring, but for isopods it depends far less on the calendar than on conditions: given the right warmth, humidity and food, they'll breed more or less year-round. When a female is ready, she enters a receptive period and releases aggregation pheromones - chemical signals that attract males.
A couple of intriguing details: both sexes mate with multiple partners, and a female can store sperm from several males, holding onto it to fertilise her eggs later - in some cases for up to a year, carrying it into the next breeding cycle. This means the male you see following a female around isn't necessarily the father of her next brood, which is worth knowing if you're trying to breed for a particular colour or trait.
The Marsupium: Brooding the Eggs
After mating, the female produces eggs - but she doesn't lay them in the soil. Instead they pass into the marsupium, a fluid-filled pouch on the underside of her thorax. There the eggs are kept moist, oxygenated and protected, effectively a portable nursery that frees terrestrial isopods from needing water to breed.
She broods the eggs inside the marsupium for a few weeks to a couple of months, depending on the species and temperature (warmer conditions speed things up). During this time she often tucks herself away in a burrow or under cover. When the eggs hatch, the young can linger in the pouch for a few more days before emerging.
Enter the Mancae
The newly emerged young are called mancae, and they look like tiny, pale copies of the adults - with one telltale difference: a manca has only six pairs of legs. It gains its seventh and final pair, along with its last body segment, after its first moult. A single brood might number anything from a dozen to around a hundred young in the larger species.
Mancae are vulnerable - small, soft and easily dried out or eaten - so they stay hidden in the burrow or among leaf litter, moulting repeatedly as they grow. With each moult their exoskeleton hardens a little more, until they're robust enough to venture out and forage independently. (Isopods moult in two halves, shedding the back portion first and the front a few days later, so a half-pale individual is usually just mid-moult, not unwell.)
Growing Up and Starting Again
As the juveniles mature, they colour up, leave the shelter of the burrow, and join the colony proper. Many species reach breeding age remarkably quickly - often within a couple of months for fast-breeding isopods, though it can take up to a year in slower species. Once mature, they begin the cycle over again, and adults can go on reproducing across several seasons.
The upshot for a keeper is a colony that, given decent conditions, becomes self-sustaining: new mancae appear faster than older animals age out, and the population quietly maintains itself. There's also evidence that isopods do better in company - isolated individuals tend to fare worse - which fits their naturally gregarious, group-living nature.
Frequently Asked Questions
How long do isopods live?
Most commonly-kept species live around 2-4 years in good conditions, varying by species and setup. They breed across several seasons during that time, moulting repeatedly throughout their lives.
Do isopods lay eggs?
Not in the soil. The female broods her eggs internally in a pouch (the marsupium) on her underside until they hatch, then releases live young. So you won't usually find isopod eggs in an enclosure.
What is a manca?
A manca is a newly emerged baby isopod. It looks like a miniature adult but has only six pairs of legs; it gains the seventh pair after its first moult.
How many young do isopods have?
It varies by species and the size of the female - larger females carry more. A brood can range from a dozen or so up to around a hundred in the larger species.
How quickly do isopods reach breeding age?
Fast-breeding species can mature in a couple of months; slower species may take up to a year. Conditions - warmth, humidity and food - matter more than the time of year.
Why do female isopods store sperm?
It lets a female fertilise multiple broods from a single mating, sometimes carrying stored sperm for up to a year. It also means a brood may not be fathered by the male currently courting her - useful to know when breeding for specific traits.
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