An isopod is a type of crustacean — more closely related to crabs and shrimp than to any insect. The order Isopoda contains over 10,000 described species worldwide, living everywhere from garden soil and rotting logs to the floor of the deep ocean. On land they're the familiar woodlice, pillbugs and roly-polies; in the sea they range from tiny scavengers to the famous giant deep-sea isopod.
This guide covers what isopods are, how their bodies and life cycle work, where they live, why they matter ecologically, and the basics of keeping them as pets. If you're specifically looking to start your own colony, our beginner's guide to isopod keeping goes deeper on the practical side.
What Exactly Is an Isopod?
Isopods are crustaceans in the order Isopoda, part of a larger group called the Peracarida — animals that brood their young in a pouch beneath the body. The name "isopod" comes from Greek for "equal foot", referring to their seven pairs of similar-looking legs, which is one of the clearest ways to tell them apart from other small invertebrates.
Of the 10,000-plus known species, roughly 4,500 are marine, around 500 live in fresh water, and about 5,000 are terrestrial. The land-dwelling group, the suborder Oniscidea, is the one most people meet — and the one the pet hobby is built around. Despite living on land, these terrestrial species never fully escaped their aquatic heritage, which is why they still breathe through gill-like structures and need damp conditions to survive.
How Are Isopods Built?
An isopod's body has two main regions: the pereon (the thorax, made of seven segments, each bearing one pair of legs) and the pleon (the abdomen, made of smaller rear segments that house the breathing structures). That gives every adult isopod fourteen legs in total — a defining feature of the group. For a deeper look at how the body works, see our essential guide to woodlouse anatomy.
Their most important adaptation sits on the underside of the pleon: pleopods, flattened gill-like appendages used for gas exchange. In many terrestrial species these contain branching structures called pseudotracheae — often visible as pale "lung" patches — that let the animal absorb oxygen from air rather than water. It's this feature that allowed isopods to colonise land while their relatives stayed in the sea.
Two long antennae complete the picture, used to taste, touch and navigate the dark, damp spaces isopods favour. Unlike crabs and lobsters, isopods have no large shell-like carapace; instead their armour is a row of overlapping plates, which trades some protection for the flexibility to squeeze into crevices and, in pill species, to roll into a ball.
The Isopod Life Cycle
Isopods don't undergo the dramatic egg-larva-pupa metamorphosis that insects do. Instead they grow gradually through a series of moults. The female carries fertilised eggs in a fluid-filled brood pouch (the marsupium) on her underside, where the young develop in a protected, portable pocket of moisture. They emerge as mancae — miniature versions of the adults, but with one fewer pair of legs, gaining the final pair after their first moult.
From there, young isopods moult repeatedly as they grow, shedding the old exoskeleton to make room for a larger one. They do this in two halves — the rear first, then the front a few days later — so an isopod that looks two-toned is usually mid-moult, not unwell. Because the exoskeleton is reinforced with calcium, a steady calcium source (such as cuttlebone) is essential for healthy moulting in captivity.
Land vs Sea: The Two Worlds of Isopods
Terrestrial isopods — woodlice and pillbugs like Armadillidium vulgare — live in moist habitats such as leaf litter, rotting wood and the soil under stones, where they help recycle decaying plant matter. These are the species kept in the hobby and used as cleanup crews.
Marine isopods occupy the other extreme. The best known is the giant deep-sea isopod (Bathynomus giganteus), which can exceed 30 cm and scavenges on the cold ocean floor of the Atlantic, Pacific and Indian Oceans. Despite the wildly different environments, the body plan is recognisably the same — a testament to how adaptable this single design has proven. In fact, isopods began as marine animals; the most recent common ancestor of all isopods was almost certainly a sea-dweller, and the terrestrial woodlice we know today descend from ancestors that made the move onto land.
Why Do Isopods Matter?
Isopods are nature's recyclers. On land, woodlice are decomposers: they consume decaying leaves, wood and fungi, breaking them into smaller fragments that return nutrients to the soil and support plant growth. This is exactly the service that makes them valuable in bioactive terrariums, where a colony quietly processes waste and keeps the substrate healthy.
In the sea, scavenging species like Bathynomus clean up dead and dying organisms on the ocean floor, recycling nutrients through deep-sea food webs. And because many isopods are sensitive to pollution, scientists use them as bioindicators — their presence, absence and health can signal the condition of a habitat.
How Many Isopod Species Are There?
More than 10,000 described species, spread across eleven suborders, with new ones still being named each year. In the British Isles there are around 35 to 40 native woodlouse species, of which five — the "famous five" — are encountered most often in gardens. The hobby, meanwhile, focuses heavily on tropical species and colourful captive-bred morphs, particularly within genera like Cubaris, Armadillidium and Porcellio.
Keeping Isopods as Pets
Most pet isopods are easy to keep, which is a large part of their appeal. The essentials are straightforward: a ventilated enclosure, a substrate rich in organic matter topped with leaf litter, hiding places such as cork bark, and a humidity gradient so the animals can choose how damp they want to be.
Humidity needs vary by species — tropical species generally want it higher and more consistent than temperate ones — so it's always worth checking the requirements of the species you're keeping rather than assuming one figure fits all. Feeding is simple: a constant supply of decaying leaves and wood as the staple, supplemented with occasional vegetables, a protein source and cuttlebone for calcium. Get those basics right and a colony will largely look after itself. You can browse the full range in our isopods for sale collection.
Frequently Asked Questions
Are isopods insects?
No. Isopods are crustaceans, related to crabs, shrimp and lobsters. Insects have six legs and three body sections; isopods have fourteen legs and breathe through gill-like pleopods, which is why they need damp conditions.
How many legs do isopods have?
Adult isopods have fourteen legs, arranged in seven pairs — one pair on each segment of the thorax. Newly hatched young have twelve and gain the last pair after their first moult.
How many species of isopod are there?
Over 10,000 described species worldwide — roughly 4,500 marine, 500 freshwater and 5,000 terrestrial — divided into eleven suborders, with more still being discovered.
What is the biggest isopod?
The giant deep-sea isopod, Bathynomus giganteus, is among the largest, exceeding 30 cm. On land, species are far smaller, typically ranging from a few millimetres to around 2 cm.
Are isopods good pets?
Yes — most species are low-maintenance, fascinating to watch and useful as cleanup crews in bioactive setups. They need a humid, ventilated enclosure, leaf litter and a calcium source, but otherwise largely care for themselves.
How long have isopods existed?
Isopods are an ancient group, with a fossil record reaching back to the Carboniferous period at least 300 million years ago, when the earliest known isopods lived in shallow seas.
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