Armadillidium serratum 'Hedgehog' Isopods

Armadillidium serratum is a small pill woodlouse with a shell that has gone completely to spikes, raised into points along the back instead of the smooth dome the genus usually wears. It is the texture that earns it a place, and we should say plainly that this page used to claim something else. It opened by calling the animal a Crystal Pineapple scaled up to a size you could read from across the room, and said that size was the whole reason to bother. Budde-Lund measured it at 8 to 9mm when he described it in 1885, which is barely a millimetre or two off the Pineapple however you count it. There is no scaling up. The size difference we built the pitch on does not exist, and it has gone.

That 1885 name is the accepted one, though the animal has also appeared in places as Armadillidium spinosum. The epithet comes from the Latin serratus, meaning saw-toothed, which is a fair account of the edges. In the trade it is sold as the Hedgehog Isopod, and nobody appears to have written down where that name came from.

So: 8 to 9mm, from the addenda where the species was first described, and small for the genus. It is a Mediterranean animal, recorded from France and Spain, and it rolls into a ball like most of its genus. Unlike the smooth-shelled species in the genus, the texture does not vanish when it closes up, so a balled serratum still reads as a spiky object rather than a bead.

Mediterranean, not tropical

A range of 18 to 26°C covers it, and UK room temperature sits inside that band, so nothing needs heating in a normally warm house. Humidity should sit in the middle, somewhere around 50 to 65 per cent, and it wants to be a gradient, not an even damp. Keep one end moist and let the far end dry out between waterings, then leave the animals to pick their spot. Pair that with medium airflow. This is a drier, better-aired setup than the tropical species most general care advice is written around, and a stuffy wet box is the quickest way to lose a group.

Deep litter and a constant calcium supply

A nutritious base substrate does the heavy lifting, topped with a generous covering of dry hardwood leaf litter and plenty of bark to shelter under and clamp against. That litter is the standing meal the colony grazes on between feeds, so keep it deep and keep replacing it. A permanent calcium source matters more here than in most tubs, because a heavily sculpted shell takes a lot of building and rebuilding. Cuttlebone or crushed shell stays in the enclosure permanently, not as an occasional top-up. Vegetables are taken readily, and an occasional protein offering supports breeding.

Care is easy, which is the useful thing about it, because rarity in this hobby usually arrives attached to a demanding care sheet. This species is uncommon and still undemanding. Anyone already running a drier Armadillidium tub can add it without changing a thing, so a newer keeper can take one on without a steep learning curve. If you are assembling a shelf of textured pill woodlice, Beier's Pill Woodlouse is the spiny Greek endemic, Armadillidium peraccae is the bumpy one kept drier still, and Armadillidium granulatum brings a pebbled finish. For the smooth, patterned counterpoint, the Zebra is where most people begin with the genus. Our Armadillidium care guide goes into the husbandry, and the wider Armadillidium range holds the rest.

Packing is a ventilated deli tub, sphagnum damp beneath the animals, and two thicknesses of card around all of it. We post Monday to Thursday and stop there, next-day and tracked through Royal Mail, before 1pm, which is what stops a box of ours sitting out a Saturday. A few spares travel above the number ordered. Live arrival is covered.