Porcellio expansus 'La Sénia' Isopods

Most isopods ask you to look closely. Porcellio expansus 'La Sénia' does the opposite. This is the largest-growing locale of the species, collected around La Sénia in north-east Spain, and mature adults push toward 5cm, roughly matchbox length. Plenty of other forms of P. expansus settle a size or two smaller, but this one outgrows them all, which is why keepers chasing a true giant seek it out.

I keep them as much for the animal as for any job they do in a tank. The carapace is dark, broken up with lighter patches and a paler fringe around the edge, and the whole body is broad and heavy. You will see them sold as the La Sénia isopod, the giant Spanish isopod or simply giant expansus, and they sit in the family Porcellionidae with the familiar Porcellio scaber. Like every Porcellio, they cannot roll into a ball. They are too large and flat-bodied to conglobate, so they rely on bulk and cover rather than curling up when startled.

The point I would press hardest is moisture. These are dry-climate animals, and the common mistake is treating them like a tropical species. I aim for 50 to 60% humidity with high ventilation, and I build a gradient rather than wetting the whole enclosure. One end stays a little damp from moist sphagnum or a single watered corner, and the rest is left dry, so the colony can choose where to sit. A mesh or well-drilled lid keeps air moving and stops damp settling in. Room temperature suits them year-round, anywhere from 21 to 28°C, and only a cold home needs a thermostat-controlled heat mat on the side of the tank, never underneath.

Substrate does more work here than in most setups. I run 8 to 10cm of a soil base with plenty of crushed limestone worked through it, a little sand for drainage in the dry zone, and, above all, a good amount of decaying hardwood. They favour rotting wood, so it doubles as shelter and food. Over the top goes a deep layer of hardwood leaf litter, oak and magnolia being my staples. Moulting a 5cm shell takes real calcium, so I keep cuttlebone and limestone available at all times. They take vegetables such as carrot, courgette and squash every few days, plus a little animal protein once or twice a week from fish flake or dried shrimp.

Temperament is part of the draw and part of the work. Adult males can be territorial and will squabble in a cramped tank, so give a colony room and several well-spaced hides of cork bark, flat stone and wood. Breeding is slower than a prolific species such as Porcellio laevis, with smaller broods and a longer wait, though the young arrive large and hardy for their kind. Start with a group of ten or more and treat it as a patient project rather than a fast cleanup crew.

If the species appeals but you would rather have colour than sheer size, the Orange form of the same species is worth a look. The dry husbandry is identical, and it is only the size and the colouring that set the two apart. You will find both, and the rest of the giants, in the Porcellio collection.

Common questions

How big do La Sénia expansus really get? Mature adults reach around 4 to 5cm, roughly matchbox length, which makes this the largest-growing locale of Porcellio expansus and one of the biggest isopods in the hobby.

Do they need it damp like other isopods? No. Keep them drier, at 50 to 60% humidity with strong ventilation and a moisture gradient, and let them move to the damp end when they want it.

Can they roll into a ball? No Porcellio can. They are too large and flat-bodied to conglobate, so they use their size and plenty of cover instead of curling up.

However you keep them, these are livestock I am glad to have in the collection, and they travel out in careful double-walled packaging under our live-arrival guarantee. Give them space, keep them on the dry side, and a La Sénia colony will repay the patience.