Porcellio haasi is the big yellow Spaniard: a long-bodied isopod from the dry country of the Ebro basin, sold in the UK as the 'High Yellow' line and the 'Bardenas' locality line. It is a medium-difficulty animal, and an unusual one, because much of what the hobby teaches about keeping isopods damp does not apply to it. Keep it at 18 to 26°C in a well-ventilated tub that is mostly dry, with a fifth to a quarter of the floor damp, plenty of flat cover, a permanent calcium source and protein two or three times a week.
Names, and Where Haasi Comes From
The species is Porcellio haasi Arcangeli, 1925, described from woodlice that Fritz Haas, the Senckenberg malacologist, collected in northern and eastern Spain between 1914 and 1919; the name honours him. The holotype came from Flix, on the Ebro in Tarragona, in November 1914.
It is a Spanish endemic: every occurrence record we found is from Spain, most of them along the Ebro basin and its flanking hills, from Tarragona and the Lleida plain through Aragón, Castellón, Navarre and La Rioja, with an outlier on the plateau in Soria and Guadalajara. Navarre includes the Bardenas Reales, badlands in the south-east of the region, where the species has been photographed at least three times since 2019 and was recorded from neighbouring Caparroso in 1983. Our care panel's origin, north and north-east Spain, fits that map.
That map is most of the care sheet. We found no published account of the animal's microhabitat, but the localities are dry, much of it limestone and gypsum ground, and keepers report finding it under dead wood and moss and along rock outcrops, where crevices hold a little damp. You are copying a dry, airy place with a few damp pockets, not a forest floor.
What They Look Like
Large, for a Porcellio. We found no published measurement of the species, so the sizes quoted for it are keepers' figures, ours included: our care panels give 20 mm of body for the High Yellow and 30 mm for the Bardenas. The tails are the uropods, the paired appendages at the rear, and on some animals they are strikingly long, adding up to about 10 mm by keepers' accounts. Keepers sex a colony by them, since males are said to carry far longer uropods than females; we found no published account of the difference, and the underside check in our guide to sexing isopods is the reliable method. Like every Porcellio, it cannot roll into a ball.
The two lines differ in pattern rather than in care. In our photographs the High Yellow has bright yellow spots in paired rows down a dark back, with a pale, translucent skirt and pale legs, while the Bardenas is pale yellow over most of the back, marbled with light brown, with a white skirt and white legs.
Temperature: A Heated Room Does Most of It
Our panel gives 18 to 26°C. An English home averages around 19°C over the heating season and about 21.5°C in July and August, so a room keeps it inside that band for most of the year. Keepers disagree on the top end, some reporting it tolerating 30°C and others capping the range at 24 to 27°C, so treat 26°C as the working ceiling and keep the tub off a sunny windowsill. Our guide to seasonal care explains why a cool winter room slows most isopods rather than harming them, and how to add gentle warmth if you want young through the winter: a low-wattage mat on a thermostat, against one side of the tub and never underneath it.
Humidity: The Dry End Is the Whole Job
Our panel's 45 to 55 per cent covers the whole tub, drier than almost anything else in the hobby. Damp a fifth to a quarter of the floor, a corner where the substrate holds moisture under a piece of bark, and leave the rest dry. Then give it strong airflow: cross-ventilation, with mesh or rows of holes on opposite sides as well as in the lid. Stagnant humid air is more dangerous to this species than air that is slightly too dry, and the usual mistake is misting the whole tub.
One caveat, because the "dry Spanish species" line gets overstated: dry is not arid. The young need that damp corner most, since mancae dry out fast and a uniformly parched tub loses them first, and even dry-country isopods need a damp spot to moult, as our moulting guide explains. You are building a gradient, most of it dry and one part reliably damp, and our Giant Spanish Porcellio guide covers the same setup for five other Spanish species.
Substrate, Cover and Calcium
Five to eight centimetres of substrate is plenty: a nutritious base of flake soil, organic topsoil and crumbled white-rotted hardwood, with a little sand for drainage. Leave coco coir out; it holds water, the opposite of what this animal wants, and feeds nothing to an animal that eats its floor. Keep the mix drier than you think, damp in that one corner only.
Cover matters more than depth. These are surface animals that spend daylight pressed flat under cover, so lay flat bark pieces down and stack them in layers, add flat stones if you have them, and give a group several separate hides rather than one. Hardwood leaf litter goes over everything, as food as much as cover.
Calcium is a serious requirement. Woodlice build a calcium carbonate cuticle, a large Porcellio rebuilds a heavy one at every moult, and much of this species' range is limestone and gypsum country, so keep cuttlebone or crushed limestone in the tub permanently, in more than one place.
Feeding: Protein Is Not Optional
The everyday diet is decaying leaf litter and white-rotted wood, always available, with a little vegetable once or twice a week. What sets the Spanish Porcellio apart is protein: keepers who breed this species feed dried shrimp, fish flake or similar two or three times a week. Keepers of the Spanish Porcellio report that underfed colonies pick off members at the moult. Partly eaten bodies prove little on their own, since isopods scavenge any corpse; the tell is losses concentrated at the moult in an otherwise healthy colony, and the first thing to change is the protein. Put it down on the dry side in small amounts and lift anything uneaten after a day or two.
Space, and the Males
Give a starter group real room: fifteen litres and up, with several separate hides. One or two sellers describe mature males chasing rivals and holding their own corners, one more reason for multiple hides; we found no breeder documenting it, so treat it as a sensible precaution rather than a rule.
Breeding: In Bursts, Not a Trickle
We found nothing published on how this species breeds, so what follows is keepers' experience. It breeds seasonally, prolific through spring and summer and quiet in the cooler months, so a colony can sit unchanged for a stretch and then release a wave of young at once. Broods are moderate, and time to maturity runs from four months to nine in the reports we have seen, too wide a spread to promise either. Lifespan is given as one to two years by some keepers and two to three by others. Either way, start with ten or more, keep warmth, protein and calcium steady, and expect numbers in bursts.
High Yellow and Bardenas: What the Names Mean
'Bardenas' is a locality line, named for the Bardenas Reales of Navarre, where the species is recorded. The trade tells two stories about 'High Yellow', that it was selected over generations for intense yellow from ordinary stock, and that it comes from a naturally yellow-marked population around Zaragoza; we found no published account of which, and both may be partly true. A 2025 study of the isopod trade by Robla and colleagues, in Insect Conservation and Diversity, lists this species as an Iberian endemic offered in six named varieties across dozens of shops, and notes that locality names imply collection from wild populations, one reason to ask any seller where the parents came from. The High Yellow we sell is bred here in the UK.
Who This Species Suits
The keeper with a colony or two behind them who wants something big and bright, and the collector of Spanish Porcellio. It is not a first isopod, not because it is delicate but because its needs run against a beginner's instincts: dry where you would mist, airy where you would seal, and fed protein where you would offer a slice of carrot. If you are new to the genus, a hardy Porcellio scaber line teaches the moisture gradient at a forgiving scale first.
Frequently Asked Questions
Where do Porcellio haasi come from?
Spain. Every record we found is Spanish, mostly from the Ebro basin and its surrounding hills, and it was described in 1925 from Flix in Tarragona.
How big do Porcellio haasi get?
Keepers put adults at about 20 to 30 mm of body depending on the line; our care panels give 20 mm for the High Yellow and 30 mm for the Bardenas. Long uropods can add up to about 10 mm more. We found no published measurement.
Do Porcellio haasi need a heat mat?
Not usually. An English home sits inside their 18 to 26°C band for most of the year. For young through the winter, a low-wattage mat on a thermostat, against one side of the tub and never underneath it, gives gentle warmth.
What humidity do Porcellio haasi need?
Our panel's 45 to 55 per cent, built as a gradient: a fifth to a quarter of the floor damp, the rest dry, with strong ventilation. Misting the whole tub is one way to lose a colony; a bone-dry tub is the other.
Why are my Porcellio haasi eating each other?
Usually protein, keepers find. Spanish Porcellio want animal protein two or three times a week, and an underfed colony picks off members while they moult. Feed dried shrimp or fish flake on the dry side and add cover.
What is the difference between haasi 'High Yellow' and 'Bardenas'?
Pattern, size and origin, not care. In our photographs the High Yellow is spotted and the Bardenas marbled, keepers put the Bardenas larger, and only Bardenas is a named locality. They are the same species, kept the same way.
Where to Start
Start with a group of ten or more in a tub of fifteen litres or more, set up dry and airy with one damp corner, and give protein from the first week. Browse our Porcellio haasi 'High Yellow' and Porcellio haasi 'Bardenas' listings.
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