The Sandstone isopod is a pill woodlouse from the Dalmatian coast with a back like fine-grained stone. For years it was sold as Armadillidium omblae. It is Armadillidium scaberrimum, described in 1859, and a good deal of what gets repeated about it, its size included, comes from shop pages rather than from the published descriptions. This guide sets out what the published record says, where the hobby's figures come from, and how to keep it. The short version: an easy, dry-leaning pill woodlouse that wants a moisture gradient, plenty of dry leaf litter and calcium within reach at all times.
Which Species It Is
J. P. E. F. Stein described Armadillidium scaberrimum in 1859 from animals he found in numbers under stones near Ragusa and Cattaro, today Dubrovnik in Croatia and Kotor in Montenegro. The name is still the accepted one. WoRMS and ITIS list no synonyms, and the genus has never changed.
The hobby sold it for years as A. omblae. That is a real species, described by Verhoeff in 1900 from two animals in a ruined wall in the Ombla valley near Dubrovnik, and it is still valid, so this was a misidentification, not a renaming.
The two original descriptions separate them well. Verhoeff's animal is grey with rows of dull black spots, carries its tubercles down the middle of the back and leaves the sides largely smooth, and in his words it can never roll up completely. Stein's has its whole back densely granular. An isopod that is granular from edge to edge fits Stein's species, as the animals in our photographs are. Stein's other characters need a lens: a long, wedge-shaped plate at the front of the head, a tail tip drawn out into a triangle where Verhoeff's is broadly squared off, and elongated, somewhat spatula-shaped uropods. The one relative Stein compared it with was A. pallasii, which he called almost twice the size and lead-grey.
Several sellers now describe the stock as "formerly A. omblae", but we could find no published account of who re-identified it, when, or from which animals, and at least one American listing still sells it under the old name. Read against the two descriptions, the correction looks sound; we simply found no paper behind it.
What It Looks Like
Stein's Latin for the back is granulis densissime obtectum: very densely covered in granules. That grainy finish, with the pale colour, is presumably what the trade name describes, and the texture stays on show around the outside of the ball when the animal rolls up. If it is texture that draws you, A. serratum 'Hedgehog' goes further, its back raised into spikes.
In our photographs the animals run from cream to a peachy tan, some with a cool lilac-grey cast and a few with bluish patches on the back. Stein called his a dull clay-yellow with paler side plates, Budde-Lund a dull yellowish, and neither mentions lilac. Descriptions of the Sandstone as lilac are describing some animals, not the species.
Size is where the hobby has drifted furthest. Stein measured his animals at 15 mm long and 8 mm wide, and Budde-Lund, measuring one that Stein had sent him, gave 14 mm in 1885. Hobby listings, ours among them, have given up to 20 mm, and we have found no published measurement that supports it. If you measure one, take the body only, from the front of the head to the back of the last segment, leaving out the antennae and uropods; that is how our care panels measure length.
Where It Comes From
Stein's two localities and every museum and photographic record we could find sit on the Dalmatian coast and its islands between Primošten and Dubrovnik, plus Kotor just over the border. Museum specimens include one of Stein's own animals from Dubrovnik, now in the Natural History Museum in London, one collected at Omiš near the sea above a gravel beach, and an old lot labelled "Lessina", most likely Hvar under its Italian name. Photographic records add Split, Primošten, the island of Brač and the coast between Omiš and Podgora.
Two outliers do not change that picture. WoRMS lists Italy, but we found no Italian specimen or published Italian record behind it. And in 1885 Budde-Lund noted animals sent to him from Crimea under this name, but judged them to differ from A. pallasii only in being smaller. Beyond Stein's note that it lives under stones and that one label from Omiš, we found nothing recorded about its habitat on that coast. It is a coast of hot, dry summers and mild, wetter winters, which fits the drier set-up keepers have settled on, though we know of no test of the connection.
The same stretch of coast is home to the Clown, A. klugii, which Schmalfuss records from the island of Pašman down to Corfu, Kotor and Hvar included. The two share a coastline and very little else: the Clown's back is smooth and spotted.
Setting Up
We found nothing published on how this species lives, so every care figure here, ours included, comes from keepers rather than from study. Our care panel gives 17 to 26°C and 50 to 65 per cent humidity, drier than most people expect from a pill woodlouse. A heated UK room covers that temperature range for most of the year, though a summer hot spell can take a room past the top of it.
Build that humidity as a gradient, not a single reading: a damp patch over about a fifth of the floor and the rest left to dry, with enough ventilation that moisture leaves the tub instead of hanging in it. Vents on opposite sides of the tub move air through it rather than letting it sit. A 5 to 15 litre tub suits a starter group, as it does for the rest of the genus. A nutritious base 5 to 8 cm deep is plenty. Cover it with a generous layer of dry hardwood leaf litter, which is shelter straight away and food as it rots, and add bark for the group to gather under. Our guide to setting up a first isopod enclosure takes the tub, the vents and the substrate mix step by step.
Feeding
Leaf litter is the staple, and oak and beech are the usual choices. Keep a mix of freshly fallen and well-rotted leaves in the tub, top it up as it goes, and add rotted hardwood for the colony to work through. Vegetables go down well as an extra, and a little protein now and then, a pinch of dry fish flake or a dried insect, keeps growth steady. Keep cuttlebone or crushed shell in the tub at all times: calcium goes into every moult, and a colony short of it moults badly.
Breeding
Here too we found nothing published, so this is breeders' experience. The Sandstone is prolific, though not as a steady trickle of young: females release large broods at intervals, so numbers can sit still for weeks and then jump. Keepers report adults living two to three years.
Leave the colony alone once it settles. The mistake I made with my own colonies was fussing: I kept opening them, moving things and checking on everything, and my numbers started falling. Leaving them alone is what fixed it. Start with a group rather than a handful; our guide to how many isopods to start with explains why the number matters.
Frequently Asked Questions
Is it the same animal as Armadillidium omblae?
No. A. omblae is a separate species, still valid, and by Verhoeff's description it is grey with rows of dull black spots, carries its tubercles down the middle of the back and cannot roll up completely. Read against the two original descriptions, the stock sold under that name was misidentified.
Does the Sandstone roll into a ball?
Sellers describe it rolling into a ball when disturbed. Neither Stein nor Budde-Lund says whether it closes completely, and we found no other source that does.
How big does it get?
Stein gave 15 mm in 1859 and Budde-Lund 14 mm in 1885, both from Stein's own animals. We have found no published measurement behind the hobby's figures of up to 20 mm.
Is it easy to keep?
Yes. It is rated easy on our care panel, is comfortable in a heated room for most of the year and eats what most isopods eat, which makes it a sensible pill woodlouse to add after a first species. The one thing to get right is keeping most of the tub dry, which suits keepers who find the damp species hard work.
Can I keep it with other isopods?
Keep it in its own tub. Different isopod species very rarely cross: we know of no documented cross between two genera, the woodlouse hybrids we have found in the scientific literature are all between subspecies of one species, and the best-known hobby report of a cross between species has not been confirmed genetically. The bigger risk in a shared tub is that one species crowds out the other. Our piece on breeding two different isopods together goes through it.
Where to Start
The Sandstone is an easy isopod to keep and an easy one to misdescribe. Our Sandstone isopods carry the care panel figures above, and our Armadillidium care guide covers the rest of the genus.
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