Castle Black Isopods (Spherillo sp.)

You will see more of the Castle Black's work than of the animal itself. A settled group lives down in the substrate, and the first sign that all is well is usually a scatter of small holes and tunnels rather than isopods on show. When one does surface, the wait pays off: glossy blue-black bands, each finished with a thin white rim, run forward to a pearl-white front and a mottled amber head, and it is one of the most striking pill isopods I have had through the door.

What it is, and what to call it

Castle Black is a trade name, and the genus attached to it depends on who is selling. You will most often see it sold as Venezillo sp. 'Castle Black', and less often as Spherillo or Cubaris. I list it as Spherillo sp. 'Castle Black', but it is the same animal whichever genus the label carries. None of those placements has been confirmed, and one experienced breeder thinks none of them fits, filing it simply as Armadillidae sp., the family all three genera belong to, and suspecting a genus that has not yet been described. Most sellers give Thailand as its origin, though I have found no published collecting locality, so that rests on the trade's word. Whatever it ends up being called, the care does not change.

Appearance and size

The pattern is what sells it. Behind the head, the first body segments are a milky pearl white; from there to the tail the shell turns a glossy slate to blue-black, and every segment carries a pale rim, so the back reads as neat bands rather than a solid block of colour. The head runs from a pale cream to amber or rust-brown with darker mottling, and the legs and antennae are a translucent blue-grey. Expect some variation: the smaller animal in our photos is much paler. That same breeder thinks it looks almost like a better version of the Tapir form of the Panda King, only bigger and glossier.

Going by the keepers who have raised them, adults are bigger than a Panda King and about the size of a Zebra isopod or a touch under, so expect roughly 12 to 18mm. Uncover one and it will often roll itself into a ball.

Care and set-up

Think damp, deep and dark. This is a burrower that likes a consistently moist, organically rich substrate and spends most of its time buried in it, so give it a deep bed to dig through. Build that bed on flake soil, white-rotted hardwood and organic topsoil, work in plenty of crumbling rotten wood, and cover the top with leaf litter and a few flat pieces of bark. Keep most of the substrate damp rather than wet, about as damp as you would keep a Rubber Ducky, with a pad of sphagnum moss at the wettest end and a slightly drier end for them to choose. Ventilation wants to be moderate: a few vents so the air turns over, not a sealed box and not a breezy one.

Warmth matters. Aim for 22 to 26°C. A warm room can manage that in summer, but in most UK homes you will need gentle, thermostat-controlled heat for much of the year, at the side or back of the tub and never underneath, where it would dry and cook the damp layer they live in. I warm my own tubs with heat cable run along the back of the shelves. Keepers have them breeding anywhere from the low twenties up into the high twenties, so steadiness counts for more than the exact figure.

Diet

Rotting hardwood and leaf litter are the staples, and they need to be there all the time, topped up as the colony works through them. Beyond that, one keeper finds they take thin slices of potato and carrot more readily than protein. Offer a little fish food now and then, and take out anything fresh still there after a day or two, before it furs over in the damp. Calcium needs to be permanent: a piece of Malawi light limestone or cuttlebone in the tub supports clean moults.

Breeding

Slow on both counts. Castle Blacks grow slowly and breed slowly, and a new group can take a good while to settle before young appear. Expect to spot juveniles at the surface long before you see any of the tiniest young, which presumably stay buried. Ten or more gives you a fair chance of both sexes; then leave the substrate alone. Of everything I have learned since I started keeping isopods in 2021, letting a slow species get on with it is the lesson that applies most here.

Who the Castle Black suits

This is not a first isopod. It suits a keeper who has already kept a damp-loving Asian species, a Rubber Ducky for example, and who is happy to own an animal that spends most of its time out of sight. If you want something on show every time you lift the lid, choose something bolder. If slow, rare Asian rollers are your corner of the hobby, the Starsky isopod sits well beside it, and our Castle Black care guide covers set-up and breeding in more depth.

Your Castle Blacks will be captive-bred and, as with every isopod order, counted out with at least 20% extra. The box is double-walled, the tub inside is vented and packed with damp sphagnum, and a Castle Black care sheet travels with them, plus a heat pack in cold spells. Parcels leave Monday to Thursday by Royal Mail, and orders sent Royal Mail Next Day are covered by my live arrival guarantee.