Giant Cannonball Isopods (Helleria brevicornis)

If you have ever watched a documentary about the giant isopods that scavenge the deep-sea floor, the Giant Cannonball is the nearest thing in feel you can keep on dry land. Helleria brevicornis is a Mediterranean oddity: one of the largest rolling isopods in the world at up to around 27 mm, and one of the oldest branches of the land isopod family tree. Roll one into its defensive ball and you understand the name at once, a dense, heavy sphere with real weight to it, quite unlike the light little pillbugs most keepers start with.

It is a remarkable animal beyond the size. Helleria is the sole member of its genus and the only member of its family to live inland away from the shore, it lives in large subsocial groups rather than loose colonies, and males perform an ancestral "nuptial ride", gripping and guarding females through the breeding period, a hangover from its aquatic ancestry that no other land isopod keeps. Keeping it is a real brush with evolutionary history, and it stands apart from anything else on a shelf of giants like the Titan and the big Jumbo Gestroi or its standard Gestroi.

Care is where it asks for a little more thought, because it is a woodland and scrub species of the Mediterranean islands and their nearest coasts rather than a tropical or dry-cliff one. It prefers cooler conditions, somewhere around 18 to 22°C, and dislikes heat, so a cool room suits it far better than any heat source. Give it a deep substrate to burrow into and a moisture gradient that stays damp but never waterlogged, with one reliably moist zone and gentle ventilation. In the wild it shelters by day in leaf litter and loose soil, from the shore up into the Corsican mountains, and forages after dark, so a deep, quiet, cool setup with plenty of cover matches its habits, and it does best kept as a group rather than singly.

The substrate is the key to keeping Helleria well. It is a burrower, so go deep, at least eight to ten centimetres, using a living mix of flake soil, rotted hardwood and organic topsoil loosened with a little sand so it holds a burrow without compacting. Keep plenty of calcium through it via cuttlebone, crushed limestone or oyster shell for a large animal's moults, and leave a plain coco-coir base out of the mix. A layer of magnolia and oak leaf litter and some bark on top provides cover and grazing.

Feeding is unfussy, but calcium and rotting wood matter most: leaf litter and decaying hardwood as the always-available base, soft veg such as courgette, squash and sweet potato in small amounts, and the occasional trace of protein. Remove uneaten fresh food promptly, since the cooler, damp setup Helleria likes is also one mould enjoys, and a springtail culture alongside earns its place keeping the substrate clean.

Breeding is slow and deliberate, in keeping with the species' unusual biology: family groups are stable, broods are small and infrequent compared with the weedy isopods, and patience is very much the order of the day. That said, a well-kept, cool, deep-substrate setup with good calcium will see a group establish and slowly grow, which for a species this remarkable is its own reward. Our Giant Cannonball care guide goes through the research behind all of this, including the nuptial ride and why the species has to be kept as a group. Every order is captive-bred and packed with care in a double-walled box around a ventilated tub of damp sphagnum, posted Monday to Thursday by tracked next-day Royal Mail and covered by our live-arrival guarantee.