Quick answer: The Giant Cannonball isopod, Helleria brevicornis, is a heavy-bodied Mediterranean roller from the oak forests of Corsica, Sardinia and the nearby coasts, reaching up to around 27mm. Keep it cool, at 18 to 22°C with no heat source, in a deep tub with at least eight to ten centimetres of living substrate to burrow into, humidity in the 60 to 75 percent range held as a gradient, and gentle ventilation. Keep it as a group, never singly, feed it leaf litter and rotting hardwood, leave its droppings alone, and expect breeding to be slow. It is the only land isopod known to still ride its mate, and the research on it says females kept alone breed late and badly. Ours are captive-bred here in the UK.
Most of the isopods on this site are variations on a theme: a Porcellio, an Armadillidium, a Cubaris, each with its own colour and its own quirks but all recognisably built the same way and living the same sort of life. The Giant Cannonball is not one of those. Roll one up and you are holding a dense sphere with real weight to it, nothing like the light little pillbugs most people start with, and the animal inside has kept habits from its aquatic ancestry that every other land isopod has dropped. It is also slow, cool-loving and unfussy in a way that suits a patient keeper and frustrates an impatient one. This guide covers what it is, what the research actually says about it, and how we keep it.
What It Is
Helleria brevicornis was described by Victor von Ebner in 1868, the genus named for the zoologist Camill Heller and the species name meaning short-horned, which you will understand the moment you look at the stubby antennae. It is the only species in its genus. The family it belongs to, Tylidae, is otherwise made up of Tylos, beach-dwelling isopods that live in the sand and wrack of shorelines around the world. Helleria is the one member of that family that went inland and stayed there, and the taxonomic work on Corsican isopods by Ferrara and Taiti places the group near the base of the land isopod family tree. If you want the wider picture of how a woodlouse came to live on land at all, our piece on the move from water to land is the place to start.
Its range is small and specific. It is endemic to the islands and coasts of the northern Tyrrhenian Sea: widespread across Corsica from sea level up to about 1,200 metres, present on Sardinia, on the Tuscan Archipelago islands of Elba, Pianosa and Capraia, on the Lérins Islands off Cannes, and along the nearby French and Italian mainland coasts, reaching about 600 metres on the French side. The French national inventory records its habitat as the humus of coastal holm oak forest, and lists the species as not protected. A 2010 study of its genetics by Gentile and colleagues, in Molecular Phylogenetics and Evolution, describes it as an ancient lineage with both old and recent splits between its island populations. It is a relict, and it behaves like one.
Appearance
Vandel's 1960 account gives a maximum length of about 27mm, though length is not what strikes you. The body is deep, domed and heavily built, so a big adult has the bulk of an animal much longer than it is. The colour is a yellow-brown to tan, often with faint darker mottling, and it changes with age: juveniles are covered in small bumps and bristles, and adults are almost smooth with a scatter of paler spots. The antennules are vestigial, the antennae are short and tuck away completely, and the animal rolls into a near-perfect ball with nothing protruding, which is where the cannonball name comes from and why a rolled adult is so much heavier in the hand than an Armadillidium of the same length.
How It Lives, and What That Means for the Tub
This is a litter animal and a digger. It lives in and feeds on the leaf litter and humus layer of the forest floor, and it digs burrows around ten centimetres deep. In the wild it shelters by day and forages after dark, and keepers report that the immature animals in particular spend most of their time below the surface. Everything about the enclosure follows from that.
The substrate is the whole setup. Go deep, at least eight to ten centimetres and more if the tub allows, using a living mix of flake soil, crumbling rotted hardwood and organic topsoil loosened with a little sand so that a burrow holds its shape without the mix compacting. Leave a plain coco coir base out of it: coir holds water and feeds nothing, and this is an animal that eats its substrate. Work a permanent calcium supply through the mix, whether crushed limestone, oyster shell or cuttlebone, because a 27mm isopod moults hard and our calcium guide explains why that supply cannot be occasional. Then cover the surface with a thick layer of oak and magnolia leaf litter and some bark, which is both the cover it wants and most of what it eats. There is more on choosing and preparing litter in our leaf litter guide.
Hold a moisture gradient through the depth rather than across the surface: damp below and at one end, never waterlogged, with the top layer allowed to dry a little between waterings. A cool, deep, damp tub is also one that mould enjoys, so give it gentle ventilation and do not seal it up.
Keep It Cool
Our ideal is 18 to 22°C, and the direction of the risk is heat. This is a coastal Mediterranean and mountain forest species, not a tropical one, and it dislikes warmth: a cool room suits it far better than any heat source, and there is no case for a heat mat at all. A British house sits inside that range for most of the year, and the animal comes from forests that see cold winters, so a room that dips over January is no threat to it. What you watch for is the July heatwave, and the answer to that is a cooler room, deeper substrate and shade rather than equipment.
Keep It as a Group, and Leave the Droppings Alone
Here is where Helleria stops being just another roller. It is found in large groups in the wild, and the researchers who studied it treat it as a subsocial animal rather than a loose collection of individuals. In 1988 Françoise Mead and Danielle Gabouriaut published a study in the International Journal of Invertebrate Reproduction and Development showing that females kept in isolation reproduced later and produced less than females kept with others, and that their reproduction recovered when faeces from other Helleria were added to their surroundings. The species eats its own kind's droppings, and that coprophagy is part of how it works.
Two practical rules fall straight out of that. Keep it as a group, never as a pair or a single animal, and our guide to how many isopods to start with applies here with more force than usual. And do not strip the tub down to clean it. Change substrate in part, not in whole, and treat the frass in the litter layer as part of the environment rather than as dirt.
The Nuptial Ride
Every other land isopod mates briefly and moves on. Helleria is, as far as the published record goes, the only fully terrestrial isopod that has kept the ancestral habit of guarding a mate, a behaviour its marine and shoreline relatives take for granted. Mead and Gabouriaut described it in the journal Behaviour in 1977: the male climbs onto a female and grips her, riding her about for anything from a day to nearly three weeks before her parturial moult, the moult that opens her brood pouch. Mating happens within five to twelve hours of her shedding, and the ride carries on afterwards until she has laid her eggs into the pouch, at which point the male lets go. If you see a Cannonball apparently stuck to the back of another for days on end, nothing is wrong. That is the species doing the thing it is famous for.
There is a second oddity that follows from the first. Work on the female reproductive tract in terrestrial isopods, published by Longo and colleagues in 2011, found that Helleria females lack the sperm storage organ most other species have. A female cannot bank sperm from one mating and use it for later broods; she needs a male for every brood. So a group with a sensible balance of the sexes matters here in a way it does not for a Porcellio, and our sexing guide will help you check what you have.
Breeding, Slowly
Breeding is slow and deliberate, in keeping with everything else about the animal. Broods are small and infrequent by the standards of the weedy species, growth is slow, and the juveniles spend most of their early lives underground where you will not see them. Keepers who have run the species for several years report that a cooler spell over winter followed by spring warmth is what sets the annual cycle going. That is keeper observation rather than published work, but it fits an animal from Corsican oak forest, and a British room that drops a few degrees in winter provides it without any effort on your part. I could not find a published lifespan for the species, and I would not trust anyone who quotes one confidently. Treat it as a long-term project: a well-kept, cool, deep group with constant calcium will establish and grow, and for a species this remarkable that is its own reward.
Feeding
Unfussy, with two things that matter more than the rest: leaf litter and decaying hardwood as the always-available base, and calcium through everything. On top of that, offer soft vegetables such as courgette, squash and sweet potato in small amounts, and the occasional trace of protein. Lift anything fresh that is still sitting there after a day or two, since the cool, damp conditions Helleria likes are the same ones mould likes, and a springtail culture alongside earns its place keeping the surface clean without disturbing the burrows.
Is This the Right Isopod for You?
It suits a keeper with a cool room, a deep tub and patience. It is not difficult in the way a Scarlet or a cave Cubaris is difficult; nothing about it demands precision, and it forgives a lot as long as it is kept cool and left alone. What it does not do is reward you quickly. If you want a roller from the same sea with a good deal more colour, Armadillo officinalis 'Red Brick' is the other big Mediterranean roller we keep. If you want numbers, a Dairy Cow will give you them in a season. The Cannonball is for the keeper who wants the strangest animal on the shelf and is happy to wait for it.
Frequently Asked Questions
How big do Giant Cannonball isopods get?
The literature gives a maximum of around 27mm. The bulk is what you notice: a rolled adult is a dense, heavy ball, far heavier than any Armadillidium of the same length.
Are they related to the deep-sea giant isopods?
Not especially. Bathynomus, the deep-sea giant, is a marine isopod from a different branch of the group entirely, and every land isopod is roughly as distant from it as every other. What Helleria does share with the marine and shoreline isopods is a set of habits the rest of the land species lost, the mate-riding above all. The giant sea isopods are covered in their own article, and there is a plain-language explainer on what isopods are if the family tree is new to you.
Do they need a heat mat?
No. Our ideal is 18 to 22°C and the species dislikes heat. A cool room is the whole of the heating plan.
Why is one riding on the back of another?
Because it is a male guarding a female before she moults and mates. It can last up to three weeks and it is normal. Do not separate them.
Can I keep one on its own?
You can, but the research says it will breed late and poorly, if at all, and this is a species that seems to need its own kind around it. Start with a group.
How fast do they breed?
Slowly. Small broods, long intervals, slow growth, and juveniles that live underground. Buy it for what it is, not for the numbers.
Where to Start
Our Giant Cannonball colonies are captive-bred here in the UK, and the rest of our unusual species sit in the other species collection. Every order travels in a ventilated tub of damp sphagnum inside a double-walled box, is dispatched Monday to Thursday by tracked next-day post so nothing sits over a weekend, and is covered by our live arrival guarantee.
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