In some populations of the common pill woodlouse, sex is not decided by chromosomes. It is decided by a bacterium, and in one lineage the bacterium's remains have been absorbed into the animal's own genome and now work as a sex chromosome. Meanwhile the gut microbes that woodlice were long said to depend on for digesting wood turn out not to carry the enzymes for it, a British woodlouse lives permanently inside ant nests without ever being attacked, and the isopod most people have heard of is the one that eats a fish's tongue and takes its place. This is a tour of the partnerships that run isopods, and of the ones where isopods are the junior partner.
The Bacterium That Decides Sex
Wolbachia is a bacterium that lives inside the cells of a great many arthropods and is passed down through the egg, never the sperm. Because only females transmit it, anything it can do to make more females is to its advantage. In Armadillidium vulgare it does the most direct thing possible: it turns genetic males into functional, breeding females.
The mechanism is specific. An uninfected male expresses a hormone called insulin-like androgenic gland hormone continuously from birth, and that hormone is what makes him male. In an infected animal, expression stops at the fourth juvenile stage, exactly the point where male differentiation would begin, and it stops alongside a roughly fivefold jump in the bacterial load. The current reading is that the bacterium induces something like insulin resistance, leaving the animal's tissues unable to respond to the hormone.
Nobody has yet found the gene that does it. The feminising factor was first proposed in 1984; three feminising strains have since been sequenced from this one host species, narrowing the search to three candidate proteins. More than forty years on, candidates is what they remain. If you keep Armadillidium vulgare in any quantity, some of your animals are very likely carrying this.
A Bacterium's Corpse, Working as a Chromosome
In 1984 two French researchers proposed that the mysterious feminising factor in some pillbug lines was not the living bacterium at all, but a piece of Wolbachia DNA that had got into the animal's own chromosomes. That was roughly twenty years before anyone documented a single case of Wolbachia DNA transferring into an animal genome.
In 2016 they were shown to be right. The factor is a nuclear insert of about three million base pairs containing around 83 per cent of a feminising Wolbachia genome, sitting in the chromosomes of A. vulgare. It is degraded: hundreds of the bacterium's genes are missing or truncated, including some it could not live without. It is inherited exactly as a chromosome is, it is completely linked to female sex, and it sits in a genetic background that had already lost the animal's original female sex chromosome. A dead bacterium has become a new sex chromosome.
Part of why this was possible is that pillbug sex chromosomes are evolutionarily young. The female-specific sequence in the A. vulgare assembly comes to about 673 kilobases, 0.04 per cent of the genome, and the authors date the divergence of the two sex chromosomes to roughly three million years. There was very little there to displace.
How Common Is Any of This?
Less common than the story implies, which is worth saying because it gets retold as though every pillbug in the garden were running on bacterial sex determination. In a survey of French populations sampled across fourteen years, just under 30 per cent of animals carried the nuclear insert alone, 15 per cent carried living Wolbachia alone, half a per cent carried both, and 54 per cent carried neither.
Prevalence varied enormously between sites. Living Wolbachia reached 63 per cent in one city and sat at 2 to 10 per cent elsewhere, while the insert dominated four other populations. Within each population, though, the picture held steady over time. A large minority, then, not a species-wide condition.
Is Wolbachia a Partner or a Parasite?
Both words get used, and the evidence has moved. On the cost side, a 2024 study of one-year-old females found infected animals more than fifteen times as likely to die over the year as uninfected ones, with significantly fewer of them producing any offspring at all, reduced immune cell density and a raised ageing marker. A 2026 study found that both sex and Wolbachia carriage reshape the animal's lipids, with infected females shifting towards neutral storage lipids and away from cardiolipin and from cuticular hydrocarbons. That last one raises the possibility that infected animals are more vulnerable to drying out, which is the bit a keeper might actually notice.
On the benefit side, infected animals survive injected Listeria and Salmonella better than uninfected ones. The protection is narrow rather than general: against the iridescent virus that turns woodlice blue, a 2026 study found no Wolbachia-mediated protection at all, and animals carrying only the nuclear insert, without live bacteria, get no survival benefit either. The balance sits on the parasite side of the ledger, with a real but specific benefit attached.
What Else the Bacterium Does, and What It Does Not
Feminisation is not the only trick. Cytoplasmic incompatibility, where infected males and differently infected or uninfected females produce dead embryos, is documented in Cylisticus convexus and in the two subspecies of Porcellio dilatatus, where two different strains produce incompatibility in both directions.
One thing you will read that is not supported: that isopods show feminisation, cytoplasmic incompatibility and male killing. The first two are documented. Male killing is well established in moths, fruit flies and ladybirds, and has not been reported in any terrestrial isopod.
There is also a transmission route worth knowing about if you run crowded cultures. Uninfected woodlice that ate infected ones carried live Wolbachia in their haemocytes, ovaries and nervous systems three and six months later, although at much lower densities than in naturally infected animals. The bacteria survived digestion and crossed the gut wall.
The Dwarf White Question
Given all that, it would be reasonable to assume Wolbachia is behind the other reproductive oddity every keeper meets: dwarf whites, in which only females are known and the culture is taken to reproduce without males. It is not, and the assumption is worth dismantling.
Wolbachia does induce parthenogenesis in wasps and mites. That specific effect has never been documented in any isopod, and when a large South American survey screened Trichorhina tomentosa it found the bacterium in only about five per cent of animals, which is the opposite of what you would expect if it were running the species' entire reproductive mode.
The woodlouse where parthenogenesis is fully worked out is a European one, Trichoniscus pusillus, with triploid all-female populations reproducing clonally. British distribution work on it dates from the 1980s, and it was confirmed genomically in 2024, in an American population. That study did not test for Wolbachia either, and said so.
The Wood Problem, and a Story That Turned Out Backwards
Here is the piece of received wisdom most worth revising, and it is one we have repeated ourselves.
For years the standard account was that woodlice cannot digest wood and depend on bacteria in their midgut glands to do it for them. Genome and transcriptome work has since shown that isopods carry their own cellulase genes: endo-beta-1,4-glucanases turn up in more than 93 per cent of the 64 isopod transcriptomes surveyed in one study, along with a range of host-encoded hemicellulases. A marine relative, the gribble Limnoria, bores through wood with a gut that carries no resident microbial community, using its own enzymes.
Then, when the best-known midgut symbionts were finally sequenced in full in 2024, the four genomes assembled encoded no plant-polysaccharide-degrading enzymes whatsoever. The authors concluded the symbiosis is not characterised by carbohydrate degradation at all.
And there is an experiment that questions the whole framing. Porcellio scaber was fed diets of 30, 60 and 90 per cent cellulose, with and without a visible biofilm on the food. The biofilm promoted growth regardless of how much cellulose was present, and gut cellulase activity was unaffected by either variable. The authors concluded that neither the animal's own enzymes nor ingested ones made a significant contribution to cellulose use, and that it is the biofilm community that determines the quality of a plant diet.
Put those four strands together and the old story is not so much wrong as pointed in the wrong direction. The animal turns out to have the enzymes it was said to lack, the symbionts turn out not to have them at all, and the nutrition may not be coming from the cellulose in the first place. The microbes may matter chiefly as food.
That is the argument for a nutritious substrate base of flake soil, rotted hardwood and topsoil rather than an inert one. It is also why a substrate needs replacing rather than topping up forever: what the colony is eating is partly the living film growing through it. Our piece on the nutritional value of isopods looks at the same animals from the other end.
What the Gut Symbionts Actually Are
Isopods do have characteristic bacteria. Candidatus Hepatoplasma crinochetorum and Candidatus Hepatincola porcellionum live in the digestive glands; Candidatus Bacilloplasma grips the hindgut wall and has never been found in soil, in leaf litter or in isopod faeces. But characteristic is not the same as inherited or irreplaceable. These bacteria are picked up from the environment, with food, from droppings and from eating each other, rather than passed down from the mother.
The community is really two communities. The one in the digestive glands and caeca looks stable and host-linked, protected even under experimental stress. The one in the hindgut is highly labile: under starvation the dominant group collapsed from over 80 per cent of the community to under 4 per cent while others surged, then recolonised on refeeding.
Most telling of all, when laboratory and wild pillbugs were compared, their very different bacterial communities delivered largely the same enzymatic capability. If different bacteria do the same job, the identity of the bacteria is not the point.
One detail for anyone who likes this sort of thing: those midgut symbionts carry what looks like an intact CRISPR-Cas9 system. Bacterial adaptive immunity, inside a woodlouse's digestive gland. Since they do not appear to be digesting anything for their host, one suggestion is that they earn their keep by physically crowding out pathogens.
Why Woodlice Eat Their Own Droppings
The explanation most often given is copper recovery. Isopods use copper-based haemocyanin to carry oxygen, copper is lost in the faeces, and eating them gets it back. That comes from work in the 1960s. The study that set out to test it, in 1982, found that copper is not normally a limiting nutrient for terrestrial isopods even when dietary copper was manipulated.
What that same study did find was that preventing coprophagy slowed growth, and that the reason is microbial: droppings are a substrate that microbes colonise and improve, and re-eating them recovers what the microbes have made. A later experiment found the benefit inconsistent and dependent on the litter: a gain on oak, nothing on alder, and reduced digestibility of the faeces derived from alder.
The safest statement is that faecal pellets are re-ingested because microbes have improved them, that the gain is real but conditional on diet quality, and that the copper explanation is an old hypothesis later work did not support. The same microbial conditioning is why well-rotted litter beats fresh, and why tannins in the substrate matter to what grows on it.
The Woodlouse That Lives With Ants
Turn over a stone on British grassland and you may find a yellow ant nest with small, blind, white woodlice moving unhurriedly among the ants. That is Platyarthrus hoffmannseggii, and it is an obligate ant associate: it is not found away from nests, and the ants do not attack it. In Britain it is recorded with Lasius and Myrmica species.
The neatest finding about it is that its body size depends on its landlord. Females from red wood ant nests had heads 1.30 times larger than those from Lasius flavus nests, and the authors attribute that to plasticity rather than to separate genetic lineages. The same animal grows to a different size depending on which ant it lives with.
What it eats is another matter. The standard account, that it feeds on ant droppings and nest refuse, comes from old natural history rather than from a feeding study, and the modern work on the species does not address diet at all. Whether it costs the ants anything has never been measured either, so commensal is a label rather than a finding.
This corner of the subject is still opening up. In 2022 a pill woodlouse was described from central Spain, Cristarmadillidium myrmecophilum, found only under stones holding ant nests. Along with three species described in the same paper it represents the first known cases of strict ant-nest specialisation in the Armadillidiidae: the family that gives us the common pillbug turns out to contain ant specialists, and we only learned that four years ago.
Parasite and Prey
Cymothoa exigua is the animal every article like this ends up at. It enters a snapper through the gills, feeds on the tongue until the tongue withers away, and remains attached to the stump. The original 1983 description reports that the fish then uses the isopod's body to manipulate food, and that sentence has been repeated ever since as though the fish had been fitted with a working prosthetic.
It has never been tested. No feeding-efficiency measurement, no controlled comparison. What has been measured, in a close relative in the same genus, is that infected largespot pompano grow markedly more slowly than uninfected ones, with no difference in what they eat, which points at respiratory distress and damage rather than impaired feeding. Across the family the recorded effects include tongue destruction, gill damage, skull deformation, reduced growth, impaired reproduction and death, with prevalence up to 73 per cent in wild populations and 98 per cent in fish farms, where mass mortalities have been recorded. A few studies have recorded no obvious harm, so the picture is not uniform, but the replacement tongue is a striking observation rather than a demonstrated favour.
The family is large: 40 genera and 383 accepted species, characteristically protandrous hermaphrodites that settle as free-swimming juveniles, mature as males and become female as circumstances require. Their cousins the Bopyridae are the lump you sometimes see bulging from one side of a shrimp's carapace. In female shrimp they prevent the ovaries maturing without changing external appearance; in males they alter the external sex characters while leaving the testes alone.
Then there is the reversal. The most famous mutualism in marine biology, cleaner wrasse picking parasites off reef fish, runs largely on isopods: gnathiid isopods are the principal thing being removed. Isopods are not the cleaners in that relationship. They are what gets cleaned off. Nearer home our own animals are prey too, as the isopod-eating spider shows.
They are manipulated as well as eaten. One of the founding demonstrations that a parasite can alter host behaviour was done on a woodlouse: Armadillidium vulgare carrying a spiny-headed worm spend more time on pale surfaces and out in the open, which makes them likelier to be taken by the birds the parasite needs to finish its life cycle.
Spores, and Genes That Used to Be Bacteria
Two quieter partnerships to finish with. Isopods disperse fungal spores: Oniscus asellus fed black truffle fruitbodies was still shedding intact, undamaged ascospores in its droppings 18 days later, with gut passage doing almost no mechanical damage. The study did not test whether those spores would germinate, so dispersal is demonstrated and successful inoculation is not.
Then the deep-sea giants. Giant marine isopods in the genus Bathynomus carry genes acquired by horizontal transfer from bacterial or viral donors. One of them, derived from a bacterium, is duplicated, unusually highly expressed and involved in regulating energy metabolism; inserted into zebrafish it extended survival under a cold-induced low metabolic rate by 37 per cent. Together with a huge food-storing stomach, it is part of how these animals go years without eating.
Bacterial genes rather than bacterial partners, which is exactly what the feminising insert in the pillbug is as well. Some partnerships end with the partner being absorbed into the genome, and the arthropods are full of the leftovers.
The Bioactive Vivarium: What We Can and Cannot Claim
Here is the position plainly. There is no published study of isopods and springtails working together in a vivarium. Not one. We looked.
What exists is soil ecology, and it is suggestive. In a study of urban soil columns, Porcellio scaber and two springtail species were run alone and together: the isopods broke litter down and raised magnesium and nitrate availability, the springtails carried the mobilised compounds deeper into the profile, and with both groups present calcium and magnesium mobilisation rose substantially while organic carbon leaching fell. The authors describe functional complementarity between litter-shredding macrofauna and the smaller animals that follow them. Separately, isopods have been shown to increase bacterial abundance and shift microbial community structure in leaf litter, and to make litter more usable to the fauna that comes after.
That is a good theoretical basis for what keepers do. It is not the same as evidence that a cleanup crew works the way we all say it does, and the claim that isopods and springtails occupy separate niches and so never compete has, as far as we can find, never been tested directly in either direction. The practice is ahead of the literature.
One published finding bears directly on enclosures, and it cuts the other way: other woodlice, along with millipedes, are recorded as glasshouse pests of seedlings. That is not an argument against a cleanup crew. It is an argument for giving one enough leaf litter to work on.
Frequently Asked Questions
What is symbiosis?
Two different species living in close, persistent association. It covers mutualism, where both benefit, commensalism, where one benefits and the other is unaffected, and parasitism, where one benefits at the other's expense. Isopods appear in all three, and in some cases which one applies has never been measured.
Can bacteria really change an isopod's sex?
Yes. Wolbachia turns genetically male Armadillidium vulgare into functional breeding females by shutting down the hormone that drives male development. In the lineages where this has gone furthest, the host's own female sex chromosome has been lost and sex is decided instead by whether the animal carries a nuclear insert of old Wolbachia DNA.
Can Wolbachia spread between woodlice in a culture?
It can. Uninfected animals that ate infected ones carried live Wolbachia in their tissues three and six months later, though at much lower densities than naturally infected animals. Cannibalism in a crowded tub is a transmission route.
Do isopods need gut bacteria to digest wood?
Less than we used to think. Isopods encode their own cellulase enzymes, a wood-boring marine relative manages with no resident gut community at all, and the best-known midgut symbionts turn out to carry no plant-digesting enzymes. One feeding experiment suggests microbial biofilm matters more as food than as machinery.
Why do isopods eat their own droppings?
Because microbes colonise the droppings and improve them. The older explanation, recovering copper for their blood pigment, was tested and copper turned out not to be limiting. The microbial benefit is real but depends on what the animals have been eating.
Is there a woodlouse that lives with ants?
Yes, and one of them is British. Platyarthrus hoffmannseggii, the ant woodlouse, is small, white and blind, lives only in ant nests, and is not attacked by its hosts. Its body size varies with which ant species it lives with.
Is the tongue-eating louse harmless to the fish?
Mostly not. The idea comes from a single observational description from 1983 that has never been tested. Measured work on a close relative shows severely reduced growth in infected fish, and across the family the recorded effects include gill damage, impaired reproduction and death, although a few studies have recorded no obvious harm.
Do isopods and springtails compete in a bioactive setup?
Nobody has tested it directly. The nearest evidence, from soil columns rather than vivaria, found the two groups complementing each other rather than competing, with better nutrient handling together than either alone.
Why Any of This Matters on a Shelf of Tubs
Most of it changes something practical. If the microbes matter as food rather than as enzymes, a nutritious substrate is not a preference but the diet itself. If gut communities are acquired from the environment and the hindgut turns over with the menu, a sterile setup is working against the animal. If a bacterium can be eaten and cross the gut wall, a crowded culture is doing more than crowding.
None of which means throwing out a setup that works. It is worth knowing that the animal in the tub is not one organism making a living from leaf litter. It is a negotiation, and a fair amount of it is still being worked out.
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