Oniscus Asellus Occidentalis Isopods

Oniscus asellus ssp. occidentalis is one of the more scientifically interesting British isopods in the UK hobby — a genetically distinct subspecies of the common woodlouse, described and formally named in 1994 by David T. Bilton in the Zoological Journal of the Linnean Society. Unlike most "rare" hobby isopods, this isn't a recent trade name or a colour morph — it's a properly peer-reviewed taxonomic entity with its own scientific provenance. For collectors interested in genuine British and Atlantic European fauna rather than the dominant Thai Cubaris, it's a meaningful addition to the small selection of properly-documented native UK isopods available captive-bred.

The subspecies's most striking feature is its biogeography. O. asellus occidentalis is described in the literature as a "relict Atlantic taxon, largely restricted to damp woodland" in the far western parts of the species's range — the British Isles, Ireland, and Western France. It's thought to represent an ancient lineage that has persisted in these damp Atlantic refugia while the nominate subspecies (Oniscus asellus asellus) has spread across the rest of Europe. In south-west Britain specifically, occidentalis partially replaces the nominate form. For naturalists, that's a properly fascinating native species with real conservation and biogeographic significance — and the captive-bred provenance from PostPods means no impact on wild populations.

Like all Oniscus, the occidentalis is flat-bodied and cannot conglobate — they're runners and clampers rather than rollers when disturbed. Browse the full Oniscus collection to compare options.

Quick Care Summary

• Scientific Name: Oniscus asellus ssp. occidentalis Bilton, 1994
• Common Names: Western Shiny Woodlouse, Atlantic Shiny Woodlouse, Occidentalis
• Family: Oniscidae
• Origin: South-west Britain, Ireland, Western France (Atlantic Europe)
• Adult Size: Up to approximately 16 mm
• Lifespan: 2–3 years typical (long-lived for a temperate isopod)
• Difficulty: Easy — hardy and forgiving, like the nominate subspecies
• Temperature: 15–25°C (cool-preferring; UK room temperature works year-round)
• Humidity: 60–75% — they like it on the damper side, reflecting native woodland habitat
• Ventilation: Good — important; prevents stagnation in their preferred damp conditions
• Conglobation: No — flat-bodied; runs/clamps rather than rolling
• Appearance: The familiar "shiny woodlouse" body with a more pronounced arch than the nominate; subtle metallic flecks visible on close inspection
• Behaviour: Nocturnal; shy initially but becomes more visible once a colony establishes
• Breeding: Moderate; colonies build steadily once settled
• Rarity: Uncommon — captive-bred stock of named subspecies is genuinely scarce

What Makes O. a. occidentalis Special

Several factors make this subspecies a properly worthwhile keep:

Genuine peer-reviewed taxonomic provenance. This is the headline. Bilton's 1994 paper in the Zoological Journal of the Linnean Society formally described and named the subspecies, distinguishing it from the nominate O. a. asellus through male sexual modifications and other morphological characters. Properly scientific, not a hobby trade name — and one of the more recent named subspecies among UK native isopods.

British native fauna with conservation interest. Most "interesting" isopods in the hobby come from elsewhere — Thailand, the Mediterranean, the Philippines. The occidentalis is genuinely British — a documented native of South-west Britain, Ireland, and Western France that has persisted in these damp Atlantic woodlands as a relict population. For UK keepers interested in their own native fauna, this is a properly meaningful species.

"Relict Atlantic taxon." The Bilton paper frames the subspecies specifically as an ancient lineage that has survived in damp woodland refugia in the western fringe of the species's range. It's a small but interesting piece of European evolutionary history — and not the kind of provenance you can fabricate.

Distinct body shape. Compared to the nominate O. a. asellus, the occidentalis is reported to be more highly arched, with subtle metallic flecks visible in good light. Set expectations toward refined naturalistic appearance rather than vivid colour — properly handsome rather than showy.

Easy care. Despite the scientific significance, the occidentalis is as easy to keep as the common woodlouse — it's a hardy temperate species adapted to UK room conditions, and forgiving of minor husbandry variation. The rarity is supply-side rather than care-side.

Captive-bred from established stock. All PostPods occidentalis are captive-bred, meaning no impact on the wild relict populations the subspecies is described from. For a species with documented conservation interest, that's a properly important detail.

How O. a. occidentalis Compares to Other Oniscus and Native UK Isopods

• vs nominate Oniscus asellus asellus: Same species, different subspecies. The nominate is the widespread "common shiny woodlouse" found across most of Europe; occidentalis is the genetically distinct Atlantic-relict western form. Same easy care, but occidentalis is more highly arched and noticeably more interesting for naturalists. Browse the Oniscus collection for comparison.
• vs Porcellio scaber Mix: Both are common native UK woodlice. P. scaber is the "rough woodlouse" with a granular surface; O. asellus occidentalis is the smoother, shinier-bodied Atlantic relict. Different texture, similar care, natural companions in a native-UK collection.
• vs Trachelipus ratzeburgii: Both are native European isopods at the easier care end. T. ratzeburgii is the larger striped European Trachelipus; O. a. occidentalis is the smaller, more refined Oniscus subspecies. Different genera, both worth keeping for a UK-and-European native collection.
• vs Dairy Cow (P. laevis): Both are easy beginner-tier non-conglobating isopods. Dairy Cow is the bold black-and-white smooth Porcellio at vivid contrast; occidentalis is the subtle native Oniscus subspecies with refined naturalistic looks. Same care principles, opposite aesthetic ends.

Browse the full Oniscus collection and the wider Porcellio collection to compare related species.

Setting Up the Enclosure

A 6–12 litre plastic container with a secure lid suits a starter colony of 5–10 individuals, with larger enclosures for established colonies. The 3L Braplast tub works for small groups; O. asellus doesn't need huge spaces.

Drill ventilation holes on opposite sides for cross-ventilation, covered with fine mesh. Good airflow is important — they prefer damp conditions but need ventilation to prevent stagnation in the humid setup. Provide plenty of hides — cork bark flats, rotting wood, leaf litter, and flat stones replicate their natural damp-woodland habitat. Browse our accessories collection for appropriate enclosures, vents, and other essentials.

Important husbandry note: O. asellus do not need a standing water dish. Misting and a moist corner provide all the moisture they need — open water risks drowning small individuals and encourages mould in the damp setup. Skip the water dish.

Substrate

Use a moisture-retentive substrate that drains well, reflecting their native damp-woodland habitat:

• Organic topsoil base (pesticide-free) as the foundation
• Sphagnum peat moss mixed throughout for moisture retention
• Composted hardwood leaf litter (oak, beech, sycamore — UK woodland species)
• Flake soil mixed in for added nutrition
• Crushed limestone or eggshells distributed throughout for calcium
• Rotting hardwood pieces — particularly important for this species, mirroring their preferred woodland habitat
• Forest moss patches in the moist section

We recommend a topsoil and sphagnum-based mix rather than coco coir. Substrate depth: 5–8 cm for burrowing and to maintain moisture gradients.

Top layer: Generous hardwood leaf litter — magnolia leaves, oak, and beech all work well — plus cork bark, decaying wood, and a sphagnum moss patch on the humid side. Plenty of cover encourages natural behaviour.

Humidity and Temperature

Maintain humidity around 60–75% with a clear moisture gradient — keep approximately half the enclosure consistently damp using sphagnum moss and damp leaf litter, while the rest stays slightly drier with leaf litter and bark cover. O. asellus occidentalis are damp-woodland specialists and appreciate the wetter side of the temperate-isopod range, but still benefit from a gradient and good ventilation rather than uniformly waterlogged conditions.

As one PostPods customer noted about following the website's care guidance, getting moisture right is the key to keeping isopods successfully — and for damp-woodland species like the occidentalis, that means damp without being waterlogged, with a proper gradient.

Temperature should be 15–25°C — comfortably within UK room temperature year-round. They handle the cooler end of the range comfortably, reflecting their Atlantic woodland origins where temperatures rarely climb high. Avoid sustained extremes.

Diet

O. asellus occidentalis are unfussy detritivores with broad woodland appetites:

• Primary diet (always available): Mixed deciduous leaf litter (oak, beech, sycamore, magnolia), rotting hardwood pieces, decaying organic matter, mosses, lichens — they're proper woodland detritivores
• Vegetables (1–2x weekly): Carrot, courgette, sweet potato, cucumber, leafy greens. Replace within 24–48 hours.
• Fruit (occasionally): Small amounts of soft fruit
• Protein (1x weekly): Fish flakes, dried shrimp, dried daphnia, reptile/invertebrate shed skin. Browse our accessories collection for the full range of protein supplements.
• Calcium (essential — always available): Cuttlefish bone, crushed limestone, oyster shell, eggshells. Important for healthy moulting — provide a constant source.

Feeding approach: Maintain a base of leaf litter and rotting wood as the dietary foundation, supplementing with vegetables, occasional fruit, weekly protein, and a constant calcium source. They have decent appetites and aren't picky.

Breeding

Oniscus asellus occidentalis are moderate breeders. Once established under stable conditions, colonies build steadily — not as explosively as some Porcellionides, but reliably over time.

Breeding basics:

• Females carry developing young in a marsupium (fluid-filled brood pouch) and release fully-formed live juveniles
• Juveniles take 8–12 months to reach maturity at this temperate end of the range
• Males can be distinguished from females by sexual modifications described in Bilton (1994)
• A pure occidentalis colony breeds the subspecies reliably; keep separated from any nominate O. a. asellus stock to maintain genetic integrity

For breeding success:

• Stable cool temperatures (18–22°C is ideal)
• A proper moisture gradient with a damp main section
• Adequate calcium for breeding females
• Regular protein supplementation
• Plenty of bark, cork, and leaf-litter hides
• A larger starter group establishes faster and provides genetic diversity
• Minimal disturbance during early establishment

Pair With Springtails

Add a thriving springtail culture to any O. asellus occidentalis setup. Springtails handle mould and microbial growth at a scale isopods can't manage — particularly useful in the damp conditions occidentalis prefer, and around protein foods. They coexist peacefully and form a helpful cleanup partnership in a native-UK bioactive setup.

Who Should Buy O. asellus occidentalis Isopods?

Ideal for:

• UK and Irish keepers interested in their own native fauna
• Naturalists who appreciate proper peer-reviewed taxonomic provenance (Bilton 1994)
• Collectors building a native British/European isopod cluster
• Anyone fascinated by the "relict Atlantic taxon" biogeographic story
• Bioactive setup builders wanting hardy, native, cool-tolerant cleanup species
• Beginners wanting a forgiving but properly documented species

Not ideal for:

• Keepers wanting an isopod that conglobates — Oniscus don't roll (try Magic Potion or other Armadillidium instead)
• Anyone wanting vivid colour morphs — this is a naturalistic native species with subtle metallic flecks rather than bright colour
• Very dry setups (they're damp-woodland specialists)
• Hot tropical setups (they prefer cool temperate conditions)

Realistic Expectations

The appearance is subtle. Set expectations toward a refined naturalistic look — the familiar "shiny woodlouse" body with a more pronounced arch than the common form, plus subtle metallic flecks visible on close inspection. Not vivid colour but genuinely handsome.

They don't conglobate. Like all Oniscus, they're flat-bodied and run/clamp rather than rolling.

The science is genuine. The Bilton 1994 taxonomic description, the "relict Atlantic taxon" framing, and the documented restricted distribution in South-west Britain, Ireland, and Western France are all directly from peer-reviewed literature — not marketing language.

They're a native species. Set expectations toward "fascinating documented British fauna" rather than "exotic foreign novelty." For some keepers that's a positive; for others looking for tropical glamour, the standard Cubaris route may suit better.

Captive-bred means no impact on wild populations. Important for a species with documented conservation interest — and a meaningful detail worth knowing.

Building Your Setup

A complete O. asellus occidentalis setup needs a roomy enclosure, a moisture-retentive damp-woodland substrate, abundant calcium, generous leaf litter and bark hides, and protein supplements. Browse our accessories collection for everything you need — enclosures, ventilation, leaf litter, calcium (cuttlebone, limestone, oyster shell), and protein supplements.

Browse the full Oniscus collection for related species — or explore other native European isopods like the Porcellio scaber Mix and Trachelipus ratzeburgii for a complete native UK/European cluster.