Oniscus asellus

Linnaeus, 1758

common woodlouse, common shiny woodlouse, European sowbug

Oniscus asellus is a large terrestrial isopod native to Western and Northern Europe, and one of the most widespread in the British Isles. It reaches up to 16 mm in length and inhabits diverse moist environments, including rotting wood, gardens, and human structures. The species exhibits biphasic moulting, consuming its shed , and has been documented to fragment weathered polystyrene plastic into microplastics. Two are recognized: the widespread O. a. asellus and the smaller, more colorful O. a. occidentalis in western France and southeastern Britain.

Oniscus asellus by (c) Katja Schulz, some rights reserved (CC BY). Used under a CC-BY license.

Oniscus asellus by (c) Alexis Tinker-Tsavalas, some rights reserved (CC BY), uploaded by Alexis Tinker-Tsavalas. Used under a CC-BY license.

Oniscus asellus - male side 2 (aka) by André Karwath aka Aka
. Used under a CC BY-SA 2.5 license.

Pronunciation

How to pronounce Oniscus asellus: //oʊˈnɪskəs əˈsɛləs//

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Distinguished from Porcellio scaber by smoother (juveniles may be confused due to rough bumps), shiny rather than matte appearance, and inability to roll completely into a ball. Three-segmented antennal and wide with lateral lobes on are -level diagnostic features. Pale calcium patches often visible. O. a. occidentalis distinguished by smaller size, more vivid coloration, and slightly different body shape; found only in western France and southeastern Britain.

Images

Habitat

Requires moist conditions; found under rotting wood, rocks, leaf litter, and in soil crevices. Occupies diverse including woodlands, grasslands, scrub, road verges, cliff , quarries, gardens, greenhouses, walls, and building interiors. Especially prevalent in decaying wood, allowing of calcium-poor environments like moorlands. Avoids dry habitats.

Distribution

Native to Western and Northern Europe from Portugal to Sweden, east to Ukraine, including British Isles, Azores, and Madeira. Widespread throughout British Isles, recorded in every vice-county of England, Scotland, Wales, and Ireland. Introduced to Americas, predominantly in Mexico and United States (east of Mississippi River, west of Rocky Mountains). Less common in Mediterranean Basin.

Seasonality

Active year-round in suitable microhabitats; may seek deeper shelter during cold or dry periods. Specific seasonal patterns not documented.

Diet

feeding on decaying plant matter, particularly rotting wood and leaf litter. In laboratory conditions, has been observed to fragment and consume weathered polystyrene foam, preferring plastic previously exposed to soil suspension over pristine or UV-weathered material.

Life Cycle

Development through direct development (no larval stages); young resemble miniature . Biphasic moulting cycle with 15 distinguishable stages: half of body moults before half. Calcium stored in deposits during premoult for post- hardening. Shed typically consumed. involves growth, formation, nurse development, and formation. Females possess two tube-shaped ovaries running much of body length; males have two three-lobed .

Behavior

surface activity with daytime sheltering under cover objects. Locomotory altered by heavy metal exposure, including reduced activity time, decreased distance traveled, and disrupted turning patterns. Aggregates in moist microhabitats. Consumes own shed after moulting.

Ecological Role

Decomposer contributing to breakdown of plant detritus, particularly woody material. Bioindicator for heavy metal pollution; accumulates lead, , zinc, and cadmium in hepatopancreas, with documented concentrations among highest recorded in animal soft tissues. May contribute to microplastic in soil through fragmentation of weathered plastic debris.

Human Relevance

Frequently encountered in gardens and buildings; generally considered harmless. Used in scientific research on moulting physiology, heavy metal bioaccumulation, , and microplastic fragmentation. Occasionally referenced as edible with flavor compared to crab or shrimp. O. a. occidentalis classified as Near Threatened in Britain due to fragmentation and hybridization with O. a. asellus.

Similar Taxa

Porcellio scaberRougher, matte ; does not possess shiny appearance of O. asellus. O. asellus with rough bumps may be misidentified as P. scaber.
Armadillidium vulgareCan roll completely into ball (conglobation); O. asellus cannot.

More Details

Subspecies

Two recognized: O. a. asellus (widespread) and O. a. occidentalis (western France, southeastern Britain). The latter described 1994, smaller, more colorful, slightly different body shape; capable of interbreeding with O. a. asellus producing hybrids.

Plastic Fragmentation

Laboratory study demonstrated ability to fragment weathered polystyrene foam into microplastic particles within 24 hours; showed strong preference for polystyrene previously exposed to soil suspension over pristine or UV-weathered material.

Heavy Metal Accumulation

Hepatopancreas accumulates exceptionally high levels of lead, , zinc, and cadmium from contaminated environments; concentrations among highest recorded in animal soft tissues. Woodlice from contaminated sites show enlarged hepatopancreata and altered locomotory .