Trichoniscus pusillus

T. pusillus may be distinguished from other British woodlice chiefly by its small size, which reaches no more than {{convert|5|mm|1}}.{{cite web |url=http://www.nhm.ac.uk/woodlice/key.html |title=Woodlouse Wizard: an identification key |publisher=Natural History Museum |accessdate=November 8, 2010}} Its body is elongate and quite rounded in cross section, and typically purplish.{{cite web |url=http://www.tiscali.co.uk/reference/encyclopaedia/wildlife/crustaceans_terrestrial_and_freshwater_crustaceans_-_trichoniscus_pusillus.html |publisher=Encyclopaedia Tiscali |accessdate=February 10, 2009 |title=Trichoniscus pusillus |archive-url=https://web.archive.org/web/20081201170909/http://www.tiscali.co.uk/reference/encyclopaedia/wildlife/crustaceans_terrestrial_and_freshwater_crustaceans_-_trichoniscus_pusillus.html |archive-date=December 1, 2008 |url-status=dead }} It may be separated from related species in North America by its eyes of three ocelli each, rather than the single ocellus in the eyes of its relative Hyloniscus riparius.{{cite journal |journal=Bulletin of the Biological Society of Washington |volume=15 |issue=1 |pages=41–43 |year=2008 |doi=10.2988/0097-0298(2008)15[41:TTICIO]2.0.CO;2 |title=The terrestrial isopods (Crustacea: Isopoda: Oniscidea) of Plummers Island, Maryland |author=Arnold W. Norden |s2cid=83483162 }}

There are two distinct reproductive strategies within the species Trichoniscus pusillus. Many populations are, like most metazoans, bisexual and reproduce sexually; in other cases, females reproduce parthenogenetically, creating clones of themselves.{{cite journal |doi=10.1111/j.1601-5223.1983.tb00594.x |title=Genetic variation in coexisting sexual diploid and parthenogenetic triploid Trichoniscus pusillus (Isopoda, Crustacea) |author=Bengt Christensen |journal=Hereditas |volume=98 |issue=2 |pages=201–207 |year=1983 |pmid=6874390 |doi-access=free }} The sexually reproducing form is diploid while the parthenogenetic form is triploid; since parthenogenesis always produces females, males are always diploid and can only be produced by sexual reproduction.

The frequency of males in the population decreases from south to north (a latitudinal cline) and in increasingly open habitats, with no males observed in most of Scotland and Scandinavia, but more than 15% males in the Iberian and Apennine Peninsulas.{{cite book |title=The Masterpiece of Nature: The Evolution and Genetics of Sexuality |author=Graham Bell |publisher=Cambridge University Press |year=1982 |isbn=0-85664-753-5 |pages=635|author-link=Graham Bell (biologist) }}

The reproductive season lasts from March to September, and one to three breeding waves may be observed. Females are gravid for 4–5 weeks before releasing 4–18 mancae from the brood pouch.{{cite journal |journal=Pedobiologia |volume=44 |issue=1 |pages=75–85 |year=2000 |title=Responses of the parthenogenetic isopod, Trichoniscus pusillus (Isopoda: Oniscidea), to changes in food quality |author=Guido Kautz, Martin Zimmer & Werner Topp |doi=10.1078/S0031-4056(04)70029-3|bibcode=2000Pedob..44...75K }}

Like other woodlice, T. pusillus eats decaying plant matter of various kinds, although only alder litter is capable of sustaining a stable reproducing population.

Predators of T. pusillus include the common shrew Sorex araneus, lycosid and dysderid spiders, centipedes such as Lithobius variegatus and perhaps carabid beetles.{{cite journal |author=S. L. Sutton |title=Predation on woodlice; an investigation using the precipitin test |journal=Entomologia Experimentalis et Applicata |volume=13 |issue=3 |year=1970 |doi=10.1007/BF00334313 |pages=279–285}}

Trichoniscus pusillus is susceptible to infection by isopod iridescence virus, or Iridovirus (Iridoviridae). This is first apparent as a blue sheen on the unpigmented underside of the animals, but soon spreads to give the entire exoskeleton a bluish iridescence.{{cite journal |url=http://openurl.ingenta.com/content?genre=article&issn=0011-216X&volume=72&issue=2&spage=145&epage=156 |doi=10.1163/156854099503249 |title=Some notes on the distribution and ecology of Iridovirus (Iridovirus, Iridoviridae) in terrestrial isopods (Isopoda, Oniscidae) |author=H. Wijnhoven, M. P. Berg |journal=Crustaceana |volume=72 |issue=2 |year=1999 |pages=145–156|url-access=subscription }}

Several former subspecies of T. pusillus are now treated as separate species. These include Trichoniscus alticola, Trichoniscus baschierii, Trichoniscus provisorius, Trichoniscus noriucs and Trichoniscus pygmaeus.

• List of woodlice of the British Isles

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• {{Commons category-inline|Trichoniscus pusillus|Trichoniscus pusillus}}

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Category:Woodlice

Category:Crustaceans described in 1833

Category:Woodlice of Europe