Metatrichoniscoides celticus Oliver & Trew, 1981


  • GB IUCN status: Vulnerable
  • GB rarity status: Nationally Rare

ID Difficulty


Metatrichoniscoides species are very small (up to 2.5 mm long) lacking pigment (white), lacking ommatidia (eyes) and the dorsal surface is covered in coarse tubercles. This is a difficult genus and only male specimens of M. celticus can be distinguished from M. leydigii or preserved (faded) specimens of Trichoniscoides.  Identification is made by microscopic examination of male 2nd endopod. 

The diagnostic male pleopods of Metatrichoniscoides celticus are figured in Ashwood & Gregory (2021).

Video footage of a live specimen of this tiny woodlouse seen at Ogmore-on-Sea can be viewed here.

Mark Telfer
Steve Gregory
Keith Lugg
Keith Lugg
Frank Ashwood


Metatrichoniscoides celticus appears to be endemic to the British Isles. Confirmed males are mainly known from a handful of sites south Wales, but single sites have recently been discovered in north Wales (Hughes, 2019) and south-west England (Ashwood & Gregory, 2021, which includes an updated distribution map, Fig.4).  

Records from Cumbria and Northern Ireland are based on female specimens and may prove to be M. leydigi (which has been recently discovered on the Lancashire coast (Hughes, 2020)). 


It was originally found associated with sparsely vegetated erosion banks occurring just above the supralittoral zone, but has also been recorded 7km inland in a limestone quarry at an altitude of 170m asl. The English site is a working allotment in Bristol.

Characteristically, small numbers of specimens are found with extreme difficulty under large stones firmly embedded into damp humus-rich soil. It is typically associated with other trichoniscids, usually Haplophthalmus mengii, Trichoniscoides saeroeensis and Trichoniscus pygmaeus

This summary is based on the detailed account in Gregory (2009).


Gregory, S. (2009) Woodlice and Waterlice (Isopoda: Oniscidea & Asellota) in Britain and Ireland.  Field Studies Council/Centre for Ecology & Hydrology.

BRC code