GB IUCN status: Vulnerable
GB rarity status: Nationally Rare
Metatrichoniscoides is a difficult genus and only male specimens of M. celticus can be distinguished from M. leydigii or preserved specimens of Trichoniscoides. Identification is made by microscopic examination of male 2nd endopod (figures are given in Ashwood & Gregory, 2021).
M. celticus is very small (up to 2.5mm long), pure white, entirely lacking ommatidia and the dorsal surface is covered in coarse tubercles.
Video footage of this tiny woodlouse at Ogmore-on-Sea can be viewed on the BMIG Facebook page:
Distribution and Habitat
Confirmed males are mainly known from south Wales, but single sites have recently been discovered in north Wales (Hughes, 2019) and south-west England (Ashwood & Gregory, 2021). This woodlouse appears to be endemic to the British Isles. It is primarily 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).