Trachelipus rathkii (Brandt, 1833)
Common name
Rathke's Woodlouse
Synonyms
Trachelipus rathkei (Brandt, 1833)
Status:
GB IUCN status: Least Concern
ID Difficulty
Identification
Trachelipus rathkii is a large woodlouse up to 15 mm body length. It is often slate-grey in colour, with usually with pale areas giving the impression of three thin longitudinal stripes, but females may be attractively marked with orange mottling. The combination of five pairs of pleopodal lungs (see images) and two flagellal segments distinguish T. rathkii from similar species.
Despite this, it is frequently confused with the common Oniscus asellus (which has three flagellal segments) and the equally common Porcellio scaber (which has two pairs of pleopodal lungs).
Distribution
Trachelipus rathkii has a ‘clumpy’ distribution across south-eastern England, with isolated records in Wales (Jones, 2008), but is probably under-recorded. For example, it has been recently found by Jon Daws in Yorkshire (BMIG Newsletter 30, pg 3) and along the river Avon in Worcestershire (Farmer, 2021). It has not been recorded from Ireland.
Recently a single male specimen was collected from a garden centre near Edinburgh (BMIG Newsletter 47, pg 13) suggesting that it may be spread further afield via the horticultural trade.
The NBN records on the English west coast - shown on the map opposite - have been shown to be erroneous - see updated distribution map in Gregory (2024).
Habitat
It favours sites with impeded drainage and is tolerant of flooding, making it a characteristic woodlouse of riverside meadows. It also inhabits poorly drained scrub, eroding ‘soft’ cliffs, gravel pits, churchyards, etc.
It occurs under stones and dead wood, beneath bark on logs, amongst grass litter, within tussocks and amongst flood debris. On flood plains it may be the dominant species, but elsewhere often associated with other ‘large’ woodlice, such as Porcellio scaber and Oniscus asellus and Philoscia muscorum.
This summary is based on the detailed account in Gregory (2009), updated in Gregory (2024).
References
Daws, J. (2014) Trachelipus rathkei found in Yorkshire. BMIG Newsletter 30: p3.
Farmer, G. (2021) The discovery of the woodlouse Trachelipus rathkii during 2020 in the Vale of Evesham and surrounding area. Worcestershire Record 48: 43-45.
Gregory, S. (2009) Woodlice and Waterlice (Isopoda: Oniscidea & Asellota) in Britain and Ireland. Field Studies Council/Centre for Ecology & Hydrology.
Links
World List of Marine, Freshwater and Terrestrial Isopod Crustaceans: https://www.marinespecies.org/isopoda/aphia.php?p=taxdetails&id=263585
BRC code
83
idBmigTaxa
Cru_1716