Proasellus coxalis (Dollfus, 1892)
Synonyms
Asellus coxalis Dollfus, 1892
Status:
GB IUCN status: Not applicable (non-native)
ID Difficulty
Identification
Waterlice (Asellidae) are distinguished from the terrestrial woodlice by their plate-like pleotelson, consisting of four pleonites and the telson fused together.
Adult Proasellus coxalis are similar in size to P. meridionalis, with males reaching 10mm and females, 8mm. This contrasts with the larger Asellus aquaticus, which can measure up to 20mm.
The pale marking along the posterior head margin can be used to differentiate P. coxalis from the other waterlice. Both Proasellus species have a single, continuous marking, while A. aquaticus usually has two distinct rectangular markings separated by an area of pale dots. In P. meridionalis, the marking has a neatly delineated border, whereas in P. coxalis the pale area has an ill-defined, speckled border (see image to right).
Although a useful feature, head markings can be variable and difficult to discern, especially in young specimens. To reach a definitive identification, it is necessary to microscopically examine the first and second male pleopods.
Text prepared by Dr. Stephanie Skipp.
Distribution
The majority of records to date have been from the Severn river basin district in the west midlands. However, there have also been isolated records in Devon and Essex, suggesting that there may be undiscovered populations of P. coxalis across the UK, making it more widespread than currently known.
Habitat
The habitat requirements of P. coxalis are similar to that of the other two UK waterlice, both of which may co-occur with this species. The majority of P. coxalis records are from smaller watercourses up to 30cm in depth, with varied sediment compositions. P. coxalis may also occur in stagnant ponds and can tolerate brackish water.
Text prepared by Dr. Stephanie Skipp.
References
Kemp, J. L., Ballot, A., Nilssen, J. P., Spikkeland, I., & Eriksen, T. E. (2020). Distribution, identification and range expansion of the common Asellidae in Northern Europe, featuring the first record of Proasellus meridianus in the Nordic countries. Fauna norvegica, 40 : 93-108.
Wouters, K., & Vercauteren, T. (2009). Proasellus coxalis sensu auct.(Crustacea, Isopoda) in a lowland brook in Heist-op-den-Berg: first record in Belgium. Lauterbornia, 67: 53-61.
Links
World List of Marine, Freshwater and Terrestrial Isopod Crustaceans: https://www.marinespecies.org/aphia.php?p=taxdetails&id=1673532
BRC code
91