CRUSTACEA > MALACOSTRACA > ISOPODA > Asellota > Asellidae

The Asellota suborder of aquatic isopod crustaceans is represented in Britain by just four freshwater species, all of which are placed in the Asellidae family.

Realistically I'm only likely to find two of these species, the Two-spotted Waterlouse as featured here, and the not dissimilar One-spotted Waterlouse, Proasellus meridianus.  The remaining two are Proasellus cavaticus, a 'nationally scace' blind species which, as its name indicates, lives underground in caves and subterranean streams; and Caecidotea communis, which is an introduced waterlouse that has only been recorded from a single location in Northumberland.

Asellus aquaticus, also known as the Two-spotted Water Slater or Water Hog-louse, can usually be distinguished from Proasellus meridianus by the two pale patches at the back of the head that are separated by a dark pigmented band. In some specimens these markings are very clear, but in others, particularly paler individuals, the patches can be almost merged together. This can make identification less clear leading to possible confusion with Proasellus meridanus which, by comparison, has a continuous pale bar running across the back of the head.

The general description of Asellus aquaticus doesn't seem to go much beyond the head markings apart from confirmation that the shape of the male first pereopod (leg) is diagnostic, but the same description is also noted for Proasellus meridanus which doesn't help. In a couple of the photos below you can see the claw, but I'm not sure whether it's the same in both species. I shall update the page with a clearer description if I find one, but otherwise can only go from my limited experience of looking at a few different individuals and noting that they are rather variable, both in terms of size and colour, which ranges from pale brown to dark brown, to almost black. Initially I thought that the smaller pale specimens may have been females, but now believe they're just immature.  

Both species are widespread occurring in a range of aquatic habitats from ditches and canals to small ponds and lakes. Asellus aquaticus though is by far the most frequently recorded, which is probably due to the fact that it is commonly found in garden ponds, whereas Proasellus meridianus has only rarely been recorded from this type of environment.

Finally, and by way of explanation, I though I'd better note that the individuals featured below from the New Forest, were collected at the same time as the water bugs (see Hemiptera > Nepomorpha), photographed at home and returned the next day. The others from our small garden pond were all found in the silt after it was emptied for cleaning. There were so many and it was finding them, together with a few other small aquatic creatures, that prompted me into purchasing a mini aquarium so that they could be properly observed and photographed. That said, the photography proved to be a little bit more difficult than I envisaged, but I think the effort was worth it in the end.