Proasellus: Life Under Water

Proasellus slavus, photographed by Hans Jürgen Hahn K. Grabow (see comments below re credits).

The animal in the picture above is not quite the animal that I was planning on telling you about today, but I couldn't find an image of my particular target species. As long-time readers of this page will know, once a week I pick some random taxon to look at, and for this week I picked out the freshwater isopod Proasellus vignai. Most of you will know isopods as the woodlice that you may find in your garden, but the woodlice are really only one small part of the broad range of mostly aquatic isopod diversity. Proasellus belongs to a group of isopods known as the Asellota; as you can see in the picture above, asellotes differ from woodlice in (amongst other things) having the dorsal shields of each segment less tightly pressed together.

Proasellus is a genus of freshwater asellotes found around the Mediterranean: Europe, western Asia, northern Africa. Proasellus vignai is one of a number of species of Proasellus that are found in subterranean habitats, like P. slavus shown above. Both P. slavus and P. vignai, like most other subterranean animals, have lost the pigment and eyes of their surface-dwelling relatives. However, not all subterranean habitats are equal, and not all subterranean animals live in 'caves' as you might usually imagine them. Some Proasellus species are indeed found living in caves, but P. vignai and P. slavus are inhabitants of the hyporheic zone, the ground around rivers and streams where the water from the river soaks into the surrounding groundwater. Cave-dwelling Proasellus species tend to be broader and have more elongate limbs, so that they can maximise their chances of finding food in the nutritionally sparse cave waters. Hyporheic species, on the other hand, are narrower and more elongate, making them better suited for squeezing through the gaps between sediment particles.

Proasellus vignai seems to be a little known species (hence the lack of an available illustration). It is only known from the hyporheic zone of the Melfa river, in the Appenine mountains of the Lazio region of Italy (Bodon & Argano 1982). The Melfa is not a long river, only about 40 km long, so P. vignai may be a very localised species. It is a close relative of P. slavus, which lives in the water catchment of the Danube River. Other related species include P. ligusticus in the Ligurian Alps, P. sketi in Greece and P. boui in Languedoc in southern France. The scattered nature of the species of the P. slavus group, all of them hyporheic, suggests a certain degree of relictualism. Like other habitats that represent the edge of things, the hyporheic environment can be an uncertain one, vulnerable to outside influences. Should something change the nature of the Melfa river, Proasellus vignai might be taken with it.


Bodon, M., & R. Argano. 1982. Un asellide delle acque sotterranee della Liguria orientale: Proasellus ligusticus n.sp. (Crustacea, Isopoda, Asellota). Fragm. Entomol. 16 (2): 117-123.


  1. Well, I at least knew that there were lots of other isopods, but I confess to being one of the uneducated who know them primarily from woodlice/sowbugs/pillbugs/slaters: so thank you for this post! (And the picture, even if it isn't quite the right species, is very pretty!)

    Question: how big is this creature? I assume it's pretty small ("squeezing through gaps between sediment particles" sounds close to microscopic), but there's no scale bar in the photo.

    Off topic question: I'm interested in the vernacular names of terrestrial isopods. Growing up in North America I never heard "slater," which I take to be common in British English. "Pill bug" and "sow bug," on the other hand, seemed unknown to my Australian informants when I lived in Melbourne. You are from New Zealand originally, aren't you? Which name(s) is (are) used there?

  2. I've just found another version of the same photo with scale bar here (if you have access—it's in Nature so it may be paywalled, sorry) that indicates its a little over five millimetres, so it's small but not remarkably so. Funny thing, though: the Nature page attributes the photo to a different person from the news page I originally got it from. Seeing as the person I originally had credited is one of the authors on the Nature article, I suspect that the newspaper just assumed the person it received the photo from was the original photographer.

    We always said 'slater' in New Zealand. 'Pill bug' is something I only know from books.


Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="">FoS</a> = FoS