The occurrence of Mesenchytraeus (Enchytraeidae: Oligochaeta) in riffle habitats of north-west American rivers, with description of a new species

Mesenchytraeus species are widespread and often abundant in rivers of northwest USA where they are associated with cobble-gravel beds in fast-running water and particularly with riffle habitats. Mesenchyraeus rhithralis n. sp. was the most frequent and was often the dominant macroinvertebrate. The species is characterized by the presence of primary septal glands at 3/4–5/6, short, stout vasa deferentia, absence of atria, atrial glands or accessory glands surrounding the penial bulbs, and free, long spermathecae without diverticula. The description of M. pedatus Eisen is amended. Failure to record enchytraeids from riffle habitats in the past may be due to a lack of familiarity with the family and the absence of suitable keys for identification.