Five, possibly six, north European species are confirmed in the prosobranch genera Rissoa and Pusillina: R. parva (Da Costa); R. lilacina Récluz; R. membranacea (Adams) (types A & B); P. inconspicua (Alder) and P. sarsi (Lovén).
The five species differ ecologically in salinity tolerances, with R. membranacea and P. sarsi being the most tolerant and regularly occurring at salinities of 7–10%0, while R. parva, R. lilacina, and P. inconspicua tolerate salinities down to 20%0 (for shorter periods 15%0). There are also differences in the ability to survive occasional, short exposures to highly reduced salinity with 50% survival for R. membranacea after 80 h; R. lilacina, 30 h; P. sarsi, 25–30 h; and for P. inconspicua after 15 h exposure to freshwater at 8°C. At higher temperatures the survival is lower. There is a difference in the requirements on water circulation, with R. parva and R. lilacina preferring more current-swept and exposed environments.
Rissoa membranacea occurs in two types, possibly species, with planktotrophic and lecithotrophic larval development respectively. In Scandinavia, populations with lecithotrophic development have a restricted occurrence between Sjaelland and Jylland in Denmark, but may be more common in the UK and France. Such specimens are always accompanied by the planktotrophic form, and it can not be excluded that the two forms are conspecific and this is a case of poecilogony. There is also a possibility that P. sarsi actually constitutes a series of populations of P. inconspicua adapted to local, more brackish conditions by larger egg size and veligers, but the two are regarded as distinct species.