Maximum height in Roskilde Fjord is 5mm (Rasmussen, 1973).
Pusillina sarsii, often found accompanying R. membranacea on Zostera in the Baltic, may be almost indistiguishable from small R. membranacea unless the radulae are compared (Warén, 1996). Features in the images that appear to differ from R. membranacea are: The foot lacks opaque white spots, and it has a broad, black, transverse band on the dorsal surface of foot that extends onto the edge of the sole (1) (in common with R. parva and P. inconspicua). There is no dark pigment on the side of the snout (2). Roskilde Fjord, Denmark. September 2015. Leg. & © K. Jensen. In coll. National Museum Wales.

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