See nature22.com/estran22/estran.html
Ribbed specimens, like this one, are far more frequent than smooth ones in Bretagne (in litt. M. Cochu). It seems that ribbed ones become more frequent in southern populations, but they do occur as far north as the Lofoten Islands in northern Norway. So ribbing may be promoted by other factors as well as warmth. R. membranacea feeds by grazing detritus, diatoms and other microphytes from the surface of Zostera and plants with similar structure, such as Posidonia in the Mediterranean (image M. Mattana flic.kr/p/nPiS2k ) and, sometimes, other seaweeds. Grazing probably benefits the plants by keeping them clean for photosynthesis but, when the population is dense, they do feed on the substance of the plants and, occasionally, have a damaging effect. On Zostera. Bretagne, France. November 2012. © Marc Cochu.
32 Rissoa membranacea.
