 In Europe there are about nine species in this genus, one of which is the rough periwinkle, "Littorina saxatilis" (Olivi, 1792).Another closely related (supposed) species "Littorina tenebrosa" (Montagu 1802) was thought to be distinct because of its different ecological requirements, but current studies have shown that it is not at all clear whether the two are indeed separate species or whether "L. tenebrosa" is perhaps merely an ecotype (an ecological form) of "L.