The intention of this paper is to draw attention to an interesting phenomenon in marine gastropods, of which the importance has not yet been as widely appreciated as, in my opinion, it should be. I came across the phenomenon for the first time when studying ten recent species of the subgenus Turboella Gray, 1847, of the genus Rissoa Desmarest, 1814 (Verduin, 1976a). Eight of these species have similar apical dimensions of about 0.09/0.15 mm, though otherwise they differ conchologically. The first of these measurements is the diameter of the nucleus, the second one that of the first half whorl (fig. 1). The two remaining species, however, are conchologically so similar to two of the eight species mentioned, that the representatives can often only be separated reliably from their sibling partners by the distinctly larger apical dimensions being about 0.14/0.24 mm. There are strong indications that, of each pair of these sibling species, the partner with the larger apical dimensions lacks a swimming larval phase, in contrast to that with the smaller ones. The sibling species are sympatric in large parts of their range. In complete agreement with the above are Rehfeldt’s (1968) findings that in the Roskildefjord in Denmark two types of Rissoa membranacea (J. Adams, 1800) with very similar shells must be distinguished, which, however, also show the differences in apical dimensions and larval phase discussed above. Both forms, which apparently must be considered separate species, also occur in England and Bretagne, France, but the one with the coarse apex seems to be absent in the Mediterranean.