Rissoa membranacea (J. Adams, 1800) (Gastropoda, Prosobranchia) from the Dutch Wadden Sea

Rissoa membranacea disappeared from the Dutch Wadden Sea together with the large subtidal stands of eelgrass Zostera marina L. in the early 1930s, due to a still not completely understood ‘wasting disease’ in Zostera. A small population of R. membranacea survived together with Zostera marina in an inland brackish water ‘de Bol’ on Texel until 1981. Empty shells of R. membranacea can still be found at these localities. The relatively large size of the top-whorl of Wadden Sea R. membranacea indicates that the larvae were lecithotrophic. In this respect they are comparable to R. membranacea type B from the Roskilde Fjord of Rehfeldt (1968) and Warén (1996) and to R. membranacea s.s. as described by Verduin (1982b). It is, however, questionable whether R. membranacea Type A of Rehfeldt and R. labiosa sensu Verduin can be separated from R. membranacea s.s. They differ only in the smaller size of the embryonic whorl indicating planktothrophic pelagic larvae. This might as well indicate two different types of larval development, lecithotrophic and planktotrophic in one and the same species R. membranacea s.l. Apical dimensions in samples from the Ria de Arosa (Spain) and from Dutch Eemian deposits show a gradual change from small ‘R. labiosa’ to larger ‘R. membranacea’ values, which supports the merging of both nominal species and corroborates Warén’s suggestion of a continuum in larval development in R. membranacea between lecithotrophy and planktotrophy as well as newer ideas of a gradually and easily crossed transition between lecithotrophy and planktotrophy of McEdward (1997).