Reproductive and morphological variations in the prosobranch ‘Rissoa membranacea’

Abstract

Rissoa membranacea of the Roskildefjord, Denmark, is divided into two distinct types as a result of investigations of its external morphology and reproduction. In the A type the initial embryonic shell is small and the larval development pelagic. Shell length seems to vary according to environmental factors, particularly to salinity. Although the A type tolerates salinities below 15‰ a salinity higher than 18–19 ‰ is needed for development of maximal shell length. In the B type the initial embryonic shell is large and the development non-pelagic with ‘nurse-embryos’, not previously described. It has little variation in morphology and seems to prefer a salinity range of 15–19 ‰