The coastal prosobranch Hydrobia ulvae is known to occur in a wide range of marine and brackish habitats and to display great variation in its breeding and life-history characteristics. Several hypotheses have been advanced to account for the latter, including that the variation is environmentally induced and that the species can be divided into ecotypes. Comparison of two adjacent populations in Norfolk, U.K., one from the marine intertidal zone and the other from a non-tidal, landlocked, brackish coastal lagoon, however, disclosed that although shell form differed markedly (including a mean height ratio of 1:1.2), there was no difference in such otherwise variable features as numbers of eggs per capsule, size at hatching and larval type. In both, each capsule contained an average of 21-22 eggs, which hatched at a shell length of 152-154 μn to liberate relatyyively long-lived, free-swimming veligers. The two populations were also interfertile. In no respect had the isolated lagoonal population diverged away from the parent marine one towards the contrasting reproductive strategies characterizing the specifically lagoonal species of Hydrobia that occur nearby. The ‘displacement’ of shell size observed in the lagoon in the absence of sympatric hydrobiids is considered to cast further doubt on competitive character displacement in this genus.
