DISTRIBUTION, ABUNDANCE AND SHELL MORPHOLOGY OF LITTORINA-SAXATILIS (OLIVI) AND LITTORINA-ARCANA HANNAFORD ELLIS AT ROBIN-HOODS BAY, NORTH YORKSHIRE

The distributions of the rough periwinkles L. saxatilis and L. arcana are considered in the northern half of Robin Hood's Bay, North Yorkshire, England. The relative proportions of these two species at different sites have been determined, using the criterion of the female reproductive system for their identification. The commonest shell colour morphs are grey (both species) and orange-banded (L. saxatilis). Hence information has been obtained not only on the proportion of banded animals in the total population, but on the proportion of banded L. saxatilis. The northern end of the bay comprises a long, almost continuous, boulder field within which the proportion of L. arcana increases with greater exposure. Most of the boulder field comprises largely sandstone boulders but shale predominates at the most exposed site and, at the most sheltered site, which is separated from the rest of the boulder field by a short stretch of shingle, the boulders are exclusively shale. Banded morphs predominate in those regions where most of the boulders are sandstone but grey morphs predominate in the shale areas. A possible explanation for the distribution of these morphs is given in terms of gene flow. One site near the centre of the bay is unusual in that it contains L. arcana exclusively. However it comprises an artificial vertical substratum which is probably quite exposed. © 1990 Kluwer Academic Publishers.