Aspects of the molluscan fauna of the rocky shores of the Firth of Forth and Forth estuary, Scotland

Abstract

SynopsisMolluscan species diversity decreased westward from the North Sea into the Forth with decreasing diversity of habitats, decreasing algae and increasing turbidity of water. Species diversity varied little from year to year or seasonally at individual sites. Adult Littorina littorea in particular were larger at lower than at higher shore levels, and larger in the outer than the inner firth or estuary, while Nucella lapillus adults were of more uniform size throughout the area.Population parameters and reproductive output of L. littorea and L. saxatilis in the estuary varied without strict relation to mere position between open firth and up-estuary sites. On the other hand, shells of L. saxatilis became progressively wider, breeding seasons of L. littorea were longer and less defined, breeding in L. saxatilis became more sharply seasonal and eggs of L. saxatilis had greater weight, calorific value and inorganic content at up-estuary sites. L. littoreashowed two-weekly peaks in spawning on spring tides while L. saxatilis showed monthly peaks in release of young near times of full moon.Comparison of fresh material and stranded shells with material from beach deposits up to 5500′ years old indicates changes in populations with changes in climate about 550 BC. Size of Littorina spp. continued to decrease while numbers of Mytilus edulis increased greatly near the onset of the industrial revolution.