Some aspects of the biology of Gibbula Cineraria (L.) with observations on Gibbula Umbilicalis (Da Costa) and Gibbula Pennanti (PHIL.). (Mollusca : Prosobranchia)

Abstract

The primary aim of this study was to establish the breeding season, growth rate and life history of Gibbula cineraria in north-east England. Sampling was carried out; over a period of 13 months on the rocky shore at Whitburn, County Durham. The population was sampled at five different tidal levels and on different substrata. The principal conclusions were that breeding took place over a long period from September to March. The young animals settled from the plankton mainly below tidal levels and moved up into the tidal zone at about 18 months of age when they I became sexually mature for the first time. Rapid growth continued for about three years. There was good evidence that the population at higher shore levels was derived by immigration from lower levels and it was also clear that there was a general movement of animals downshore in the autumn and upshore in the ' spring. The boulder areas of the beach supported a much higher density of animals than the open rocks and this was probably due to greater protection from predators. Density also declined with increase in level above Chart Datum. The rate of growth was in every case greater where the population density was low which suggested that intra-specific competition for resources was occurring. G. cineraria populations, together with Gibbula umbilicalis and Gibbula pennanti where they occurred, were examined at many other stations between the Faeroe Islands(620 N.) and the Canary K Islands (28ºN.). It was concluded that where two or three species of Gibbula were present on the same shore inter-specific competition occurred which was variously manifested by a restricted distribution, slower growth rate or reduced density. Two biogeographical effects were also noted: a gradual size diminution and restricted Intertidal range from north to south.