Annual variation in soft-body dry weight, reporductive cycle and sex ratios in populations of Patella vulgata at adjacent sites in the Orkney Islands

Abstract

The relationships of shell length and internal volume with soft-body dry weight for three populations of Patella vulgata L. at Sandwick Bay in the Orkney Islands were studied between August, 1979 and March, 1982. Seasonal and annual fluctuations in the regression constants were explained by age-related variations both in overall growth rates and in the amount of effort directed toward gonad production. The reproductive cycle at all three sites was well synchronised, rapid development occurring between late June and November, with spawning following sometime between December and February; the whole population then remaining in the neuter/resting stage until July. The sex ratios within the population with respect to size (age) were examined; the majority of small, mature individuals were male, the number of females increased with size, although the largest individuals at each site were males. The sex ratio in the different populations varied from 2:1 to 6:1 in favour of males, but the “functional” ratio was much less in the latter due to the population structure.