| Summary: | The three Patella species ( P. vulgata Linnaeus, P. depressa Pennant, and P. ulyssiponensis Gmelin) inhabiting rocky shores in the south west of England provide an accessible model for studies of co-existence and competition in closely related species. They overlap in geographical distribution in the north east Atlantic where limpets play an important part in structuring rocky shore communities through their herbivory. P. ulyssiponensis occurs mainly low down and in pools on more exposed shores. P. vulgata and P. depressa overlap considerably in their distribution on the midshore of all but the most sheltered shores where P. vulgata predominate. This thesis undertakes comparisons of the biology of P. vulgata and P. depressa in order to understand how co-existence is maintained in regions of overlap of distribution and their role in controlling algal growth on the shore. The biology of each species is reviewed (Chapter 1). In Chapter 2 their distribution and degree of overlap are quantified on shores in the south and west of England. Proportions and densities of both species varied on shores across the south coast, distribution of populations was influenced geographically by physical factors and at an individual level by behavioural controls. Chapter 3 describes the population biology and dynamics of P. vulgata and P. depressa covering population structure, density, growth rates, recruitment, immigration, emigration and mortality. An offset in the timing of the life cycle between species was found, P. depressa begins growth and spawns before P. vulgata annually. The influence of recruitment on population densities and replenishment was shown through an increase in populations in a year of increased recruitment. Comparisons of foraging activity (Chapter 4) showed that exposure and moisture influence grazing during low tide.
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