Competitive interactions within and between size-classes of the limpet Patella depressa Pennant were investigated in central Portugal. Experimental enclosures of 25 x 25 cm containing marked limpets were set up at mid-tidal level on the shore. Twelve treatments with different combination of densities and size-classes were assigned to the plots, with three replicates of each. Mortality was recorded every 15 days and length was measured monthly during the approximately 6 months of the experiment. <br/><br/>2. At the end of the experiment limpets were collected for determination of length, height, dry weight, sex and gonad development. The availability of food was assessed indirectly by determination of chlorophyll concentration with spectrophotometric analysis of rock chips. Reduced density treatments showed very low mortality and substantially increased growth. Both size-classes of limpets showed increased mortality and reduced growth in increased density treatments. This effect was, however, more marked for large than small limpets when caged separately.<br/><br/>3. Although both size-classes could negatively affect each other, the effect of large limpets on small was greater than the reverse. Large limpets are superior competitors that may modulate the abundance of small limpets on the shore. It is unlikely, however, that they will totally exclude small limpets due to intensity of competition within the large size-class. Niche differentiation and high recruitment at low shore levels are other possible factors that can contribute to reduce competition between the size-classes.<br/><br/
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