Evidence of self-thinning on two coexisting mussel species in tropical rocky shores: A methodological approach

Intraspecific competition can limit the growth of organisms in populations, so that maximum values of biomass per unit area are attained for a given density – the self-thinning (ST) relation. These relationships are more commonly found in sessile species, where space is the main limiting resource, although for animals food limitation can also occur. Here we studied ST relationships in two coexisting mussel species common in SE Brazil, Brachidontes solisianus and B. darwinianus, and evaluated different geometric methods to determine the projected area of each individual onto the substrate (parallelepiped projection vs. apical projection through image analysis), and different statistical approaches to fit the self-thinning models. We found that for the apical projection through image analysis, different statistical methods result in different conclusions in relation to ST relationships in B. darwinianus, whereas no differences were found when using the parallelepiped projection. No significant differences among geometric or statistical methods were found for B. solisianus. The ST results suggest that B. darwinianus, which occurs lower in the intertidal, is limited by space, whereas B. solisianus, which occurs higher in the intertidal, is limited by both space and food, so that intraspecific competition is important for both species. The results also suggest that the geometric methods used for mussels in suspended cultures may not be adequate to study mussels on rocky shores, due to large environmental differences between these systems. •We studied self-thinning relationships in two coexisting mussel species.•Different conclusions can be found depending on the statistical method used.•Brachidontes darwinianus, which occurs lower in the intertidal, is limited by space.•B. solisianus occurs higher in the intertidal and is limited by both space and food.•Differences in the type of resource limitation influences self-thinning.