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.

种内竞争会限制种群中生物的生长，因此对于给定的密度，即自稀化（ST）关系，可以获得单位面积生物量的最大值。这些关系更常见于无柄物种中，其中空间是主要的限制资源，尽管对动物而言，食物也可能受到限制。在这里，我们研究了巴西东南部常见的两种共存贻贝物种Brachidontes solisianus和B. darwinianus的ST关系。，并评估了不同的几何方法来确定每个人在基板上的投影面积（通过图像分析来确定平行六面体投影还是顶角投影），以及使用不同的统计方法来拟合自减薄模型。我们发现，对于通过图像分析进行的顶端投影，不同的统计方法得出的关于达尔文牛体ST关系的不同结论，而使用平行六面体投影时则没有发现差异。几何或统计方法之间没有显著差异，发现B. solisianus。所述ST结果表明，B. darwinianus，其发生在潮间下，通过空间的限制，而B. solisianus，在潮间带较高的位置，受空间和食物的限制，因此种内竞争对这两个物种都很重要。结果还表明，悬浮培养物中贻贝的几何方法可能不足以研究多岩石的海岸上的贻贝，因为这些系统之间存在很大的环境差异。