Salinity gradients and interspecific competition determine the distribution of chthamalid barnacles in a subtropical estuary

Distribution patterns in the upper intertidal zone, where diversity is low, are often explained by the variation in physical factors. However, certain environmental conditions may hinder resource monopolization by a dominant species, allowing for the coexistence of species. Under these conditions, interspecific interactions may become relevant. In southeastern Brazil, the upper intertidal zone of exposed rocky shores is typically colonized by the barnacle Chthamalus bisinuatus. In estuaries, two other species, Chthamalus proteus and Microeuraphia rhizophorae, may occur on hard substrates, although patterns of coexistence at relevant spatial scales are presently unknown, which prevents a better understanding of the ecology of rocky shore systems in the region. This study aimed to (i) describe the distribution patterns of chthamalid barnacles along the Paranaguá Estuarine Complex (PEC) and (ii) evaluate the vertical distribution of species in places where substantial coexistence was observed to preliminarily test whether their distribution is compatible with a lottery model of resource acquisition or compatible with a process of interspecific interference competition. A well-defined barnacle community was observed along the upper intertidal zone, from the euhaline to the oligohaline sector, with the cover area being larger in the euhaline sector. The effects of hydrodynamic exposure were limited to the euhaline sector, where barnacles covered larger extensions of the rocky intertidal habitat at more exposed sites. Barnacle density did not vary consistently among sectors or levels of hydrodynamic exposure but changed according to certain combinations of these factors. As a result, the realized habitat tended to decrease from the mouth of the estuary to the oligohaline sector. Three species were recorded in this study: Chthamalus bisinuatus, practically limited to the euhaline sector; Microeuraphia rhizophorae, limited to the polyhaline and mesohaline sectors; and Chthamalus proteus, limited to the polyhaline, mesohaline and oligohaline sectors. Habitat sharing was substantial only in 3 of the 16 sampled sites and only between Microeuraphia rhizophorae and Chthamalus proteus. At low barnacle densities, up to 40% substrate cover, there was no apparent vertical segregation between these two species. Above 40–50% cover, Microeuraphia rhizophorae occupied a higher position on the shore than Chthamalus proteus. These results suggest a lottery model when vacant space is still abundant and interspecific interference competition when barnacle abundance surpasses a critical abundance threshold. In this latter case, Chthamalus proteus displaces Microeuraphia rhizophorae to upper zone levels, where conditions for growth and reproduction are probably less favorable.