The reef coral Siderastrea stellata thriving at its range limit: population structure in Arraial do Cabo, southeastern Brazil

Marginal reef sites are typically characterized by a low diversity of coral species; however, they are considered high conservation priorities. Despite their importance, the ecology of edge populations is poorly known. Arraial do Cabo, which is located within an upwelling region, represents the southernmost limit of the reef coral Siderastrea stellata Verrill, 1868, a species endemic to Brazil. The present study characterizes the population structure of S. stellata in Arraial do Cabo in terms of colony size, density, and percentage cover, in relation to the density of other coral species and seawater temperature. When compared to previous studies on S. stellata, populations within the species geographical range, a decrease in the abundance and colony size was expected to be observed in this edge population. However, S. stellata thrives in Arraial do Cabo and population parameters did not seem to decrease toward the species southern range limit. Siderastrea stellata may have found an ecological niche in a marginal area that is not available with the species range, allowing this population to expand in colony size and number. Siderastrea stellata was the most abundant species within the coral community and appears to be resilient to low temperatures. Recruits and small-sized adults of S. stellata were well represented at all sampling sites, but large-size colonies were more frequent in restricted-access areas. Given its unique environmental features, Arraial do Cabo should be regarded as a hotspot for coral conservation and research in the southwestern Atlantic Ocean.