Physical contact interactions with scleractinian corals in hard substrate communities

Shallow reefs are among the most diverse ecosystems, and their substrate is densely occupied by sessile organisms that frequently contact physically and interact mutually. Nevertheless, the relative importance of species abundance in shaping physical contacts in these ecosystems remains largely unknown. The aim of this study was to evaluate physical contact interactions, by using tools derived from the complex network theory, between hard corals and other benthic organisms in four areas along the Brazilian coast comprising coral reefs and rocky shores with different physical and biotic structures. It was also investigated whether interactions with corals occurred as expected by the abundance of the benthic organisms in each area, and whether corals belonging to families known as more aggressive interacted less with other organisms. In all areas, the abundance of both corals and contacting organisms directly influenced the abundance of interactions between benthic organisms, regardless of the physical or biological characteristics of the habitat and of the interacting organisms. In addition, coral species interacted more with the most abundant benthic group, the epilithic algal matrix, in all areas. Nevertheless, some evidence was found about biological mechanisms helping to explain the structure of the studied interactions, although to a lesser extent. The obtained networks presented high nestedness and connectance, but low modularity. These patterns indicate the low specificity of the studied interactions and reinforce the role of abundance as an important driver of contacts between sessile organisms in shallow hard bottom ecosystems.