Coralline Hills: high complexity reef habitats on seamount summits of the Vitória-Trindade Chain

Seamounts and oceanic islands play an important role as biodiversity hotspots amid the vastness of the oligotrophic open ocean. While island ecology and evolution have received a lot of attention in the last decades, the exploration and understanding of community and habitat dynamics of seamounts remain challenging. Here, we investigate the ecology and biogeography of fish and benthic communities of a recently discovered southwestern Atlantic reef system at Davis seamount. This seamount belongs to the Vitória-Trindade Chain and is located in international waters off the Brazilian coast. We present this reef system, that also occurs on other shallow seamounts of the chain, as a new reef habitat named “Coralline Hills”: Its hill-shaped structure is mainly built by crustose coralline algae and rises up from the seamount summit at 60–70 m to 17 m depth. The benthic community is mainly composed by coralline algae and sponges. Fish biomass at Davis coralline hill is dominated by carnivores, mainly top predators such as nurse sharks and large groupers. The relatively shallow reef top presents higher species richness, abundance and distinct trophic structure (mostly omnivore and planktivore species) than the mesophotic zone (with higher abundance of carnivorous fishes). A biogeographic analysis revealed that the reef fish community structure is greatly influenced by a set of dispersal and establishment traits that strongly differs from that encountered on coastal reefs of the central Brazilian coast and on insular reefs of Trindade Island. Gathering information about the ecology and structure of such unique and remote habitat is timely, since the region is under imminent threat such as fishing and mining and lacks international attention.