Biodiversity, biogeography and conservation of mesophotic coral ecosystems in the Atlantic Ocean

Abstract

The recent popularization of technical diving by scientists has yielded a large increase in the exploration of mesophotic coral ecosystems (MCEs), deep reefs found between 30 and 150 m depth. Biodiversity assessments have disclosed rich MCE assemblages and many unknown species across the globe, with a current discovery rate of two new fish species per hour of technical diving exploration. Most biodiversity surveys suggest that species richness decrease along the depth gradient, and recent studies reported substantial changes in coral and reef fish assemblages with depth, a trend mainly driven by species turnover. However, the interactive effects of biogeography and environmental filtering in shaping vertical patterns of reef fish diversity, community and functional structure in MCEs have yet to be studied in detail. The few ecological studies available have revealed contradictory community patterns. Studies based on depth range datasets present high overlap of species between shallow and deep reefs, suggesting that MCEs could provide refuge to species of broad depth range that are threatened in shallow reefs; a concept known as Deep Reef Refuge Hypothesis (DRRH). On the other hand, studies based on empirical data show low levels of similarity between depth zones, revealing unique assemblages on MCEs, with many endemic and undescribed species. This contradiction might be a result of the DRRH being species or context dependent, and further environmental and biogeographic analyses exploring components of abundance-based dissimilarity are lacking. Oceanic islands and archipelagos are excellent natural laboratories to conduct these ecological and evolutionary studies, offering exceptional conditions to study MCEs. Environmental factors such as sea temperature, habitat complexity, coral cover and wave exposure are known to drive the ecology of shallow reef fishes, but need to be better explored in MCEs. Pleistocene sea-level fluctuations might have had stronger effects reducing and expanding insular shallow habitats than deep ones (Habitat Persistence Hypothesis), what is predicted to influence distinct patterns in biodiversity, endemism and the evolutionary history of MCE fishes. This proposal aims to create a research line for mesophotic reef exploration at the Center for Marine Biology of the University of São Paulo (CEBIMar / USP), and advance the understanding of evolutionary and ecological processes underlying biodiversity patterns at MCEs in the Atlantic Ocean. Among the innovative questions and hypothesis elaborated, this proposal aims, for the first time, to: evaluate how the environmental and biogeographical factors are driving taxonomic (alpha and beta) and functional diversity patterns in shallow, upper and lower MCEs of the Atlantic Ocean; assess if the validity of "Deep Reef Refuge Hypothesis" depends on ecological, historical and geographical contexts by analyzing fish abundance along depth gradients in multiple locations; determine the relationships between species richness and endemism with island biogeography predictors (area, isolation, and Pleistocene sea-level fluctuations) on both shallow and deep communities; and test the Habitat Persistence Hypothesis by exploring the evolutionary history of shallow and deep species using advanced genomic techniques. Other goals of this project include: record, sample, and describe the fish biodiversity, including new species, new records and exotic species, from poorly explored MCEs of the Atlantic Ocean; capacitate researchers and students to perform technical diving and modern genomic analyses; and consolidate this research line at the CEBIMar, thus contributing to scientific and conservation initiatives as well as awareness of the importance of MCEs. (AU)