THE INFLUENCE OF EUSTATIC SEA-LEVEL FLUCTUATIONS ON GORGONIAN OCTOCORALS OF THE TROPICAL PACIFIC

Tropical Pacific coral reefs are central to marine biodiversity, with the greater proportion of these reefs (~80%) existing between 30-150 m depth. Such deep reefs or Mesosphotic Coral Ecosystems (MCEs) are among the most diverse, yet least understood and most unexplored realms on the planet. Of those explored, deep reefs within the tropical Pacific are typically dominated by gorgonian octocorals. Comparisons with modern-day and historic collections across the tropical Pacific reveal an increase in gorgonian diversity with depth, and an eastward attenuation in the shallows (<30 m). With less than 50% species overlap between shallow and deep-reef taxa, coupled with area specific deep-reef diversity, it is clear that deep and shallow reef species possess fundamentally different biogeographic distributions. Moreover, attenuating biodiversity on shallow reefs away from centers of species richness is not observed on deeper (MCE) reefs. Phylogenetic reconstructions suggest depth-specific species traits and reveal new discoveries of species associated with past low sea level-stands. These results suggest the influence of eustatic sea-level fluctuations on reef biodiversity as posited, in part, by the habitat persistence hypothesis (HPH) whereby reef habitat persistence at depth maintains biodiversity, leading to higher rates of endemism. However, current estimates of endemism or species complexes are thwarted by taxonomic uncertainties throughout the Octocorallia. Furthermore, bathymetric polyphyly within certain families (e.g., Isididae, Acanthogorgiidae) prompt phenotypic trait comparisons for assessing the evolution of structure-function relationships over geological time. In an age necessitating resource protection, deep-reefs provide a natural laboratory to address questions of biodiversity, systematics, resilience, and conservation priorities.