Although the existence of coral-reef habitats at depths to 165 m in tropical regions has been known for decades, the richness, diversity, and ecological importance of mesophotic coral ecosystems (MCEs) has only recently become widely acknowledged. During an interdisciplinary effort spanning more than two decades, we characterized the most expansive MCEs ever recorded, with vast macroalgal communities and areas of 100% coral cover between depths of 50–90 m extending for tens of km2 in the Hawaiian Archipelago. We used a variety of sensors and techniques to establish geophysical characteristics. Biodiversity patterns were established from visual and video observations and collected specimens obtained from submersible, remotely operated vehicles and mixed-gas SCUBA and rebreather dives. Population dynamics based on age, growth and fecundity estimates of selected fish species were obtained from laser-videogrammetry, specimens, and otolith preparations. Trophic dynamics were determined using carbon and nitrogen stable isotopic analyses on more than 750 reef fishes. MCEs are associated with clear water and suitable substrate. In comparison to shallow reefs in the Hawaiian Archipelago, inhabitants of MCEs have lower total diversity, harbor new and unique species, and have higher rates of endemism in fishes. Fish species present in shallow and mesophotic depths have similar population and trophic (except benthic invertivores) structures and high genetic connectivity with lower fecundity at mesophotic depths. MCEs in Hawai‘i are widespread but associated with specific geophysical characteristics. High genetic, ecological and trophic connectivity establish the potential for MCEs to serve as refugia for some species, but our results question the premise that MCEs are more resilient than shallow reefs. We found that endemism within MCEs increases with depth, and our results do not support suggestions of a global faunal break at 60 m. Our findings enhance the scientific foundations for conservation and management of MCEs, and provide a template for future interdisciplinary research on MCEs worldwide.
The commercially valuable Hawaiian black coral Antipathes grandis Verrill, 1928 is redescribed based on reexamination of the holotype from the Bernice P. Bishop Museum and field collections of 34 specimens from depths of 27–127 m. The first scanning electron micrographs of A. grandis skeletal spines are provided, along with a series of in situ colour photographs and morphometric measurements of spines and polyps. Three colour morphotypes were collected in the field (red, pale red, and white), none of which could be differentiated based on morphological or genetic characters (two mitochondrial and two nuclear markers). In situ observations are used in conjunction with morphological and genetic characters to distinguish among the commercially valuable Hawaiian black coral species A. grandis and A. griggi Opresko, 2009. A. grandis is differentiated from A. griggi by its finer and more irregular branching, smaller and more closely-spaced polyps, and conical spines that are smaller and not characterised by bifurcations towards their apex. Morphologically, the species most closely resembling A. grandis is A. caribbeana Opresko, 1996 from the Caribbean. Among analysed congenerics, DNA sequences of A. grandis were likewise most similar to those of A. caribbeana for three of the four molecular markers used in this study. A combination of low genetic variability, incomplete taxonomic sampling, and unexpected similarity between A. caribbeana and the unbranched whip coral Stichopathes cf. occidentalis (Gray, 1860), hindered our ability to determine the sister relationship of A. grandis. However, in no phylogenetic reconstruction did A. grandis group sister to its sympatric congener A. griggi.
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