Diverse staghorn coral fauna on the mesophotic reefs of north-east Australia.

Paul Muir,Carden Wallace,Pim Bongaerts,Tom C. L. Bridge

doi:10.1371/journal.pone.0117933

Abstract

Concern for the future of reef-building corals in conditions of rising sea temperatures combined with recent technological advances has led to a renewed interest in documenting the biodiversity of mesophotic coral ecosystems (MCEs) and their potential to provide lineage continuation for coral taxa. Here, we examine species diversity of staghorn corals (genera Acropora and Isopora) in the mesophotic zone (below 30 m depth) of the Great Barrier Reef and western Coral Sea. Using specimen-based records we found 38 staghorn species in the mesophotic zone, including three species newly recorded for Australia and five species that only occurred below 30 m. Staghorn corals became scarce at depths below 50 m but were found growing in-situ to 73 m depth. Of the 76 staghorn coral species recorded for shallow waters (depth ≤ 30 m) in north-east Australia, 21% extended to mesophotic depths with a further 22% recorded only rarely to 40 m depth. Extending into the mesophotic zone provided shallow water species no significant advantage in terms of their estimated global range-size relative to species restricted to shallow waters (means 86.2 X 106 km2 and 85.7 X 106 km2 respectively, p = 0.98). We found four staghorn coral species at mesophotic depths on the Great Barrier Reef that were previously considered rare and endangered on the basis of their limited distribution in central Indonesia and the far western Pacific. Colonies below 40 m depth showed laterally flattened branches, light and fragile skeletal structure and increased spacing between branches and corallites. The morphological changes are discussed in relation to decreased light, water movement and down-welling coarse sediments. Staghorn corals have long been regarded as typical shallow-water genera, but here we demonstrate the significant contribution of this group to the region’s mesophotic fauna and the importance of considering MCEs in reef biodiversity estimates and management.

Highlights

Reef-building corals host symbiotic microalgae of the genus Symbiodinium that provide much of the energy required for their growth and calcification [1]
We found a total of 38 staghorn coral species and two potentially undescribed species at mesophotic depths (Table 1, Fig. 1), indicative of a highly diverse staghorn coral fauna in mesophotic coral ecosystems (MCEs) of the Great Barrier Reef (GBR) and Coral Sea
In addition to this diversity, we found staghorn corals to be relatively abundant (Fig. 2), capable of forming large mono-stands and colonies (Figs. 3 and 4) and occurring to a maximum depth of 73 m (Fig. 5), indicating this group is a significant component of the mesophotic coral assemblages of the region

Summary

Introduction

Reef-building corals host symbiotic microalgae of the genus Symbiodinium that provide much of the energy required for their growth and calcification [1] They are dependent on PLOS ONE | DOI:10.1371/journal.pone.0117933. Recent improvements in the cost and utility of small remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) have allowed researchers to begin studying depths normally out of range of SCUBA diving. These tools have identified extensive areas of previously undocumented coral reef habitat [9,10,11]. Recent analyses indicate that the area of coral habitat on the Great Barrier Reef (GBR) may be at least 50% larger than currently estimated when MCEs are taken into account [12]

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Conclusion