Diverse staghorn coral fauna on the mesophotic reefs of north-east Australia.
Bottom Line: Staghorn corals became scarce at depths below 50 m but were found growing in-situ to 73 m depth.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.The morphological changes are discussed in relation to decreased light, water movement and down-welling coarse sediments.
Affiliation: Queensland Museum, Townsville, Australia.
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 10(6) km2 and 85.7 X 10(6) 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.
Mentions: Four species were newly recorded for Australia, including A. elegans, reported in an initial survey by the authors in . Finding new species records in the GBR and Coral Sea is unusual because reef-building corals, particularly the genus Acropora, have been relatively well documented in the region [17, 18, 33, 34], highlighting the dearth of information on coral biodiversity in deeper waters. There is some prior indication that the distribution of one of these species, A. kimbeensis, includes north-east Australia (map p.352 ), although no data or specimens are provided and in-situ identification is difficult for this species [17,18]. The other three species (A.elegans, A. pichoni, A. tenella) are distinctive, having unusual morphology (Fig. 6A and B) which is unlikely to have been overlooked and they have probably remained undocumented to date as they were only recorded at or below 40 m depth. Recent data [17,28], along with these new records, extend the geographical range of each species substantially (Fig. 7), as previously they were documented only for Indonesia with one species extending to PNG and another to the South China Sea . In light of these range extensions the classification of these species as rare or globally restricted [29,35] and the IUCN listing as endangered and rare  may have to be reassessed. The presence of deep-water “Coral Triangle” species also suggests links between the deep-water fauna of the far northern GBR and Coral Sea and northern Papua New Guinea. Interestingly, the five exclusively deep-water species belong to the A. elegans species group (Table 3) and show close phylogenetic affinity . These records increase the number of species of staghorn corals in the region to 81 (Table 1). In addition, we found two potentially undescribed Acropora species exclusively below 40 m depth, which will be described pending further analysis. The new records for the region and potential undescribed species restricted to depths greater than 40 m, highlight the need to include the mesophotic zone in assessments of reef species diversity and the management of reef and inter-reef areas.