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: In the exclusively mesophotic species, colony shape was restricted to plating and free-living arborescent growth forms (Table 3). The plating morphology found in these taxa was quite different to that found in shallow reef taxa, with plates formed by lateral flattening and spacing of the branches and an increase in corallite spacing (Figs. 3 and 6A, B, E). Depth generalist species displayed hispidose, corymbose, arborescent, plating and rarely arborescent table and cuneiform colony forms (Table 3). The arborescent, digitate and table colony forms, which are common in shallow reef Acropora , were rare or absent below 30 m depth for the sites surveyed. Interception of light, hydrodynamics, competition and predation likely restrict the colony shape of staghorns [2,18,35,45]. In addition, many lower reef slopes are exposed to coarse, semi-suspended sediments advected down the reef slope. For sites with a steep slope or wall adjacent to shallow reef habitats we frequently found high levels of coarse calcareous sands that cascaded down the slope forming chutes or “rivers of sand” that settled upon corals, partially burying some colonies (Fig. 8). Similar coarse sediments and “chutes” were reported for steep reef slopes at Enewetak Atoll at mesophotic depths . Carbonate productivity of shallow reefs is estimated at up to 110 t ha-1 yr-1  and much of this material is likely to become coarse sand transported down the reef slope. While horizontal surfaces are needed by mesophotic corals to maximize interception of light at depth , such surfaces appear prone to collecting coarse sediments, particularly in habitats below the wave base with low water movement. Accumulation of sediment on coral surfaces is well documented to be deleterious , therefore extensive growth of mesophotic corals requires strategies for dealing with down-welling coarse sands. In staghorn corals, the indeterminate and “diffuse plate” morphologies commonly found in the mesophotic Acropora species probably minimize sediment accumulation while maintaining adequate photosynthesis. Thin branches and an indeterminate growth form would also allow relatively rapid rates of branch extension and therefore an ability to recover from partial burial. These extreme morphologies allow Acropora to extend to depths of 73 m at some sites (Table 3). Reef-building corals extend down to depths of 125 m in this region , although the colony morphologies found below 70 m were almost exclusively solid plate and encrusting forms.