Moderate zooxanthellate coral growth rates in the lower photic zone

Abstract

The ecology of phototrophic corals in the lower photic zone remains poorly understood. Studies to date indicate that growth rates generally decrease as available light attenuates with depth and are very slow at depths > 40 m. Here, we provide detailed evidence for moderate growth for obligate zooxanthellate corals at extreme depths. Using reliable U–Th dating techniques, Leptoseris spp. from 70 to 110 m in Hawaii were determined to grow (linear extension) from 8.0 ± 3.0 to 24.6 ± 2.7 mm yr−1. Given the modest growth rates of other agariciids in well-lit shallow waters, these deep-water growth rates are unexpectedly high, suggest specialized physiology and help alter the traditional paradigm on photo-ecology in mesophotic coral ecosystems. The thin plate-like colonies of Leptoseris spp., which dominate the coral community in the lower photic zone throughout the Indo-Pacific, primarily grow radially and do not appear to appreciably thicken over time. In the lower photic zone, this growth strategy maximizes surface area with minimal calcification, thereby enabling Leptoseris to expand planar area more quickly than thickly calcified species. Photosynthetic and growth efficiencies may also be facilitated by the optical geometry of their skeletons, increased access to inorganic nutrients and cooler temperatures at depth. These robust growth rates have important implications on the recovery potential of MCE habitat in the event of disturbance.