Deep down on a Caribbean reef: lower mesophotic depths harbor a specialized coral-endosymbiont community

The composition, ecology and environmental conditions of mesophotic coral ecosystems near the lower limits of their bathymetric distributions remain poorly understood. Here we provide the first in-depth assessment of a lower mesophotic coral community (60-100 m) in the Southern Caribbean through visual submersible surveys, genotyping of coral host-endosymbiont assemblages, temperature monitoring and a growth experiment. The lower mesophotic zone harbored a specialized coral community consisting of predominantly Agaricia grahamae, Agaricia undata and a deep-water lineage of Madracis pharensis, with large colonies of these species observed close to their lower distribution limit of similar to 90 m depth. All three species associated with deep-specialist photosynthetic endosymbionts (Symbiodinium). Fragments of A. grahamae exhibited growth rates at 60 msimilar to those observed for shallow Agaricia colonies (similar to 2-3 cm yr(-1)), but showed bleaching and (partial) mortality when transplanted to 100 m. We propose that the strong reduction of temperature over depth (Delta 5 degrees C from 40 to 100 m depth) may play an important contributing role in determining lower depth limits of mesophotic coral communities in this region. Rather than a marginal extension of the reef slope, the lower mesophotic represents a specialized community, and as such warrants specific consideration from science and management.