Mesophotic coral ecosystems are characterized by the presence of light-dependent corals and associated communities found at water depths where light penetration is low. The term mesophotic literally translates to 'meso' for middle and 'photic' for light. The dominant communities providing structural habitat in the mesophotic depth zone can be made up of coral, sponge, and algal species. [a] The fact that they contain zooxanthellae and require light distinguishes these corals from true deep-sea corals, though their depth ranges may overlap. [b]
Mesophotic coral ecosystems are typically found at depths ranging from 30-40 m and extending to over 150 m in tropical and subtropical regions. [a] Mesophotic coral ecosystems may be regarded as extensions of shallow coral ecosystems and often share common species. It has been hypothesized that mesophotic coral ecosystems may serve as potential sources to reseed or replenish degraded shallow-water coral reef species.
Little is known or understood about mesophotic coral ecosystems. Until recently, investigations into mesophotic ecosystems have been hampered by their depth. Unfortunately, the upper limit of mesophotic coral ecosystems lies below the depth limitation for the most cost-effective and widely-used underwater survey techonology, conventional SCUBA diving (30-40 m). However, advances in technical diving methods and instrumentation, such as mixed gas diving; autonomous underwater vehicles; and remotely operated vehicles, are increasingly providing easier access to study these ecosystems. [c]
The values of and threats to mesophotic coral ecosystems are thought to be similar to those of shallow-water coral ecosystems. However, the extent to which the threats to shallow-water reefs affect mesophotic coral ecosystems is unknown. In addition, many of these ecosystems are unprotected and are thus potentially at risk. [a]