1. Mesophotic Coral Ecosystems: Introduction and Overview

Although the existence of zooxanthellate corals in mesophotic coral ecosystems (MCEs; light-dependent coral ecosystems from 30 to 150 m in depth) has been known since the nineteenth century and focused scientific exploration of MCEs began over 50 years ago, more than 70% of all research on MCEs has been published only within the past seven years. MCEs represent approximately 80% of potential coral reef habitat worldwide, yet very little is known about them in comparison to shallow reefs. Many MCE species new to science have been discovered in the past decade, and many more await discovery. The term MCEs has been widely adopted by the scientific community since its 2008 inception; however, there is considerable inconsistency in how it is subdivided into “upper” and “lower” (and sometimes “middle”) zones. Moreover, doing so may lead to artificial boundaries when habitats and ecological communities at different depth zones may blend together. Growing evidence suggests that MCEs harbor proportionally more geographically endemic species than their shallow-water counterparts, and initial indications are that major biogeographic patterns described for shallow reef organisms may not apply to MCEs. Although MCEs may serve as refugia for some shallow species, they are increasingly recognized as unique ecosystems, important in their own right. Future research on MCEs should aim to address gaps in our understanding of the basic physical and biological characteristics of MCEs including geography, taxonomic composition, depth distribution, ecology, physiology, and connectivity. Improving knowledge of MCEs would benefit from combining different technologies to leverage the strengths of each.