Abstract
Mesophotic ecosystems (MEs) are characterized by the presence of light-dependent organisms, found at depths ranging from ~30 to 150 m in temperate, subtropical and tropical regions. These communities occasionally create massive reef structures with diverse but characteristic morphologies, which serve as the framework builders of those ecosystems. In many localities, MEs are physically linked with shallow and deep-sea habitats, and while taxa from both environments share this space, a unique and endemic biodiversity is also found. The main MEs studied to date are the mesophotic coral ecosystems (MCEs) and the temperate mesophotic ecosystems (TMEs), which have received increased attention during the last decade. As shallow coral reef ecosystems are among the most threatened habitats on Earth, the potential of MEs to act as refugia and contribute to the resilience of the whole ecosystem has been a subject of scrutiny. New technologies and methods have become more available to study these deeper parts of the reef ecosystems, yielding many new discoveries. However, basic gaps in knowledge remain in our scientific understanding of the global diversity of MEs, limiting our ability to recognize biogeographic patterns and to make educated decisions for the management and conservation of these ecosystems.
Highlights
Shallow marine habitats occupying photic-zone areas over a large latitudinal gradient, from tropical to temperate waters, are part of the most productive and diverse ecosystems
A few ecological and biological studies have tried to explain the boundaries between shallow and mesophotic reefs by using light-dependent coral assemblages and light levels to define the zonation along the depth gradient [13,14,15,18]
Despite a widely reported trend of a decrease in species richness along the depth gradient [12,30,31,32,33], recent contributions have suggested that extensive fish species turnover, instead of purely nestedness, characterize this spatial gradient from shallow to mesophotic depths [26,29]
Summary
Shallow marine habitats occupying photic-zone areas over a large latitudinal gradient, from tropical to temperate waters, are part of the most productive and diverse ecosystems. Tropical coral reefs are among the most diverse habitats on the globe [1,2], demonstrating high efficiency in the retention and recycling of carbon and nutrients, which contribute to the productivity of the ecosystem [3,4]. Mesophotic coral ecosystems (MCEs) and temperate mesophotic ecosystems (TMEs) have received little research attention [5,6].
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