Picture a coral reef — most people will probably imagine brightly coloured corals, fish and other animals swimming in well-lit shallow waters. In fact, the coral reefs that live close to the surface of the sea — the ones that we can swim, snorkel, or dive near and see from space — are only a small portion of the complete coral reef ecosystem. Light-dependent corals can live in much deeper water (up to a depth of 150 m in clear waters). The shallow coral reefs from the surface of the sea to 30–40 m below are more like the tip of an iceberg; they are the more visible part of an extensive coral ecosystem that reaches into depths far beyond where most people visit. These intermediate depth reefs, known as mesophotic coral ecosystems (MCEs), are the subject of this report. Although MCEs are widespread and diverse, they remain largely unexplored in most parts of the world, and there is little awareness of their importance among policy makers and resource managers. As a result, MCEs are for the most part not considered in conservation planning, marine zoning and other marine policy and management frameworks. The goal of this report is to raise awareness in policy makers and resource managers by providing an accessible summary on MCEs, including a discussion of the ecosystem services they provide, the threats they face, and the gaps in our understanding.Key questions addressed in this report include: can MCEs provide a refuge for the many species in shallow water reef ecosystems that are facing increasing threats from human activities? If shallow reefs (< 30–40 m) continue to decline, can MCEs provide the stock to re-populate them? The answer is of course that it depends on the species involved. In some situations, MCEs may provide this ecosystem service and act as “lifeboats” for nearby, connected shallower reefs that have been damaged. In other cases, however, MCEs may be just as vulnerable as shallower reefs to the range of human pressures exerted upon them. Whether or not they are lifeboats for shallow reef species, MCEs are worthy of protection, both for their inherent biodiversity and for the wide range of ecosystem goods and services they provide. The biodiversity of MCEs is comparable to that of shallow reefs, yet there are also unique species that are found only in MCEs and/or deeper water. Table 1 shows key differences between MCEs and shallow reefs.While buffered from some of the natural and anthropogenic threats faced by shallow reefs, MCEs are nevertheless vulnerable to many of the same threats, such as fishing, pollution, thermal stress, diseases and tropical cyclones, albeit to differing extents (Table 2). MCEs also face threats from oil and gas exploration and cable and pipeline laying, which are less common on shallow reefs. For light-dependent mesophotic reef organisms living at low light levels (1 per cent of that found at the sea surface), anything that inhibits light reaching the depths (e.g. sedimentation, turbidity or pollution) has an impact on their survival. In general, there remains much to be discovered about the extent of impacts from natural and anthropogenic threats on MCEs.While some pressures on MCEs are global in origin, and require a global response, many others are regional or local. It is important that measures to protect an individual MCE take an adaptive, ecosystem-based approach to address the cumulative impacts, considering both global pressures and specific local pressures. Most of the management tools used to protect and conserve shallow coral reefs can also be used to protect and conserve MCEs (Table 2). The main recommendations made in this report (see text box on guidance for resource managers) relate to this lack of awareness of MCEs, the anthropogenic threats facing them, and the immediate actions that can be taken, at the local and regional levels, to protect and conserve them.Although the study of MCEs has increased exponentially in the past 30 years, there are still large gaps in our scientific knowledge of them, especially in comparison with shallow reefs. The best way to close these information gaps is to focus research efforts on answering questions that are critical toenabling resource managers to make informed decisions about MCE protection and conservation. For MCEs, the most crucial information is what scientists would call “baseline information”, including information on their location, biological and physical characteristics, threats, condition and the causes and consequences of that condition. The key questions for resource managers and the corresponding research priorities to address them are detailed in Table 3.

Depth range
30- 150 m

Mesophotic “mentions”

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Classification
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Biodiversity
Ecology
Community structure
Evolution
Physiology
Geomorphology
Oceanography
Long-term monitoring
Fisheries
Management and Conservation
Connectivity
Paleoecology

Focusgroups
Fishes
Scleractinia (Hard Corals)
Overall benthic (groups)
Algae (Macro, Turf and Crustose Coralline)
Other invertebrates
Antipatharia (Black Corals)
Hydrozoa
Octocorallia (Soft Corals)
Porifera (Sponges)
Symbiodinium (zooxanthellae)

Locations
Australia - Great Barrier Reef
USA - Hawaii
Israel - Red Sea
Japan - Ryukyu Islands (Okinawa)
Micronesia - Caroline Islands (Pohnpei)
Puerto Rico
US Virgin Islands
USA - Pulley Ridge
Australia - Northern Australia

Platforms
Remotely Operated Vehicle (ROV)
Rebreather
Fishing
Autonomous Underwater Vehicle (AUV)
Sonar / Multibeam
Drop / Towed Video
SCUBA (open-circuit or unspecified)