Deep Coral Reefs

A manipulator arm from a Pisces submersible of the Hawaiʻi Undersea Research Laboratory (HURL), University of Hawaiʻi at 
                 Mānoa, placing a marker at one end of a transect line on a mesophotic coral reef dominated by Leptoseris hawaiiensis 
                 corals in the Auʻau Channel off Maui, at a depth of ~ 80 m. Photo courtesy of Hawaiʻi Undersea Research Laboratory.
A manipulator arm from a Pisces submersible of the Hawaiʻi Undersea Research Laboratory (HURL), University of Hawaiʻi at Mānoa, placing a marker at one end of a transect line on a mesophotic coral reef dominated by Leptoseris hawaiiensis corals in the Auʻau Channel off Maui, at a depth of ~ 80 m. Photo courtesy of Hawaiʻi Undersea Research Laboratory.

Deep coral reefs, known scientifically as mesophotic coral reefs, are light-dependent coral reefs and associated communities found at water depths where light penetration is low. Mesophotic coral reefs are found at depths of 30-40 m but are also known to extend to depths past 150 m in both tropical and subtropical regions.

The name mesophotic translates to "meso" for middle and "photic" for light.

These coral reefs can comprise species of plate corals, sponges, and algae. Mesophotic coral reefs are sometimes regarded as extensions of shallow coral reefs because they share common species with reefs found at depths < 30 m. Although their depth ranges may overlap, corals of mesophotic reefs are different from deep-sea corals in that they contain symbiotic algae (zooxanthellae).

Reef Building

Reef-building mesophotic corals, like scleractinian (stony) corals, depend on a symbiotic unicellular algae, which

  • live within the tissue of their host corals and
  • provide organic nutrients for their coral hosts.

In return, these algae receive inorganic nutrients and shelter from their hosts. Although many corals also have a capability to feed on zooplankton, they get up to 90% of their nutrients from their algal symbionts, and they grow much faster because of them.

It is natural, therefore, to expect that coral reef growth is greater at shallow depths (< 30 m), where more light is available, compared to reefs at deeper depths, where corals are expected to be rare or absent.
PIFSC scientist Frank Parrish records fish observations on a mesophotic reef dominated by macroalgae, with small coral 
                 colonies mixed in. Photo by John Rooney, PIFSC.
PIFSC scientist Frank Parrish records fish observations on a mesophotic reef dominated by macroalgae, with small coral colonies mixed in. Photo by John Rooney, PIFSC.

That expectation tends to agree with most scientists' observations and general experience with conventional open-circuit scuba diving on coral reefs, and it has been reported in scientific literature as well.

Grigg and Epp (1989), for example, reported in the highly respected journal Science that, "the critical depth for drowning for most coral reefs in the world is estimated to be about 30 to 40 m, but could be less". Because that depth range coincides with the maximum depth of conventional open-circuit, no-decompression, scuba diving, relatively few scientists have had the opportunity to observe coral reefs at depths > 40 m (e.g., Menza et al. 2007). Several papers on coral reefs found at depth > 40 m have been published (e.g., Maragos and Jokiel 1986; Frick and Knauer 1986; Hopley 1991), but such reefs were typically considered more as oddities than as important components of coral reef ecosystems.

Technical Growth and Greater Access

In a unique collaboration that took advantage of the capabilities of both submersibles and divers, technical divers from the 
                 University of Hawaiʻi at Mānoa and Honolulu's Bishop Museum work at the edge of a mesophotic reef at a depth of 80 m, 
                 with PIFSC scientists in the Pisces submersible, to stain coral colonies for coral-growth-rates studies. Photo credit: Richard Pyle, 
                 Bishop Museum.
In a unique collaboration that took advantage of the capabilities of both submersibles and divers, technical divers from the University of Hawaiʻi at Mānoa and Honolulu's Bishop Museum work at the edge of a mesophotic reef at a depth of 80 m, with PIFSC scientists in the Pisces submersible, to stain coral colonies for coral-growth-rates studies. Photo credit: Richard Pyle, Bishop Museum.
Hawaii Undersea Research Laboratory (HURL) Pisces 4 submersible in front of a mesophotic reef at a depth of 102 m in the Auʻau 
                 Channel off Maui. Access to research submersibles has been invaluable for expanding our knowledge of mesophotic coral ecosystems. 
                 Photo courtesy of HURL.
Hawaii Undersea Research Laboratory (HURL) Pisces 4 submersible in front of a mesophotic reef at a depth of 102 m in the Auʻau Channel off Maui. Access to research submersibles has been invaluable for expanding our knowledge of mesophotic coral ecosystems. Photo courtesy of HURL.

The growth of "technical" scuba diving—which involves forays to depths well below 40 m, use of breathing-gas mixtures, including helium, and staged decompression—along with the entry of closed-circuit rebreathers into the consumer market has enabled access to depths > 40 m to an increasing number of scientists over the last 20 years or so. In 2004, researchers working off the Florida Keys received extensive national and international press with their discovery of a coral reef complex at depths of 60-75 m (Jarrett et al. 2005). That discovery coincided with the discovery of other coral reefs at similar depths and spurred awareness that such communities might be ecologically important. A few years later, the term "mesophotic coral ecosystem" was coined to refer to these light-dependent communities of corals, algae, and other associated organisms (Hinderstein et al. 2010).

Scientists at the PIFSC and partner agencies in Hawaiʻi, such as the Division of Aquatic Resources of the Hawaiʻi Department of Land and Natural Resources, Bishop Museum, University of Hawaiʻi at Mānoa, the Hawaii Undersea Research Laboratory, and Hawaiʻi Pacific University have discovered and conducted research on a number of mesophotic reefs in the Hawaiian Archipelago. PIFSC scientists have found luxuriant mesophotic coral reefs not only in Hawaiʻi but also in American Samoa and the Mariana Archipelago and at depths up to 130 m (Kahng and Kelley 2007; Bare et al. 2010; Rooney et al. 2010), and others have reported on reefs throughout the Caribbean, along the Great Barrier Reef, and at many other locations (e.g., Armstrong et al. 2006; Bridge et al. 2011). Despite the recent spate of research, very basic ecological questions about mesophotic reefs remain unanswered. For instance, numerous fish species have been found on some mesophotic reefs (Costa et al. 2012). Why is it that no fish have been observed on other apparently similar reefs? There are mesophotic reefs off the west coast of Maui, but why are far fewer reefs known for the west coast of O`ahu, despite the availability of hard substrate that appears suitable for them?

Future Research

Frame grab from video recorded with a baited remote underwater video station (BRUVS) video at a depth of ~ 75 m. The reef shown 
                 here is composed almost exclusively of the coral Euphylllia paraancora off the west coast of the island of Saipan in the 
                 Commonwealth of the Northern Mariana Islands. This species is one of 82 species that have been petitioned for coverage under the 
                 Endangered Species Act. Photo by Steve Lindfield, University of Western Australia and University of Guam Marine Laboratory.
Frame grab from video recorded with a baited remote underwater video station (BRUVS) video at a depth of ~ 75 m. The reef shown here is composed almost exclusively of the coral Euphylllia paraancora off the west coast of the island of Saipan in the Commonwealth of the Northern Mariana Islands. This species is one of 82 species that have been petitioned for coverage under the Endangered Species Act. Photo by Steve Lindfield, University of Western Australia and University of Guam Marine Laboratory.

It is important for scientists and resource managers to learn more about mesophotic reefs and the organisms associated with them. They are significant ecological communities in and of themselves and may provide important services to the overall coral reef ecosystem of which they are part. Mesophotic reefs have been proposed as refugia for species that are threatened on shallow reefs from pollution, effects of global warming, overfishing, and other forms of environmental stress (Riegl and Piller 2003). The 3D structure provided by some mesophotic reefs has been observed to shelter large numbers of small or juvenile fishes. Perhaps, some of them become new recruits to shallow reefs or become targets of recreational, subsistence, or commercial fishermen. To manage these resources effectively and efficiently, much more needs to be learned.

References Cited