Mesophotic Coral Ecosystems are coral-dominated communities that occur in the deepest half of the photic zone. These deep reefs are currently understudied due to their secluded existence. Understanding these ecosystems, and specifically their supportive role in the maintenance of shallow water reefs, is vital to establishing future reef management strategies.

Mesophotic Coral Ecosystems (MCEs) are coral-dominated communities that occur in the deepest half of the photic zone (30 m to >150 m). Despite the extreme low-light conditions, mesophotic communities harbor corals that live in symbiosis with zooxanthellae (genus Symbiodinium) and are therefore still dependent on light for their energy requirements. As the red, orange, yellow and green wavelengths of the sun rapidly attenuate with depth, the light available to mesophotic corals is not only low, but also limited to the blue spectrum (450-500nm). Therefore, even though mesophotic coral communities are usually characterized by deep autumn (brown-red) colors, they appear completely blue when observed underwater (see photographs). Strategies of corals to cope with the paucity of light include specialized (such as plate- and fan-shaped) morphologies that optimize light capture and association with particular (low-light adapted) genetic strains of symbiotic algae. Additionally, these deep reef corals might benefit from nutrient–rich oceanic water that in some areas is pushed onto the shelf due to tidal forces.

Figure 1: The plating, zooxanthellate coral Agaricia grahamae on a Caribbean reef (~60 m / 200 ft. depth) under ambient light (left) and visualized using a strobe light (right). Photographs: Pim Bongaerts.

"With this research I hope to address the importance of these interesting, yet understudied ecosystems as coral reefs enter a century of unprecedented human disturbance."

Mesophotic reefs harbor a variety of coral species, some of which can also be found in shallower waters (i.e. depth-generalist species) and others unique to these deep reef habitats (i.e. deep-specialist species). Most coral species in mesophotic habitats are plating and/or encrusting species, belonging to genera such as Agaricia and Montastraea in the Caribbean, and Leptoseris and Montipora in the Indo-Pacific. Nonetheless, branching species are found as well, such as Madracis formosa in the Caribbean and several species of Acropora in the Indo-Pacific. The strong competition for space observed on shallow water reefs is often lacking on mesophotic reefs, where usually plenty of bare substrate is available. This seems to be linked to the slow growth rates of mesophotic corals and the reduced abundance of macro algae such as Halimeda, Lobophora and Dictyota spp. Besides corals, other taxa that are less or independent on light can be abundant in these habitats as well, such as gorgonians, crinoids, hydrocorals and sponges.

Figure 2: Diver collecting zooxanthellate coral specimens (Agaricia spp.) from a mesophotic reef for genetic research (~60 m depth). Photographs: Pim Bongaerts.

As with the coral community, the mesophotic fish community comprises a combination of depth-generalist and deep-specialist species. The diversity of the fish community associated with mesophotic habitats is strongly correlated with live coral cover, and more importantly the amount of relief provided by the substrate. Mesophotic reefs with great relief and many crevices usually support a relatively higher fish diversity consisting of both smaller and larger fish species, while flatter areas dominated by plating corals usually only harbor smaller fish species. Due to the reduced abundance of macro algae, there are generally few herbivorous fish species, and most species seem to be (zoo)planktivores. In the past decade, a large number of deep-specialist fish species, only occurring in the mesophotic zone and beyond, have been discovered (e.g. Chromos abyssus) by ichthyologist Dr. Richard Pyle.

Compared to shallow water reefs, extremely little is known about mesophotic coral ecosystems. This is largely due to the logistical complexity of studying these deep reef ecosystems, as they lie beyond traditional SCUBA diving limits.  However, this relative inaccessibility is now being overcome through advances in diving technology and underwater robotics. For example, the narcotic effect of nitrogen, experienced when breathing compressed air at depths in excess of 30 m, can now be eliminated through the use of helium-based diving gases. In addition, longer dive times and more effective decompression are possible through the use of closed-circuit rebreathers, which recycle (by removing CO₂ and adding O₂) exhaled breathing gas. Developments in underwater robotics have led to smaller-sized and more economic survey vehicles, which make Remotely Operated Vehicles (ROVs – tethered) and Autonomous Underwater Vehicles (AUVs – untethered) more readily available to scientists.

With these exciting advances it is expected that over the coming decade we will achieve a better understanding of the community structure and ecology of these vastly unexplored coral habitats. These insights may be crucial, as mesophotic reefs could function as important refugia during disturbances in shallow reef areas. In particular, species that occur at both shallow and mesophotic depths (i.e. depth-generalist species) may be able to seek refuge in mesophotic habitats and contribute to recovery through recruitment on the shallow reef after disturbance. In my research, I attempt to address this refugium potential of deep reefs, by studying the genetics of scleractinian corals and the associated zooxanthellae in the mesophotic zone. More specifically, I am interested in the genetic similarity between shallow and deep coral (and zooxanthellae) populations and the question whether shallow and deep coral communities are connected through the exchange of larvae. With this research, and that of many others, I hope to address the importance of these interesting, yet understudied ecosystems as coral reefs enter a century of unprecedented human disturbance.

Figure 3: A yellow tube sponge (Aplysina fistularia) with small coral colonies at the base (Madracis formosa and Agaricia grahamae) (left) and a pair of French angelfish (Pomacanthus paru) on a mesophotic reef in the Caribbean at a depth of approximately 60 m. Photographs: Pim Bongaerts.

Visit www.mesophotic.org to find out more about mesophotic coral ecosystems

References

Bongaerts P, Ridgway T, Sampayo EM, Hoegh-Guldberg O (2010) Assessing the ‘deep reef refugia’ hypothesis: focus on Caribbean reefs. Coral Reefs: on-line first

Kahng SE, Garcia-Sais JR, Spalding HL, Brokovich E, Wagner D, Weil E, Hinderstein L, Toonen RJ (2010) Community ecology of mesophotic coral reef ecosystems. Coral Reefs: on-line first

Pyle RL, Earle JL, Greene BD  (2008) Five new species of the damselfish genus Chromis (Perciformes : Labroidei : Pomacentridae) from deep coral reefs in the tropical western Pacific. Zootaxa 1671: 3–31

Mesophotic Coral Ecosystems: a resource database for science in the mesophotic realm. http://www.mesophotic.org