Mesophotic Coral Ecosystems

5.3. Recovery source for shallow populations

MCEs may be a source of recovery and resilience for degraded or perturbed shallow populations, through the movement of adult individuals and the dispersal of larvae. At present, MCEs are considered to be buffered frommany coastal and global stressors that are associated with shallow coral reef decline, and may in fact represent a refuge or refugia for specific species in specific locations. Indeed, some coral species have been found in higher densities on MCEs than on shallower reefs after periods of long- term decline or sudden local extinction on shallow reefs (Menza et al. 2008, Sinniger et al. 2013). For example, the common coral Seriatopora hystrix was thought to be extinct around Okinawa, Japan, following a severe global bleaching event in 1998, but has since been rediscovered at mesophotic depths (Sinniger et al. 2013). Refuge habitats that are capable of supporting populations of marine organisms throughout periods of stress must be sufficiently removed from stress, persistent in time and be connected through adult or larval migration to other habitats. Many commercially-important fish species have depth- generalist distributions, and are found on both mesophotic and shallow coral reefs. It may be that adults of these species freely move between shallow reefs and MCEs, and could take refuge on mesophotic reefs during times of stress. Some of these mesophotic fish populations may be very important as refugia for coral reef fish under intense fishing pressure, since many fishing techniques (e.g. free diving) are not possible or are rendered more difficult at mesophotic depths (Bejarano et al. 2014, Lindfield et al. 2014).

Many corals are also depth-generalists; however, being sessile, adults cannot move to avoid stressful environmental conditions.There is evidence that the larvae of depth-generalist corals in the Caribbean can migrate into shallow habitat in some locations (Holstein et al. 2015). Evidence of this vertical migration, in the form of genetic population connectivity, has shown that vertical migration is location and species-specific (Bongaerts et al. 2010a, van Oppen et al. 2011, Slattery et al. 2011, Serrano 2013, Serrano et al. 2014) and that speciation may be occurring with depth in some scleractinian coral species (Bongaerts et al. 2013b, 2015a, Prada and Hellberg 2013). It is important to note that mesophotic coral species’ ability to replenish shallow depths is limited to those species that are found in shallow waters (Bongaerts et al. 2010a). Although the depth and isolation of MCEs has led to interest in their potential as refugia, coral communities on MCEs have been shown to be susceptible to disease (Smith et al. 2010), thermal (both warm and cold water) bleaching (Bongaerts et al. 2010a, Smith et al. 2015) sedimentation (Appeldoorn et al. 2015) and coral-algal phase-shifts potentially driven by invasive species (Lesser and Slattery 2011). Although the degree to which MCEs may serve as recovery sources for shallow coral reef communities remains uncertain, there is little doubt that these mesophotic communities are integrated components of larger marine metapopulations, and their conservation corresponds with the goals of coral reef conservation in general.

5.4. Tourists exploring the mesophotic zone

In recent years, the technology that has enabled scientists to explore areas of the ocean below scuba depths has becomemore accessible to the general public. Closed-circuit rebreathers that allow longer and deeper dives are being used by recreational divers. Tourists can now also explore the mesophotic zone in a submersible. Following in the footsteps of DSV Alvin and other research submersibles, companies are running submersible

dive tours (e.g. in Costa Rica, Honduras, Hawai‘i and Curaçao). Tours to mesophotic depths and below are likely to increase, as researchers discover more about the deeper regions of the ocean and the infrastructure to visit these areas expands. Deep- sea tour operators are also helping to build our knowledge of the mesophotic zone, as they document their dives and analyse video footage for new species (Breedy and Guzman 2013).

5.5. Potential source of novel products

The collection of mesophotic reef organisms has been ongoing since the 1970s, resulting inmany discoveries (including the drug discodermolide, a potent anti-tumour compound; Gunasekera et al. 1990), as well as other promising natural products (reviewed byNewman andCragg 2007). Investigations in thewaters around Palau have, for example, resulted in the publication of over 100 papers on marine natural products since 2004 (Faulkner et al. 2004). A number of the promising samples collected come from mesophotic depths (Qureshi et al. 2000, Sandler et al. 2006).

It is estimated that almost half of all drugs currently in use are sourced from natural products (Newman and Cragg 2012). Most of these have been derived from terrestrial plants, animals and microorganisms, but marine biotechnology is expanding, and will increasingly move out of the shallows and into deeper water. It is difficult to estimate the value of this industry at present, but it is probably safe to assume that potential future economic benefits could be very high (Newman and Cragg 2007).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 66

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