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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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).
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
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).
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.
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).
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).
5.3.
Recovery source for shallow populations
5.4.
Tourists exploring the mesophotic zone
5.5.
Potential source of novel products