scattering, they irradiate the corals only weakly. Second,
shallow corals manufacture sunscreens [mycosporine-like
amino acids (MAAs) – Dunlap and Shick, 1998] that
shield their tissues from any ambient UV that does reach
them. These compounds are especially important for those
corals that are periodically left emerged at extreme low
tides (Figure 3) and thereby exposed to direct sunlight.
Some of the MAAs produced by the corals occurs in
mucus that is secreted by emergent corals and protects
them from dehydration.
Environmental controls on coral’s presettlement
stage
The environmental milieu of the coral’s presettlement
stage is quite different to that of adult corals. Corals
develop by two main methods (Harrison and Wallace,
1994; see also Corals: Biology, Skeletal Deposition, and
Reef-Building): asexually (from the fragments of an
existing colony that become separated by budding or
breakage from a parent colony – see Section Coral
growth) and sexually (from fertilized eggs that grow into
pelagic larvae that swim or are carried by currents to their
place of settlement). Pelagic coral larvae are released into
the water by sexually mature corals (usually >3–5 years
old) that produce sperm and eggs. Depending on species,
the production of the larvae follows one of the two pathways (1) broadcast spawning or (2) brooding. In broadcast
spawning, gravid polyps expel eggs and sperm into the
water column during a mass spawning event whose timing
is synchronized to lunar cycles. Fertilization takes place in
the water column, and the drifting fertilized egg transforms itself into a planula larva over a period of several
days (Hirose et al., 2008). During its days in the currents,
the metamorphosis from egg to larva is fueled by lipids
originating from the egg. Once it has developed its simple
gut ($4 days), it can feed on phytoplankton and detritus
and presumably does so if it is nowhere near a suitable
place to settle (Richmond, 1987). However, at this stage
it is competent to settle (Hirose et al., 2008), and it tends
to do so quickly when deprived of food in the laboratory
(Hirose et al., 2008). When food is supplied, individuals
of some species can settle within 2–3 days (Miller and
Mundy, 2003). For those that do not settle so quickly, there
can be major mortality within the first few days, but survival of others in the water column upwards of 100 days
(Graham et al., 2008). During this time they are potentially
exposed to environmental hazards such as hyposaline
areas, excessively muddy areas, or very clear areas where
they may become exposed to damaging doses of UVB
radiation (Gleason and Wellington, 1995). In open waters
between reefs, they will likely be aggregated into fronts
generated by the current’s interaction with headlands,
islands, and reefs; this aggregation on one hand attract
predatory fishes or shrimps, but on the other, take cohorts
of competent larvae to potential settlement sites (Wolanski
and Hamner, 1988). The broadcast-spawning strategy
described above favors colonization of widely dispersed
reef substrata and hence the maintenance and/or
restoration of those area’s reef-building potential (see
Coral Reef, Definition).
The brooding strategy, by contrast, leads to localized
settlement that favors persistence of the local population
and local reef-building. In brooding coral species, fertilization takes place within the polyps of the parent colony
and so does metamorphosis into the planula larva
(Harrison and Wallace, 1994). When expelled several
days after fertilization, they are strong swimmers, zooxanthellae bearing, and competent to attach to the reef within
minutes to hours of release: they avoid the hazards of days
to weeks in the water column that are faced by broadcast
pelagic larvae. Brooded larvae have much higher likelihood of finding a suitable settlement place, in this case,
close to one that has been tried and tested by the parent
colony.
When broadcast larvae are swept over a reef from the
sea on a flooding tide, they face the environmental hazard
of “the wall of mouths” (Hamner et al., 1988) belonging to
coral polyps (Fabricius and Metzner, 2004) and schools of
planktivorous fishes that ride the surge along reef edges
(Hamner et al., 2007). Those that survive will then receive
environmental cues to a potential place for settlement.
A hydrodynamic cue alone (Abelson and Denny, 1997)
may attract them to within centimeters of solid substrata.
Their choice of specific microcrevices may then be guided
by chemical cues emitted by coralline algae and microbial
films coating the substratum (Harrington et al., 2004).
Abelson and Denny (1997) suggest that when hydrodynamic forces are higher than the larva’s swimming capacity (as may often be the case in wave-swept reef
environments), final site selection may be due to desertion
of unfavorable sites rather than exploration and active
selection of an appropriate site. For many species it is only
after the coral larvae has settled and transformed itself into
a rudimentary polyp that it acquires zooxanthellae that
multiply within its tissues (Hirose et al., 2008) and contribute to the nutrition of their host.
Reef environments present a wide range of colonizable
substrata for the establishment of corals that could occupy
that place for decades or even centuries. In wave-swept
parts of the reef, the substratum to which a coral attaches
is usually big and heavy enough to stay in place against
normal hydraulic forces generated by waves (e.g., solid
framework and heavy rubble). In sheltered parts of the
reef, smaller rubble sizes (coral shingle and mollusc
shells) can be colonized by coral larvae, and even sand
and mud can be colonized by coral fragments of some
species.
Global climate change: implications for coral
growth
The net effect of global warming and increases in atmospheric CO 2 per se are detrimental to the biological calcification process (Feely et al., 2004; Hoegh-Guldberg et al.,
2007; Guinotte and Fabry, 2008). There is so much more
290
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
shallow corals manufacture sunscreens [mycosporine-like
amino acids (MAAs) – Dunlap and Shick, 1998] that
shield their tissues from any ambient UV that does reach
them. These compounds are especially important for those
corals that are periodically left emerged at extreme low
tides (Figure 3) and thereby exposed to direct sunlight.
Some of the MAAs produced by the corals occurs in
mucus that is secreted by emergent corals and protects
them from dehydration.
Environmental controls on coral’s presettlement
stage
The environmental milieu of the coral’s presettlement
stage is quite different to that of adult corals. Corals
develop by two main methods (Harrison and Wallace,
1994; see also Corals: Biology, Skeletal Deposition, and
Reef-Building): asexually (from the fragments of an
existing colony that become separated by budding or
breakage from a parent colony – see Section Coral
growth) and sexually (from fertilized eggs that grow into
pelagic larvae that swim or are carried by currents to their
place of settlement). Pelagic coral larvae are released into
the water by sexually mature corals (usually >3–5 years
old) that produce sperm and eggs. Depending on species,
the production of the larvae follows one of the two pathways (1) broadcast spawning or (2) brooding. In broadcast
spawning, gravid polyps expel eggs and sperm into the
water column during a mass spawning event whose timing
is synchronized to lunar cycles. Fertilization takes place in
the water column, and the drifting fertilized egg transforms itself into a planula larva over a period of several
days (Hirose et al., 2008). During its days in the currents,
the metamorphosis from egg to larva is fueled by lipids
originating from the egg. Once it has developed its simple
gut ($4 days), it can feed on phytoplankton and detritus
and presumably does so if it is nowhere near a suitable
place to settle (Richmond, 1987). However, at this stage
it is competent to settle (Hirose et al., 2008), and it tends
to do so quickly when deprived of food in the laboratory
(Hirose et al., 2008). When food is supplied, individuals
of some species can settle within 2–3 days (Miller and
Mundy, 2003). For those that do not settle so quickly, there
can be major mortality within the first few days, but survival of others in the water column upwards of 100 days
(Graham et al., 2008). During this time they are potentially
exposed to environmental hazards such as hyposaline
areas, excessively muddy areas, or very clear areas where
they may become exposed to damaging doses of UVB
radiation (Gleason and Wellington, 1995). In open waters
between reefs, they will likely be aggregated into fronts
generated by the current’s interaction with headlands,
islands, and reefs; this aggregation on one hand attract
predatory fishes or shrimps, but on the other, take cohorts
of competent larvae to potential settlement sites (Wolanski
and Hamner, 1988). The broadcast-spawning strategy
described above favors colonization of widely dispersed
reef substrata and hence the maintenance and/or
restoration of those area’s reef-building potential (see
Coral Reef, Definition).
The brooding strategy, by contrast, leads to localized
settlement that favors persistence of the local population
and local reef-building. In brooding coral species, fertilization takes place within the polyps of the parent colony
and so does metamorphosis into the planula larva
(Harrison and Wallace, 1994). When expelled several
days after fertilization, they are strong swimmers, zooxanthellae bearing, and competent to attach to the reef within
minutes to hours of release: they avoid the hazards of days
to weeks in the water column that are faced by broadcast
pelagic larvae. Brooded larvae have much higher likelihood of finding a suitable settlement place, in this case,
close to one that has been tried and tested by the parent
colony.
When broadcast larvae are swept over a reef from the
sea on a flooding tide, they face the environmental hazard
of “the wall of mouths” (Hamner et al., 1988) belonging to
coral polyps (Fabricius and Metzner, 2004) and schools of
planktivorous fishes that ride the surge along reef edges
(Hamner et al., 2007). Those that survive will then receive
environmental cues to a potential place for settlement.
A hydrodynamic cue alone (Abelson and Denny, 1997)
may attract them to within centimeters of solid substrata.
Their choice of specific microcrevices may then be guided
by chemical cues emitted by coralline algae and microbial
films coating the substratum (Harrington et al., 2004).
Abelson and Denny (1997) suggest that when hydrodynamic forces are higher than the larva’s swimming capacity (as may often be the case in wave-swept reef
environments), final site selection may be due to desertion
of unfavorable sites rather than exploration and active
selection of an appropriate site. For many species it is only
after the coral larvae has settled and transformed itself into
a rudimentary polyp that it acquires zooxanthellae that
multiply within its tissues (Hirose et al., 2008) and contribute to the nutrition of their host.
Reef environments present a wide range of colonizable
substrata for the establishment of corals that could occupy
that place for decades or even centuries. In wave-swept
parts of the reef, the substratum to which a coral attaches
is usually big and heavy enough to stay in place against
normal hydraulic forces generated by waves (e.g., solid
framework and heavy rubble). In sheltered parts of the
reef, smaller rubble sizes (coral shingle and mollusc
shells) can be colonized by coral larvae, and even sand
and mud can be colonized by coral fragments of some
species.
Global climate change: implications for coral
growth
The net effect of global warming and increases in atmospheric CO 2 per se are detrimental to the biological calcification process (Feely et al., 2004; Hoegh-Guldberg et al.,
2007; Guinotte and Fabry, 2008). There is so much more
290
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH