usually kills the remaining polyps and the fragment
becomes rubble. In habitats where a fragment is
immobilized by lodgment in a crevice or falling into
a sheltered place, or is inverted only at intervals of weeks
to months at most, the living veneer of polyps survives and
deposits an increasing mass and volume of skeleton. In
shallow flat habitats with reversing currents, a ball-like
corallith with live polyps on all sides may develop
(Glynn, 1974). But in most viable habitats, the fragment
will produce skeletal processes that attach it to adjacent
stable substratum and prevent further rolling. Once
attached, the environmental needs and vulnerabilities are
the same as for other attached corals of comparable size.
Modes of nutrition and coral growth forms
Corals need both large supplies of energy-rich compounds
and small amounts of nutrients (N, P) for their growth (tissue and skeleton) and reproduction. Modes of nutrition of
corals can be arranged along a spectrum of relative reliance on ingested food as a primary source of energy-rich
compounds (Hallock, 2001). In “phototrophic” corals, it
is small (but necessary for intake of N and P), and the
corals primarily use “photosynthate” (the product of photosynthesis by their symbiotic zooxanthellae). In “heterotrophic” corals, at the other end of the spectrum, there is
major reliance on ingested food (zooplankton, organic
detritus, and dissolved organic matter). In the middle are
“mixotrophic” corals, which use both sources more
equally.
Corals build exoskeletons in shapes that facilitate their
particular mode of resource use, i.e., adequate interception
of solar radiation and particulate food and the water-borne
precursors for calcification; use of the moving water to
remove the waste products of metabolism. In calm, clear,
shallow waters, there is so much light that virtually any
shape will be effective in harvesting sufficient PAR.
Indeed, these phototrophic corals have to expend energy
producing compounds to protect themselves against damaging excess irradiation. However, in wave breaking and
surging zones, the range of coral shapes is more limited.
These include encrusting forms and/or small colonies of
species that adopt a top-heavy form in calmer waters and
streamlined and elongated variants of growth forms that
are more radially symmetrical in calm waters: e.g.,
Acropora palmata in the Atlantic and several Acropora
groups in the Indo-Pacific: “robusta,” “humilis,” “nasuta,”
and “palifera.” Only those shapes that can resist dislodgement by normal fair-weather waves can survive
for long, and even they will eventually reach a size where
their dislodgement is likely (Massel and Done, 1993;
Madin and Connelly, 2006; see also Tropical Cyclone/
Hurricane).
Depth and light
In deep or turbid water, phototrophic corals often adopt
horizontal growth forms to maximize the interception of
PAR. However, in places where this exposes them to
a rain of sediments falling out of the water column, corals
survive by adopting shapes that shed sediments passively
or with minimal energy expenditure by the polyps: low
convex profiles from which gentle water motion can easily
entrain sediment particles; dense networks of flattened
branches, perforated with gaps through which sediments
can drop and vases whose interaction with the currents
sets up a vortex that lifts accumulated sediments clear
(Reigl et al., 1996). In some species, notably in the genera
Pocillopora and Acropora, colonies in very sheltered habitats have thinner and more open branches than shallower
colonies (Veron, 1995; Wallace, 1999), thereby maximizing extension without investing unnecessarily in a strong
skeleton.
The PAR dose reaching corals down a reef slope has
been attenuated by absorption and scattering as it passes
down through the water column (Baker and Smith,
1982). Deeper phototrophic corals compensate for
reduced PAR by elevating zooxanthellae densities, chlorophyll content per zooxanthella, or both. These changes
cause them to appear more darkly colored than shallow
water corals. In clear waters, corals as deep as several tens
of meters can remain saturated and function normally
(Chalker, 1983).
In turbid waters, this depth may be only a few meters.
There, the day-to-day and week-to-week variation in irradiation a few meters down the slope can, due to frequent
resuspension of mud from the adjacent shallow sea floor,
be more than an order of magnitude greater than it is at
the surface and at equivalent depths in offshore, clearwater benthic habitats (Anthony, 2000; Anthony et al.,
2004). Corals on slopes of turbid-water reefs may thus
alternate between periods of light deficiency and light
excess. Phototrophic corals are therefore confined to the
relatively well-lit shallower parts of such reefs, whereas
deeper, darker, and muddier parts of these reefs are populated by corals with specific adaptations for those conditions. In very dark (deep or very turbid) waters, only
mixotrophic or heterotrophic corals can survive (Anthony,
2000). These corals largely replace the now minimal
zooxanthellae-derived photosynthate as the primary food
source with dissolved organic matter (saprophagy) or particulate organic matter (detritus and/or zooplankton)
(Anthony et al., 2004). In deep (50–4,000 m), dark coldwater coral reefs (4–12
C), approximately ten species of
scleractinian corals have been discovered, all completely
lacking zooxanthellae (Roberts et al., 2006). They rely
entirely on food transported from surface waters to the
sea floor, frequently in sites with food supply enhanced
by locally accelerated currents.
Ultraviolet radiation (UVR – 390–400 nm) is potentially damaging to corals. However, a number of things
protect corals from UVR damage. First, is depth itself:
shorter wavelengths are scattered by particles and
absorbed by water molecules and dissolved organic compounds more readily than PAR wavelengths (Baker and
Smith, 1982). As a result, UVR attenuates to near zero
within a few meters at most, and because of the extreme
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
289
becomes rubble. In habitats where a fragment is
immobilized by lodgment in a crevice or falling into
a sheltered place, or is inverted only at intervals of weeks
to months at most, the living veneer of polyps survives and
deposits an increasing mass and volume of skeleton. In
shallow flat habitats with reversing currents, a ball-like
corallith with live polyps on all sides may develop
(Glynn, 1974). But in most viable habitats, the fragment
will produce skeletal processes that attach it to adjacent
stable substratum and prevent further rolling. Once
attached, the environmental needs and vulnerabilities are
the same as for other attached corals of comparable size.
Modes of nutrition and coral growth forms
Corals need both large supplies of energy-rich compounds
and small amounts of nutrients (N, P) for their growth (tissue and skeleton) and reproduction. Modes of nutrition of
corals can be arranged along a spectrum of relative reliance on ingested food as a primary source of energy-rich
compounds (Hallock, 2001). In “phototrophic” corals, it
is small (but necessary for intake of N and P), and the
corals primarily use “photosynthate” (the product of photosynthesis by their symbiotic zooxanthellae). In “heterotrophic” corals, at the other end of the spectrum, there is
major reliance on ingested food (zooplankton, organic
detritus, and dissolved organic matter). In the middle are
“mixotrophic” corals, which use both sources more
equally.
Corals build exoskeletons in shapes that facilitate their
particular mode of resource use, i.e., adequate interception
of solar radiation and particulate food and the water-borne
precursors for calcification; use of the moving water to
remove the waste products of metabolism. In calm, clear,
shallow waters, there is so much light that virtually any
shape will be effective in harvesting sufficient PAR.
Indeed, these phototrophic corals have to expend energy
producing compounds to protect themselves against damaging excess irradiation. However, in wave breaking and
surging zones, the range of coral shapes is more limited.
These include encrusting forms and/or small colonies of
species that adopt a top-heavy form in calmer waters and
streamlined and elongated variants of growth forms that
are more radially symmetrical in calm waters: e.g.,
Acropora palmata in the Atlantic and several Acropora
groups in the Indo-Pacific: “robusta,” “humilis,” “nasuta,”
and “palifera.” Only those shapes that can resist dislodgement by normal fair-weather waves can survive
for long, and even they will eventually reach a size where
their dislodgement is likely (Massel and Done, 1993;
Madin and Connelly, 2006; see also Tropical Cyclone/
Hurricane).
Depth and light
In deep or turbid water, phototrophic corals often adopt
horizontal growth forms to maximize the interception of
PAR. However, in places where this exposes them to
a rain of sediments falling out of the water column, corals
survive by adopting shapes that shed sediments passively
or with minimal energy expenditure by the polyps: low
convex profiles from which gentle water motion can easily
entrain sediment particles; dense networks of flattened
branches, perforated with gaps through which sediments
can drop and vases whose interaction with the currents
sets up a vortex that lifts accumulated sediments clear
(Reigl et al., 1996). In some species, notably in the genera
Pocillopora and Acropora, colonies in very sheltered habitats have thinner and more open branches than shallower
colonies (Veron, 1995; Wallace, 1999), thereby maximizing extension without investing unnecessarily in a strong
skeleton.
The PAR dose reaching corals down a reef slope has
been attenuated by absorption and scattering as it passes
down through the water column (Baker and Smith,
1982). Deeper phototrophic corals compensate for
reduced PAR by elevating zooxanthellae densities, chlorophyll content per zooxanthella, or both. These changes
cause them to appear more darkly colored than shallow
water corals. In clear waters, corals as deep as several tens
of meters can remain saturated and function normally
(Chalker, 1983).
In turbid waters, this depth may be only a few meters.
There, the day-to-day and week-to-week variation in irradiation a few meters down the slope can, due to frequent
resuspension of mud from the adjacent shallow sea floor,
be more than an order of magnitude greater than it is at
the surface and at equivalent depths in offshore, clearwater benthic habitats (Anthony, 2000; Anthony et al.,
2004). Corals on slopes of turbid-water reefs may thus
alternate between periods of light deficiency and light
excess. Phototrophic corals are therefore confined to the
relatively well-lit shallower parts of such reefs, whereas
deeper, darker, and muddier parts of these reefs are populated by corals with specific adaptations for those conditions. In very dark (deep or very turbid) waters, only
mixotrophic or heterotrophic corals can survive (Anthony,
2000). These corals largely replace the now minimal
zooxanthellae-derived photosynthate as the primary food
source with dissolved organic matter (saprophagy) or particulate organic matter (detritus and/or zooplankton)
(Anthony et al., 2004). In deep (50–4,000 m), dark coldwater coral reefs (4–12
C), approximately ten species of
scleractinian corals have been discovered, all completely
lacking zooxanthellae (Roberts et al., 2006). They rely
entirely on food transported from surface waters to the
sea floor, frequently in sites with food supply enhanced
by locally accelerated currents.
Ultraviolet radiation (UVR – 390–400 nm) is potentially damaging to corals. However, a number of things
protect corals from UVR damage. First, is depth itself:
shorter wavelengths are scattered by particles and
absorbed by water molecules and dissolved organic compounds more readily than PAR wavelengths (Baker and
Smith, 1982). As a result, UVR attenuates to near zero
within a few meters at most, and because of the extreme
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
289