particulate matter on which corals feed (especially zooplankton); and temperature is the same as the adjacent
sea (Figure 2c).
As water crosses the shallow reef, key properties are
changed by its interaction with the shallow reef and its
biota (see Hydrodynamics in Coral Reef Systems). For
example, the margin-inhabiting corals (and other calcifying organisms) extract calcium carbonate precursors and
nutrients from the water, reducing the levels available to
corals “down-stream” (i.e., toward the middle of the reef
flat). Calcification by a community reduces the water column’s total carbon and alkalinity, resulting in a decrease
of CO 3
2À (reviewed in Nakamura and Nakamori, 2007).
When different reef communities are aligned across
a reef from their wave-exposed to their sheltered side, their
exchanges with the passing water set up seaward to leeward gradients in pH, total carbon and total alkalinity. In
effect, the upstream communities strip CO 3
2À from the
water, limiting its supply to sheltered backwaters, making
calcification there impossible. Where corals do grow in
such back reef areas, it indicates that seawater with the correct pH, total carbon, total alkalinity, and composition of
carbonate species is reaching that place via routes other
than a transit across a productive reef flat (e.g., refraction
around reef flanks or through broad channels).
Shallow water on reef flats can be easily heated beyond
the range preferred or tolerated by corals. This greater
temperature volatility is amplified by low-tide ponding
and other restrictions on its rate of flushing caused by prior
growth of the reef itself (Macintyre, 2007). Where backreef waters are highly impounded, they can become sinks
for rainfall and land-runoff, and for inorganic and organic
detritus from the reef upstream, with regimes of temperature, oxygenation, salinity, and turbidity that are inimical
for coral growth. Where, by contrast, back-reef habitats
are well flushed with ocean waters, corals can grow profusely on sandy lagoon floors and on the margins of
back-reef pinnacles and walls.
Solar radiation reaching a coral varies in both space and
time, and the key drivers of variability have been reviewed
by Anthony et al. (2004): (1) seasonal pattern of daily surface irradiance; (2) variations in clouds; (3) transmittance
through the water column, which depends on the optical
properties of the water (Baker and Smith, 1982), most
notably turbidity; and (4) tides, whose daily and seasonal
cycles always affect the depth of the water column and
in some circumstances influence turbidity. In a clear oceanic setting, a gradual depth-attenuation of PAR allows
corals that are primarily phototrophic to grow to depths
as much as 100 m. By contrast, in turbid coastal waters,
dissolved and suspended matter attenuates PAR to below
useful levels at <20 m. However, corals that gain most
of their food by feeding (i.e., heterotrophic corals) can
thrive in low light settings, be it deep on a clear water reef
or much shallower on turbid water reefs. In both cases,
something else may set the lower depth limit to coral distribution; a steep unstable talus in the oceanic reef; a flat
muddy sea floor in the coastal reef.
Coral growth
From primary polyp to coral colony
There are approximately 700 species of Scleractinian
corals globally (Veron, 2000). Based on their adult shapes,
they can be divided into six broad “growth-form” categories (Veron, 1996): massive (similar in all dimensions);
columnar (forming columns); encrusting (adhering to
the substrate); branching (arborescent or tree-like to digitate or finger-like); foliaceous (leaf-like); and laminar
(plate-like). There is considerable variability in the
detailed morphology among and within species: for example, massive corals may be taller or squatter; columns may
be thick or thin, long or club-like; encrustations may be
thick or thin; and branches may have different shapes,
thicknesses, densities, taper and departure angles of secondary and tertiary branches, which may or may not fuse.
A major contributor to this growth form variability is the
local environmental setting, which has a comparable
effect to that of the coral’s genetic makeup (Veron,
1996) in transformation of the microscopic primary polyp
into a flat, fingernail-sized spat, and thence up and out into
a particular locally adapted variant of the basic growth
form.
The primary polyp develops from a soft-bodied planula
larva (see below) that transforms itself into a single softbodied polyp within hours of settlement (Hirose et al.,
2008), when it glues itself onto something solid (e.g., mollusc shell, dead coral in situ, rubble, rock, or reef framework). The “glue” becomes a perforated shallow calcium
carbonate saucer with radial costae (spiky ridges) on top
that support the polyp’s tissues. This single act of transformation from soft-bodied larva to skeleton-secreting
“zooid” (polyp + skeleton) marks the commencement of
a period occupancy that may continue for centuries and
produce a colony meters high and meters across.
The adult coral colony that eventually develops has one
of the basic growth forms listed above. All except “solitary” corals (in which the original zooid just continues to
grow, reaching the size of a desert plate or hefty fruit bowl
in a matter of a few years) undergo a process of modular
growth. Modular growth occurs through vegetative replication (e.g., budding or expansion, and subdivision) of
the original zooid and all of its descendants – Rosen,
1986; Kim and Lasker, 1998). While the basic growth
forms are constrained genetically, actual colony morphologies adopted by a particular colony of a particular species
in a particular place will in large measure be a phenotypic
response to its microenvironment: viz, the regimes of
chemistry, nutrition, temperature, and light and wave
energy that are incident upon it.
Some large coral colonies originate from an unattached
fragment, essentially a small version of the adult colonies,
for which the environmental milieu is qualitatively similar
to that of the adult. However, being unattached means it
can be dislodged, projected, or rolled by strong currents
(Fox et al., 2003). In wave-swept reef habitats, where
oscillating flows make such motion a daily occurrence, it
288
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH
sea (Figure 2c).
As water crosses the shallow reef, key properties are
changed by its interaction with the shallow reef and its
biota (see Hydrodynamics in Coral Reef Systems). For
example, the margin-inhabiting corals (and other calcifying organisms) extract calcium carbonate precursors and
nutrients from the water, reducing the levels available to
corals “down-stream” (i.e., toward the middle of the reef
flat). Calcification by a community reduces the water column’s total carbon and alkalinity, resulting in a decrease
of CO 3
2À (reviewed in Nakamura and Nakamori, 2007).
When different reef communities are aligned across
a reef from their wave-exposed to their sheltered side, their
exchanges with the passing water set up seaward to leeward gradients in pH, total carbon and total alkalinity. In
effect, the upstream communities strip CO 3
2À from the
water, limiting its supply to sheltered backwaters, making
calcification there impossible. Where corals do grow in
such back reef areas, it indicates that seawater with the correct pH, total carbon, total alkalinity, and composition of
carbonate species is reaching that place via routes other
than a transit across a productive reef flat (e.g., refraction
around reef flanks or through broad channels).
Shallow water on reef flats can be easily heated beyond
the range preferred or tolerated by corals. This greater
temperature volatility is amplified by low-tide ponding
and other restrictions on its rate of flushing caused by prior
growth of the reef itself (Macintyre, 2007). Where backreef waters are highly impounded, they can become sinks
for rainfall and land-runoff, and for inorganic and organic
detritus from the reef upstream, with regimes of temperature, oxygenation, salinity, and turbidity that are inimical
for coral growth. Where, by contrast, back-reef habitats
are well flushed with ocean waters, corals can grow profusely on sandy lagoon floors and on the margins of
back-reef pinnacles and walls.
Solar radiation reaching a coral varies in both space and
time, and the key drivers of variability have been reviewed
by Anthony et al. (2004): (1) seasonal pattern of daily surface irradiance; (2) variations in clouds; (3) transmittance
through the water column, which depends on the optical
properties of the water (Baker and Smith, 1982), most
notably turbidity; and (4) tides, whose daily and seasonal
cycles always affect the depth of the water column and
in some circumstances influence turbidity. In a clear oceanic setting, a gradual depth-attenuation of PAR allows
corals that are primarily phototrophic to grow to depths
as much as 100 m. By contrast, in turbid coastal waters,
dissolved and suspended matter attenuates PAR to below
useful levels at <20 m. However, corals that gain most
of their food by feeding (i.e., heterotrophic corals) can
thrive in low light settings, be it deep on a clear water reef
or much shallower on turbid water reefs. In both cases,
something else may set the lower depth limit to coral distribution; a steep unstable talus in the oceanic reef; a flat
muddy sea floor in the coastal reef.
Coral growth
From primary polyp to coral colony
There are approximately 700 species of Scleractinian
corals globally (Veron, 2000). Based on their adult shapes,
they can be divided into six broad “growth-form” categories (Veron, 1996): massive (similar in all dimensions);
columnar (forming columns); encrusting (adhering to
the substrate); branching (arborescent or tree-like to digitate or finger-like); foliaceous (leaf-like); and laminar
(plate-like). There is considerable variability in the
detailed morphology among and within species: for example, massive corals may be taller or squatter; columns may
be thick or thin, long or club-like; encrustations may be
thick or thin; and branches may have different shapes,
thicknesses, densities, taper and departure angles of secondary and tertiary branches, which may or may not fuse.
A major contributor to this growth form variability is the
local environmental setting, which has a comparable
effect to that of the coral’s genetic makeup (Veron,
1996) in transformation of the microscopic primary polyp
into a flat, fingernail-sized spat, and thence up and out into
a particular locally adapted variant of the basic growth
form.
The primary polyp develops from a soft-bodied planula
larva (see below) that transforms itself into a single softbodied polyp within hours of settlement (Hirose et al.,
2008), when it glues itself onto something solid (e.g., mollusc shell, dead coral in situ, rubble, rock, or reef framework). The “glue” becomes a perforated shallow calcium
carbonate saucer with radial costae (spiky ridges) on top
that support the polyp’s tissues. This single act of transformation from soft-bodied larva to skeleton-secreting
“zooid” (polyp + skeleton) marks the commencement of
a period occupancy that may continue for centuries and
produce a colony meters high and meters across.
The adult coral colony that eventually develops has one
of the basic growth forms listed above. All except “solitary” corals (in which the original zooid just continues to
grow, reaching the size of a desert plate or hefty fruit bowl
in a matter of a few years) undergo a process of modular
growth. Modular growth occurs through vegetative replication (e.g., budding or expansion, and subdivision) of
the original zooid and all of its descendants – Rosen,
1986; Kim and Lasker, 1998). While the basic growth
forms are constrained genetically, actual colony morphologies adopted by a particular colony of a particular species
in a particular place will in large measure be a phenotypic
response to its microenvironment: viz, the regimes of
chemistry, nutrition, temperature, and light and wave
energy that are incident upon it.
Some large coral colonies originate from an unattached
fragment, essentially a small version of the adult colonies,
for which the environmental milieu is qualitatively similar
to that of the adult. However, being unattached means it
can be dislodged, projected, or rolled by strong currents
(Fox et al., 2003). In wave-swept reef habitats, where
oscillating flows make such motion a daily occurrence, it
288
CORALS: ENVIRONMENTAL CONTROLS ON GROWTH