are also less prominent guilds, forming a hierarchy down
to less conspicuous tasks such as parasite control, sediment mixing, and nutrient cycling. The essential point
about guilds is that they are functional units selected for
because they perform functions efficiently. It matters little
what species perform the function, only that they do it with
the necessary speed.
Corals are the most prominent reef-builders today
because, with their zooxanthellae, they can harness the
energy of sunlight to make building blocks sufficiently
quickly to outstrip erosion. This is not to say that all
zooxanthellate corals contribute to reef-building; perhaps
half of all species make no significant contribution to reefs
at all for they do not grow in environments suitable for reef
growth, especially where the water is too turbid or too cold
or where there is limited hard substrate, or because their
skeletons are fragile and are easily removed by wave action.
The reason why corals build reefs has as much to do
with the physical environment as with organisms: it is
a matter of ecology.
Light
Reef-building corals depend on photosynthesis for they
use the unlimited resources of solar energy and air to produce food. However, about half of all Scleractinia (the
azooxanthellate species) do not have symbiotic algae.
Some azooxanthellate corals live on coral reefs, especially
under overhangs or in caves, but with the exception of
a few species that are both symbiotic and nonsymbiotic,
all zooxanthellate corals need light, and it is only these
taxa that build reefs. As a result, reefs are restricted to shallow sunlit waters. Azooxanthellate corals are not limited
by light or by temperature, nor are they confined to shallow sunlit water; they live in the ocean depths where there
is less competition for space. Therefore, they cannot build
reefs and must live without food from photosynthesis:
food can only come from detritus and the chance of capturing passing plankton.
Algal symbiosis
Symbiosis, the interdependence of different organisms for
the benefit of one or both participants, is much more prevalent in the oceans than on land. Within the spectrum of
symbioses, zooxanthellae clearly have a special place.
They are not just found in Scleractinia; they occur in other
cnidarians (soft corals, anemones and their allies) as well
as in an assortment of other animals including singlecelled ciliated protists, sponges, flatworms, and molluscs
(including giant clams). Once thought to be a single species, zooxanthellae have been found to be genetically
diverse (consisting of many genetic types or “clades”)
(Trench, 1979; Rowan and Powers, 1992), even though
under a microscope they all look much the same
(Figure 2). They can all live independently, although not
in such concentrated numbers nor with such long-term
security as they can live in the tissues of hosts. In the case
of corals (but not clams), they live inside the cells of the
host organisms – in the innermost (gastrodermal) layer
of the two cell layer body wall (illustrated above). All zooxanthellae are tiny, around one hundredth of a millimeter
in diameter. Seldom does more than one occur in
a single gastrodermal cell.
Zooxanthellae photosynthesize as do other green
plants, releasing up to 95% of the nutrients they produce
to the host organism (Muscatine, 1990). This is a curious
arrangement because most corals are voracious feeders
on zooplankton and therefore have two very different food
sources. Nevertheless, many if not most corals that are
kept in darkness (so that their zooxanthellae cannot
photosynthesise) will start to die after a few months no
matter how much food they have. Somehow, the zooxanthellae have made themselves indispensable. Just how or
why remains unresolved.
In brief, important points about algal symbiosis are as
follows (Hoegh-Guldberg, 1999 and many subsequent
articles). Corals acquire their zooxanthellae either directly
from the parent colony or through infection of freeswimming or newly settled larvae. Uptake of nonparental
zooxanthellae in early life may be by random chance, giving different advantages to different colonies: some colonies might be infected with temperature-tolerant
symbionts, others with more productive ones (this hypothesis has yet to be confirmed). Zooxanthellae readily
change in abundance depending on conditions such as season, position on the coral, and light level. More than one
genetic type of zooxanthellae can occupy a single colony.
The abundance of genetic types varies geographically on
any scale (Ulstrup and Van Oppen, 2003) and some
genetic types facilitate a faster growth rate than others
(Little et al., 2004).
Water depth, turbidity, and latitude
Any factors which alter light in the marine environment
will have a significant effect on calcification rates and reef
development. Depth is a primary constraint as only a few
zooxanthellate corals live below 100 m, even where the
water is very clear and the substrate does not slope so
steeply that it is shaded. Leptoseris commonly forms
extensive beds to at least 160 m in the Red Sea and
Hawaii, and there are several records of moderately
diverse coral communities at depths of over 100 m elsewhere, including the outermost reef faces of the GBR.
Turbidity has a dominant role to play in controlling
light levels in all except clear-water habitats. Where the
water is not very clear, as is the case with most reefs near
major land masses, coral diversity drops off sharply at
depths below about 50 m. Where the water is particularly
muddy, the depth limit for any coral can be as little as 5 m.
Turbidity, especially that caused by fine clay particles
which are easily resuspended by wave action, has other
effects on corals besides reducing light.
Latitude also has an effect on light availability, much
more in the ocean than on land due to the refraction of
sunlight as it enters water. The higher the latitude the
278
CORALS: BIOLOGY, SKELETAL DEPOSITION, AND REEF-BUILDING
to less conspicuous tasks such as parasite control, sediment mixing, and nutrient cycling. The essential point
about guilds is that they are functional units selected for
because they perform functions efficiently. It matters little
what species perform the function, only that they do it with
the necessary speed.
Corals are the most prominent reef-builders today
because, with their zooxanthellae, they can harness the
energy of sunlight to make building blocks sufficiently
quickly to outstrip erosion. This is not to say that all
zooxanthellate corals contribute to reef-building; perhaps
half of all species make no significant contribution to reefs
at all for they do not grow in environments suitable for reef
growth, especially where the water is too turbid or too cold
or where there is limited hard substrate, or because their
skeletons are fragile and are easily removed by wave action.
The reason why corals build reefs has as much to do
with the physical environment as with organisms: it is
a matter of ecology.
Light
Reef-building corals depend on photosynthesis for they
use the unlimited resources of solar energy and air to produce food. However, about half of all Scleractinia (the
azooxanthellate species) do not have symbiotic algae.
Some azooxanthellate corals live on coral reefs, especially
under overhangs or in caves, but with the exception of
a few species that are both symbiotic and nonsymbiotic,
all zooxanthellate corals need light, and it is only these
taxa that build reefs. As a result, reefs are restricted to shallow sunlit waters. Azooxanthellate corals are not limited
by light or by temperature, nor are they confined to shallow sunlit water; they live in the ocean depths where there
is less competition for space. Therefore, they cannot build
reefs and must live without food from photosynthesis:
food can only come from detritus and the chance of capturing passing plankton.
Algal symbiosis
Symbiosis, the interdependence of different organisms for
the benefit of one or both participants, is much more prevalent in the oceans than on land. Within the spectrum of
symbioses, zooxanthellae clearly have a special place.
They are not just found in Scleractinia; they occur in other
cnidarians (soft corals, anemones and their allies) as well
as in an assortment of other animals including singlecelled ciliated protists, sponges, flatworms, and molluscs
(including giant clams). Once thought to be a single species, zooxanthellae have been found to be genetically
diverse (consisting of many genetic types or “clades”)
(Trench, 1979; Rowan and Powers, 1992), even though
under a microscope they all look much the same
(Figure 2). They can all live independently, although not
in such concentrated numbers nor with such long-term
security as they can live in the tissues of hosts. In the case
of corals (but not clams), they live inside the cells of the
host organisms – in the innermost (gastrodermal) layer
of the two cell layer body wall (illustrated above). All zooxanthellae are tiny, around one hundredth of a millimeter
in diameter. Seldom does more than one occur in
a single gastrodermal cell.
Zooxanthellae photosynthesize as do other green
plants, releasing up to 95% of the nutrients they produce
to the host organism (Muscatine, 1990). This is a curious
arrangement because most corals are voracious feeders
on zooplankton and therefore have two very different food
sources. Nevertheless, many if not most corals that are
kept in darkness (so that their zooxanthellae cannot
photosynthesise) will start to die after a few months no
matter how much food they have. Somehow, the zooxanthellae have made themselves indispensable. Just how or
why remains unresolved.
In brief, important points about algal symbiosis are as
follows (Hoegh-Guldberg, 1999 and many subsequent
articles). Corals acquire their zooxanthellae either directly
from the parent colony or through infection of freeswimming or newly settled larvae. Uptake of nonparental
zooxanthellae in early life may be by random chance, giving different advantages to different colonies: some colonies might be infected with temperature-tolerant
symbionts, others with more productive ones (this hypothesis has yet to be confirmed). Zooxanthellae readily
change in abundance depending on conditions such as season, position on the coral, and light level. More than one
genetic type of zooxanthellae can occupy a single colony.
The abundance of genetic types varies geographically on
any scale (Ulstrup and Van Oppen, 2003) and some
genetic types facilitate a faster growth rate than others
(Little et al., 2004).
Water depth, turbidity, and latitude
Any factors which alter light in the marine environment
will have a significant effect on calcification rates and reef
development. Depth is a primary constraint as only a few
zooxanthellate corals live below 100 m, even where the
water is very clear and the substrate does not slope so
steeply that it is shaded. Leptoseris commonly forms
extensive beds to at least 160 m in the Red Sea and
Hawaii, and there are several records of moderately
diverse coral communities at depths of over 100 m elsewhere, including the outermost reef faces of the GBR.
Turbidity has a dominant role to play in controlling
light levels in all except clear-water habitats. Where the
water is not very clear, as is the case with most reefs near
major land masses, coral diversity drops off sharply at
depths below about 50 m. Where the water is particularly
muddy, the depth limit for any coral can be as little as 5 m.
Turbidity, especially that caused by fine clay particles
which are easily resuspended by wave action, has other
effects on corals besides reducing light.
Latitude also has an effect on light availability, much
more in the ocean than on land due to the refraction of
sunlight as it enters water. The higher the latitude the
278
CORALS: BIOLOGY, SKELETAL DEPOSITION, AND REEF-BUILDING