54
2. ENVIRONMENTALLY DRIVEN PLASTICITY
ogy and reproduction in addition to the traditional taxonomic studies. This
led to the finding that morphological species boundaries did not necessarily correspond to the ecological or behavioral differences found in the field.
In several Indo-Pacific coral species reproductive boundaries did not correspond to morphological classifications. Furthermore hybridization between
presumed morphological species is found frequently (Veron 1995) which indicates a need to allow for much greater skeletal plasticity than is permitted
by classical coral taxonomy within presumed species.
Corals are not restricted to shallow, well-lit tropical seas. They survive
and grow without light. The branching coral Lophelia pertusa grows at depths
around 250 m in the North Sea, most commonly in Norwegian waters (70 ° N).
It forms frameworks up to 10 m high. Some species grow in abyssal Antarctic
waters and have to extend their tissues around their entire skeleton to prevent
skeletal dissolution at high pressure. Although these examples illustrate their
adaptive capabilities, it is in warm, mostly tropical seas (> 18 "C) that corals
are most diverse and common. The major calcifying organisms on coral reefs,
such as corals, have developed an endosymbiotic relationship with unicellular plants. The most widely distributed endosymbiotic alga is the unicellular
dinoflagellate, Symbiodinium microadriaticum, which in its various species
resides in a large range of reef invertebrates including hydrozoan corals, scleractinian corals, and the various species of giant clam in the Pacific. The
symbiotic algae, or zooxanthellae, within the tissues of most tropical corals
provide their hosts with carbon products of photosynthesis and considerably
enhance their rate of skeletal growth (calcification). Photosynthesis allows
reef organisms to precipitate calcium carbonate faster than physical, chemi -
cal, and biological agencies can disperse it. The success of this symbiosis has
allowed corals to form reefs, which are major geological structures on the
earth's surface. Living reefs cover about 15% of the seabed in the 0-30 m depth
range and they form about 0.2% of the world's ocean area. Geologists originally used the term "hermatypic" for corals that form reefs. Biologists have
used this term to describe corals with zooxanthellae, which again emphasizes
the importance of zooxanthellae to reef formation and maintenance. Light
is considered to be the single most important environmental factor affecting coral growth. Light levels change most profoundly over the first 10-15 m
of the water column. Over these depths, 60-75% of the surface light is being
absorbed or scattered. The decrease in light intensity becomes more gradual
deeper in the water column. The presence of zooxanthellate corals at 100 m
depth indicates the ability of corals to live under very low light levels. Reduced growth rates with depth have been found for many corals. Increasing
depth results in changes in growth form (mostly from rounded to flattened
morphologies), changes in polyp and zooxanthellar densities, and changes
in the types and concentrations of pigments associated with photosynthesis
(Falkowski and Dubinsky 1981).
Corals acquire essent ial nutrients other than organic carbon by capturing zooplankton from the water column. The relative contribution of
autotrophy (photosynthesis) and heterotrophy (particle feeding) may depend upon local availability from each source. Porter (1976) suggested that
morphological variation might be related to this. In his model, corals with
larger polyps are better able to feed heterotrophically while corals with high
surface-to-volume ratios (e.g. branching species) depend more on autotrophy
because of better light capturing capabilities.
2. ENVIRONMENTALLY DRIVEN PLASTICITY
ogy and reproduction in addition to the traditional taxonomic studies. This
led to the finding that morphological species boundaries did not necessarily correspond to the ecological or behavioral differences found in the field.
In several Indo-Pacific coral species reproductive boundaries did not correspond to morphological classifications. Furthermore hybridization between
presumed morphological species is found frequently (Veron 1995) which indicates a need to allow for much greater skeletal plasticity than is permitted
by classical coral taxonomy within presumed species.
Corals are not restricted to shallow, well-lit tropical seas. They survive
and grow without light. The branching coral Lophelia pertusa grows at depths
around 250 m in the North Sea, most commonly in Norwegian waters (70 ° N).
It forms frameworks up to 10 m high. Some species grow in abyssal Antarctic
waters and have to extend their tissues around their entire skeleton to prevent
skeletal dissolution at high pressure. Although these examples illustrate their
adaptive capabilities, it is in warm, mostly tropical seas (> 18 "C) that corals
are most diverse and common. The major calcifying organisms on coral reefs,
such as corals, have developed an endosymbiotic relationship with unicellular plants. The most widely distributed endosymbiotic alga is the unicellular
dinoflagellate, Symbiodinium microadriaticum, which in its various species
resides in a large range of reef invertebrates including hydrozoan corals, scleractinian corals, and the various species of giant clam in the Pacific. The
symbiotic algae, or zooxanthellae, within the tissues of most tropical corals
provide their hosts with carbon products of photosynthesis and considerably
enhance their rate of skeletal growth (calcification). Photosynthesis allows
reef organisms to precipitate calcium carbonate faster than physical, chemi -
cal, and biological agencies can disperse it. The success of this symbiosis has
allowed corals to form reefs, which are major geological structures on the
earth's surface. Living reefs cover about 15% of the seabed in the 0-30 m depth
range and they form about 0.2% of the world's ocean area. Geologists originally used the term "hermatypic" for corals that form reefs. Biologists have
used this term to describe corals with zooxanthellae, which again emphasizes
the importance of zooxanthellae to reef formation and maintenance. Light
is considered to be the single most important environmental factor affecting coral growth. Light levels change most profoundly over the first 10-15 m
of the water column. Over these depths, 60-75% of the surface light is being
absorbed or scattered. The decrease in light intensity becomes more gradual
deeper in the water column. The presence of zooxanthellate corals at 100 m
depth indicates the ability of corals to live under very low light levels. Reduced growth rates with depth have been found for many corals. Increasing
depth results in changes in growth form (mostly from rounded to flattened
morphologies), changes in polyp and zooxanthellar densities, and changes
in the types and concentrations of pigments associated with photosynthesis
(Falkowski and Dubinsky 1981).
Corals acquire essent ial nutrients other than organic carbon by capturing zooplankton from the water column. The relative contribution of
autotrophy (photosynthesis) and heterotrophy (particle feeding) may depend upon local availability from each source. Porter (1976) suggested that
morphological variation might be related to this. In his model, corals with
larger polyps are better able to feed heterotrophically while corals with high
surface-to-volume ratios (e.g. branching species) depend more on autotrophy
because of better light capturing capabilities.