Scleractinian corals

Scleractinian corals evolved from soft-bodied ancestors in the Middle Triassic period. By late Triassic times they had begun to form small patch reefs, and their importance as reef builders has been continuous since then. They are facilitated in this role by the following morphological adaptations: they have a basal plate which acts as a holdfast, they build porous skeletons of aragonite (which are more readily secreted than the massive calcite skeletons of more primitive corals), they are able to add material to the outside of the corallite to cement it to a firm substrate or adjacent colony, and they can live in a symbiotic association with photosynthetic zooxanthellae.

Solitary scleractinian corals evolved in the early Jurassic, and inhabited deeper water. They became diverse and important late in the Cretaceous. Many genera of both types of coral disappeared as a result of the end-Cretaceous extinction event. Generalists, with a wide ecological range, appear to have had a better chance of survival than specialist forms, though there is no apparent difference in survival rates between those with zooxanthellae and those without.

Internally, the corallite of a scleractinian coral tends to be dominated by septa (Fig. 4.7). Dissepiments and central columnar structures may also be developed. Septa are added in cycles of 6, 12, or 24, each with a regular spacing. The walls of the polyp hang over the edge of the calice, explaining how aragonite can be secreted on the outer surface of the corallite. Colonial scleractinian corals have well-integrated soft tissues and often lack corallite walls. These are replaced by a shared zone of perforated aragonite, similar to the coenenchyme of rugose and tabulate corals, but known as the coenosteum.

Scleractinian corals are amongst the most important reef builders of the Mesozoic and Cenozoic. Reefs commonly develop as fringes around small islands. If sea levels rise, corals can often keep pace with the rate of change, building upwards and outwards towards the high energy zone of wave action. In

Central structure: frequently developed from a range of other

Colonial Scleractinian Coral Fossil

in most scleractinian on environmental factors.

corals. Inserted regularly Lightly constructed from around the corallite porous aragonite in most scleractinian on environmental factors.

corals. Inserted regularly Lightly constructed from around the corallite porous aragonite doing so they migrate away from the shoreline over time and may eventually be the only remnant of a sunken island, forming a ring-shaped atoll around the drowned land.

Reef-forming corals are amongst the small number of organisms capable of modifying their environment, changing the topography of the sea bed in such a way as to promote their own survival. Incidentally, this increases local biodiversity by generating a range of spatially distinct niches. For example, structures such as the Great Barrier Reef are of enormous extent and geological importance.

These diverse ecosystems exist within low nutrient regions of the oceans, such as around mid-ocean islands. In earlier geological periods these appear to have been regions of much lower diversity.

Scleractinian corals reproduce without undergoing a plank-tic phase. This leads to problems of dispersal, as the gametes cannot travel far from their parents before settling. This in turn has led to a pronounced provincialism within modern corals, with distinctive modern Indo-Pacific and Caribbean provinces being developed. Diversity is much higher in the Indo-Pacific region (700 species, compared to 62 in the Caribbean), suggesting that this region was a refuge for corals during the low sea level of the major Pleistocene glaciations, and the location from which their subsequent radiation has occurred.

Corals at risk

Scleractinian corals are unusual animals because of their symbiotic relationship with algae that provide them with food and with a favorable chemical environment for precipitating calcium carbonate for their skeletons. This relationship breaks down if sea temperatures become too high and this puts many modern corals at risk from global warming and raises the question of how corals survived in greenhouse periods of the past.

If corals experience long-term rises in temperature of even 1°C, the algal symbionts are lost and the corals change color or bleach. Pessimistic projections suggest that all coral reefs may be bleached by 2030. However, this is at odds with the observation that corals have survived much higher global temperatures in their history. At the Palaeocene-Eocene boundary, for example, sea surface temperatures rose by 8°C in a few thousand years, and for most of the Mesozoic there is no evidence for ice at either pole.

Marine scientists have now seen a level of recovery in bleached reefs, where a different symbiont is recruited into damaged corals that can photosynthesize at much higher temperatures. This is consistent with the long geological survival of many living coral genera and shows how resilient organisms can be to changes in global and local climate.

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