CORAL: CLIMATE CHANGE INDICATORS

A lovely specimen of fossilized coral. Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps.

Annual growth bands in some corals, such as the deep-sea bamboo coral, Isididae, may be among the first signs of the effects of ocean acidification on marine life. The growth rings allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlies high-resolution records of past climatic and environmental changes using geochemical techniques.

Certain species form communities called microatolls, which are colonies whose top is dead and mostly above the waterline, but whose perimeter is mostly submerged and alive. The average tide level limits their height. By analyzing the various growth morphologies, microatolls offer a low-resolution record of sea-level change and help us to reconstruct Holocene sea levels. Fossilized microatolls can also be dated using Radiocarbon dating, a method of discovering how old something is by measuring the decay of the radioactive isotope Carbon-14. 

Increasing sea temperatures in tropical regions, ~1 degree C, over the last century have caused major coral bleaching, death, and collapsing of coral populations although they are able to adapt and acclimate. It is uncertain if this evolutionary process will happen quickly enough to prevent a major reduction in their numbers.

Though coral has large sexually-reproducing populations, their evolution can be slowed by abundant asexual reproduction. Gene flow is variable among our coral friends. According to the biogeography of coral species gene flow cannot be counted on as a dependable source of adaptation as they are very stationary organisms. Also, coral longevity might factor into their adaptivity.

However, adaptation to climate change has been demonstrated in many cases. These are usually due to a shift in coral and zooxanthellae genotypes. These shifts in allele frequency have progressed toward more tolerant types of zooxanthellae. Scientists found that a certain scleractinian zooxanthella is becoming more common where sea temperature is high. Symbionts able to tolerate warmer water seem to photosynthesize more slowly, implying an evolutionary trade-off.

In the Gulf of Mexico, where sea temperatures are rising, cold-sensitive staghorn and elkhorn coral have shifted in location. Not only have the symbionts and specific species been shown to shift, but there seems to be a certain growth rate favourable to selection. Slower-growing but more heat-tolerant corals have become more common. The changes in temperature and acclimation are complex. Some reefs in current shadows represent a refugium location that will help them adjust to the disparity in the environment even if eventually the temperatures may rise more quickly there than in other locations. This separation of populations by climatic barriers causes a realized niche to shrink greatly in comparison to the old fundamental niche.