Coral reefs are extraordinarily rich ecosystems, considered key areas of the biosphere , which grow in very specific conditions, and which, as a result of anthropogenic climate change, are at serious risk of disappearing.
The fascination with coral reefs is not recent, and many naturalists have tried to formulate a hypothesis about their formation. The first to come up with a solid theory, however, was Charles Darwin . A decade before developing his theory of evolution by natural selection, he published a paper in which he explained how, thanks to geological processes, the long passage of time and the slow growth of corals, these rich and complex ecosystems could be formed. .
The formation of the coral reef
Corals only grow in specific conditions of temperature and light . This means that they can only grow where there is enough light for it. Being sessile animals, they need to settle on the substrate to develop, therefore, they only do so in water that is always submerged, but at a shallow depth. However, some reefs can extend to depths where corals can no longer grow. This, added to the slow growth rate of reefs, led Charles Darwin to come up with his theory.
According to Darwin’s theory, and widely confirmed later, coral reefs begin with an island or a shoreline of emerged land. Corals develop at the optimum depth for them, forming the base of the reef with their exoskeletons. As time goes by, the island sinks and so does the substrate on which the corals grow. However, they continue to grow upwards, building the reef barrier, keeping the growing part always at the same depth.
Over the millennia, the island ends up disappearing to the bottom of the sea, leaving behind a ring of reefs, called an atoll . When it happens on the coast, a wall-like structure is formed, which is called a barrier reef , and leaves an area of shallow water between the shore and it.
Polyps, reef builders
Corals are colonies of closely related organisms, individual and small polyps, from the group of anthozoans , to which sea anemones also belong. Unlike other groups of cnidarians, anthozoans do not have free-living jellyfish. The polyps produce the eggs and sperm, and the egg, once fertilized, produces a larva, called a planula , which swims until it finds a new location where it can attach to the substrate and form a new polyp.
The polyps that form a coral are associated in colonies of up to millions of individuals , in symbiosis with algae , usually red. Although coral polyps can capture prey and consume suspended particles, basic nutrition relies on that relationship; the algae provide oxygen and carbon compounds to the coral, and in return, they gain protection, carbon dioxide, and waste products such as ammonia.
The polyps produce a calcareous exoskeleton , which is gradually built on previous skeletons of already dead corals, always staying at the optimum depth.
In ecological terms, corals are one of the most productive and biodiverse ecosystems, rivaling tropical rainforests . A single coral reef can support more than 3,000 different species of fish. Although they occupy a minimal area compared to the enormity of the ocean, they contain up to 25% of the biodiversity of the oceans .
Coral reefs at risk
Around a third of reef-building coral species are threatened, and the risk of extinction is increasing. Risk factors include many human activities. Overfishing, especially trawling , kills everything, and can rip polyps from their substrate. In addition, reef mining , for building materials or for jewelry , continues to be practiced in some places.
Corals are particularly sensitive to pollution . By feeding on suspended particles, certain contaminants can enter the food web.
Microplastics are a growing risk to corals. In addition, deforestation on land can lead to massive sediment entrainment events—which would otherwise be retained by plant roots—and suffocate and even bury corals.
Under certain stress conditions, such as sudden changes in temperature or in salinity or acidity , corals can expel their symbiotic algae, producing what is known as coral bleaching . Without their algae, the polyps lose their ability to fully nourish themselves and eventually die.
Some of those effects are directly associated with climate change. A minimal change in the average temperature of the water can lead to the bleaching of large masses of corals. But in addition, human activity in recent centuries is releasing large amounts of carbon dioxide into the atmosphere, and although the ocean has a great capacity to absorb and cushion this impact, this gas, dissolved in surface waters, where corals live, acidifies it. That increased acidity is a major stressor for corals because it dissolves the exoskeletons .
This is one more way in which climate change can put the maintenance of biodiversity at serious risk.
References:
Darwin, C. 1842. The structure and distribution of coral reefs. Being the first part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. Smith Elder and Co.
Hoegh-Guldberg, O. et al. 2017. Coral Reef Ecosystems under Climate Change and Ocean Acidification. Frontiers in Marine Science, 4.
Stanley, G. D. 2006. Photosymbiosis and the Evolution of Modern Coral Reefs.Science, 312(5775), 857-858. DOI: 10.1126/science.1123701
Stoddart, D. R. 1969. Ecology and Morphology of Recent Coral Reefs. Biological Reviews, 44(4), 433-498. DOI: 10.1111/j.1469-185X.1969.tb00609.x
