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All reef-building coral species enjoy a symbiotic relationship with species of algae called zooxanthellae.Residing within the coral polyps’ tissues, these photosynthetic one-celled plants are the ultimate source of the energy for the polyp to produce and exude calcium carbonate.Their relationship is such that the corals cannot survive without the zooxanthellae and the latter cannot survive without the corals.
The polyps secrete calcium carbonate which forms an exoskeleton into which the polyp withdraws for protection from predators.
While a very few such animals are solitary, the great majority are “colonial” in that they form colonies in which their individual skeleton structures are joined together in a common mass.
There are three types of reefs: fringing reefs, extending outward directly from the shore; barrier reefs, occurring further offshore with a channel between the reef and the shoreline; and atolls, essentially horseshoe-shaped coral islands enclosing a lagoon.
These large structures are not simply communities of minute polyps. Plants growing on coral surfaces feed herbivorous fish.
Recent and repeated mass bleaching events have raised widespread concern about the future of these key ecosystems and the implications that their loss could have for biodiversity, the economy, and human health.
Large-scale observations of the Great Barrier Reef first began in the late 1970s.Reef-building corals and their associated zooxanthellae are particularly sensitive to their environments.They tolerate a sustained temperature range of only 25-29 degrees Celsius.The convolutions of coral structure provide safe havens from predators.And some predators (e.g., moray eels) use these structures to hide in while awaiting passing prey.Hughes has spent the past three decades tracking changes on Australia’s Great Barrier Reef.It was in 1998 that the reef faced its first “mass bleaching event”.While algae can be reabsorbed when the water temperature drops once more, corals will die if this separation is too long.Coral bleaching is of great global concern, as around one in six people globally rely on coral reefs for food, shelter and livelihoods.“We can see that corals have been able to acclimate and recover from past bleaching events.However, the increase in bleaching frequency and the numbers of corals affected since temperatures started consistently increasing in the modern era raises serious concerns about whether corals are approaching a critical threshold beyond which their long-term survival is uncertain.” Dr Sebastian Hennige from the University of Edinburgh’s School of Geosciences is a co-author on the paper.