Ariel, in The Tempest, has little sympathy for Ferdinand, whose father is believed to have been drowned:
Full fathom five thy father lies,
And of his bones are coral made.
But the verse is misleading on two counts. As the narrative unfolds, we discover that Ferdinand's father is alive and well; and science tells us that coral is much more complex than a heap of bones.
A coral reef is a living community. Its prime inhabitant is the coral polyp, a tiny unattractive creature comprising little more than a mouth surrounded by several rings of writhing tentacles. Polyps secrete a gelatinous substance to build a hard, shell-like mass of calcium carbonate around themselves. At night they feed by capturing plankton with their tentacles; by day they rest.
The key to coral, however, lies in its symbiotic relationship with tiny single-cell algae called zooxanthellae. The host polyp provides its zooxanthellae with a protected environment in which to live, and a steady supply of CO2 for photosynthesis; byproducts of photosynthesis in turn provide nutrition to the polyp. In combination, both organisms thrive in a way they could not do alone, growing both by predation at night and photosynthesis by day.
The world's coral reefs have grown from these tiny building blocks over many, many centuries. Many other creatures and plants live in close association with them, resulting in such high productivity and biodiversity that the reefs are often thought of as the tropical rain-forests of the oceans.
But their existence is precarious. Coral can survive only in warm semi-tropical waters whose temperature lies within a relatively narrow range. And strange things have been happening to the reefs in recent decades; instead of presenting a kaleidoscope of different colours, large areas of them have turned completely white, or bleached.
The immediate cause is well known. Coral polyps themselves are colourless; the reefs take their colours from the zooxanthellae, and when the polyps become stressed - as happens, for example, if the water becomes warmer than their norm - they expel their symbiotic friends and assume a bleached appearance. Bleaching depresses coral growth and results, sometimes, in mortality.
It had been noted that severe bleaching occurred in 1982-83, and again in 1987-88, when El Nino events brought unusually warm waters to many regions of the world's oceans. But then bleaching of large areas of coral continued into the 1990s, to the extent that some 40 per cent of the world's coral reefs are now believed to have been destroyed. There are a myriad of possible causes, but the "chief suspect", to use a phrase currently in vogue, is global warming.