The Sixth Extinction: An Unnatural History

Coral polyps.
Like sea urchins and starfish and clams and oysters and barnacles,
reef-building corals have mastered the alchemy of calcification. What sets
them apart from other calcifiers is that instead of working solo, to
produce a shell, say, or some calcitic plates, corals engage in vast
communal building projects that stretch over generations. Each
individual, known unflatteringly as a polyp, adds to its colony’s collective
exoskeleton. On a reef, billions of polyps belonging to as many as a
hundred different species are all devoting themselves to this same basic
task. Given enough time (and the right conditions), the result is another
paradox: a living structure. The Great Barrier Reef extends,
discontinuously, for more than fifteen hundred miles, and in some places
it is five hundred feet thick. By the scale of reefs, the pyramids at Giza are
kiddie blocks.
The way corals change the world—with huge construction projects
spanning multiple generations—might be likened to the way that humans
do, with this crucial difference. Instead of displacing other creatures,
corals support them. Thousands—perhaps millions—of species have
evolved to rely on coral reefs, either directly for protection or food, or
indirectly, to prey on those species that come seeking protection or food.