Builders in water

Out of the rough, raw materials of the planet Earth mankind has fashioned mighty cities, towering pyramids, exquisite tapestries, translucent porcelains and ships of hardened steel. But predating the advent of humans, there were animal builders whose works were mightier and certainly more cost-effective.

Take, for example, those of the tiny sea creatures known as Foraminifera, which extract minerals from water and turn them into protective casings for themselves. They multiply by division, breaking up into many small parts that quit the original shell and then begin to make new coverings of their own. A pinch of beach sand might contain 50 000 abandoned shells, but over the ages the animals who made them have created limestone rocks and contributed not a little to the fabric of cities.

Other persistent builders in limestone are the coral polyps of tropical seas. Growing in colonies of uncountable billions, they create delicate abstract statuary, rainbow-hued underwater gardens of waving tentacles, tropical islets of breathtaking beauty and long coastal breakwaters that can rip the heart out of the weightiest storm-driven wave. They flourish only in sunlit waters, because they share their stony cells with algae that produce food by trapping sunlight and absorbing carbon dioxide from the water - which also helps the polyps to build their skeletons.

Yet some coral reefs plunge down to regions of perpetual darkness. This is because, over a long period, the seabed has gradually subsided, taking the coral upon it down into the twilight. The polyps died, but their skeletons provided a strong platform for the next layers of coral to grow upon. And so it went on, generating upon generation, the topmost one bathed in sunlight not far below the surface. In some cases coral ancestry is venerable indeed. Borings into some reefs have shown that the lowest coral layers were formed at least as much as 80 million years ago.