what’s called the thermal structure of the water column, and it’s why
tropical waters are often so beautifully clear.) As a consequence, the seas
in the tropics should be barren—the aqueous equivalent of deserts. Reefs
are thus not just underwater rainforests; they are rainforests in a marine
Sahara. The first person to be perplexed by this incongruity was Darwin,
and it has since become known as “Darwin’s paradox.” Darwin’s paradox
has never been entirely resolved, but one key to the puzzle seems to be
recycling. Reefs—or, really, reef creatures—have developed a fantastically
efficient system by which nutrients are passed from one class of
organisms to another, as at a giant bazaar. Corals are the main players in
this complex system of exchange, and, at the same time, they provide the
platform that makes the trading possible. Without them, there’s just
more watery desert.
“Corals build the architecture of the ecosystem,” Caldeira told me. “So
it’s pretty clear if they go, the whole ecosystem goes.”
One of the Israeli scientists, Jack Silverman, put it to me this way: “If
you don’t have a building, where are the tenants going to go?”
REEFS have come and gone several times in the past, and their remains
crop up in all sorts of unlikely places. The ruins of reefs from the Triassic,
for example, can now be found towering thousands of feet above sea level
in the Austrian Alps. The Guadalupe Mountains in west Texas are what’s
left of reefs from the Permian period that were elevated in an episode of
“tectonic compression” about eighty million years ago. Reefs from the
Silurian period can be seen in northern Greenland.
All these ancient reefs consist of limestone, but the creatures that
created them were quite different. Among the organisms that built reefs
in the Cretaceous were enormous bivalves known as rudists. In the
Silurian, reef builders included spongelike creatures called
stromatoporoids, or “stroms” for short. In the Devonian, reefs were
constructed by rugose corals, which grew in the shape of horns, and
tabulate corals, which grew in the shape of honeycombs. Both rugose