Eventually, this calcium, which over time may be hundreds of feet thick, becomes the actual reef structure. The coral animal essentially becomes a living veneer over it's own dead body. The calcium skeleton provides an excellent fossil record and core samples yield fascinating geological insights. Corals have survived for over 500 million years, so they must be doing something right. Changes in the elevation of the sea due to passing ice ages and tectonic land movements are accurately reflected by ancestral reefs, now stranded high above and below our modern-day sea level.
Corals leave daily growth rings in their skeleton, similar to trees.
Because corals grow so much faster during the day when the algae are active, careful study of these rings in fossilized corals have led scientists to believe the earth's rotation may be slowing down due to atmospheric friction. Coral reefs are complex, enduring, magnificent. They provide habitat for countless millions of marine creatures and doubtless harbor mysterious species still unknown to science. Fundamental to this majestic organic marvel is the vital microscopic partnership between plant and animal. Without the single-celled zooxanthelle essential to growth, coral could never attain massive proportions, and reefs, along with their immense diversity of life, simply would not exist.