Under Wonders
Article by Joel Simon

The strong equatorial sun warms my back, below me it casts a shimmering light on a grove of delicately branched pale blue soft coral trees, bright purple sea fans, and yellow, green and red sponges. Together, we move in consort with the slow rhythmic surge of the sea. What appears to be a forest is, in fact, a garden of animals. Sharp black spines of sea urchins, gaping green heads of moray eels, and orange arms of brittle stars stick out from narrow crevices. Thousands of creatures including fish of dazzling colors and bizarre shapes, giant anemones waving pink-tipped tentacles hosting spindly striped crabs and blue-spotted shrimp all participate in this complex living tapestry. I am floating above one of earth’s most diverse and remarkable ecosystems: a tropical coral reef.

For centuries corals were thought to be plants. They have no heads, tails, or feet; no eyes, no ears. They don’t crawl or swim. Close-up they look like flowers. When in 1723, naturalist Jean Andre’ Peyssonel proposed that corals might be animals, he was functionally expelled from the French Academy of Sciences. Today we know he was right. Corals, along with jellyfish and anemones, are members of a large and varied group of animals called Cnidarians: round little creatures, with tentacles surrounding a mouth. In the South Pacific alone, over 700 species of coral have been identified .... so far.

Individually, most corals measure less than half an inch across, yet collectively they have built the largest organic structures on earth. Ever. The Great barrier reef stretches over 1000 miles along Australia’s Eastern shore, and the Palancar reef bordering Mexico, Belize, and Honduras is nearly as large. Although solitary corals are found in all seas, from the fjords of Norway to the weathered shores of Patagonia, and to depths exceeding 15,000 ft., reef building corals form a vast shallow living wreath centered along the earth’s equator.

Tropical seas are nearly devoid of nutrients, which is one reason the water is usually clear. Yet reefs which thrive within them are among the planet’s most prolific living systems, veritable oases in oceanic deserts. How so much life could be sustained on so little food remained one of the greatest scientific mysteries until 1929 when researcher C. M. Yonge unraveled one of the most marvelous co-operative relationships in the animal kingdom. Living within the stomach cells of all reef building corals are microscopic plants, single-celled algae with an unwieldy multi-syllable name: zooxanthelle. These algae are doing exactly what plants do best. Taking advantage of readily available organic waste from the coral’s metabolism (basically fertilizer) and carbon dioxide from the coral’s respiration, the plants produce usable nutrients and oxygen. This is precisely what a coral animal needs to grow. Just as with plants on land, in this remarkable recipe, one more ingredient is necessary for survival: sunlight for photosynthesis. The more sunlight the better. The dependency on sunlight limits reef building corals to clear, shallow waters, making them conveniently accessible to snorkelers and SCUBA divers.

Reef building corals, like most animals, have a calcium skeleton. As a coral grows upward (or outward) to maximize exposure to sunlight, it continues to deposit calcium around its base. Fast growing corals may expand one inch per year although most are much slower. During the day, when the photosynthetic algae are most productive, corals grow approximately 14 times faster than at night. Clouds can reduce growth by half. 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 core samples from approximately 400 million years ago indicate the Devonian year had 400 days!

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. Let’s remain thankful for small things.