Coral Reef Development: History in the Making
Article by Joel Simon

Coral reefs are complex, enduring, magnificent -- dynamic living structures of geologic proportion. They are among our planet’s most prolific and intricately interrelated ecosystems. They are also the planet’s oldest natural community. Snorkelers floating above a coral reef are immediately struck with the teeming variety of life; an organic tapestry of dazzling color, bizarre form, and endless activity. Spread before them is a panorama of eons of development, the progeny of the sea’s entire evolutionary process, a history of a billion years, or more. All major taxonomic levels are present, from single-celled algae, invertebrates still without terrestrial counterparts, to fish, reptiles, and the largest mammals to ever grace this earth.

The fossil record reveals the ancestry of this glorious congress, many of which have kin embedded in sedimentary layers from 50 - 100 million years ago. Today’s reefs are simply the most recent link in a lengthy genetic chain. Understanding modern reefs and especially corals, will give us the tools to better decipher their intriguing history.

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 1,250 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 -- a setting scientists would not have expected to host such robust communities.

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 cooperative 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: zooxanthellae. 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. Although the reasons are unclear, it seems this relationship also mandates water temperatures above 72º F. Limited by dependencies on sunlight and warmth, reef building corals are constrained to clear, shallow, tropical waters, making them conveniently accessible to snorkelers and scuba divers. The origin of this vital coral/algae collaboration is a topic of speculation, but the partnership is as old the reefs themselves.

Reef building corals, like most animals, have a calcium skeleton. As a coral grows upward (or outward) to maximize exposure to sunlight, it deposits 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 its own dead body. And within these enduring dead bodies are contained significant clues to the ancient history of both coral and our world.

The calcium skeleton provides an excellent fossil record and core samples yield fascinating geological insights. Corals have survived, in one form or another, for over 500 million years, so they must be doing something right. "Modern" corals first appeared about 230 million years ago, when today’s continents were a single land mass. 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. On the border between Texas and New Mexico, the Guadeloupe Mountain Range embodies a 3000 foot-high, 360 mile-long ridge of jagged rock, once a thriving coral reef submerged beneath tropical seas.

Corals leave daily growth rings in their skeleton, similar to trees. In 1963, John Wells made a remarkable discovery, since confirmed by subsequent researchers. Because corals grow so much faster during the day when the algae are active, careful study of these rings in fossilized corals indicate the earth’s rotation may be slowing down due to atmospheric friction. Coral core samples from approximately 400 million years ago imply the Devonian year had nearly 400 days!

Other inferences are equally astounding. Fossilized corals from the Cretaceous period, some 100 million years ago, are found at 50º north latitude, roughly equivalent to Vancouver, London, and the southern tip of the Aleutian Islands. Conversely, similarly dated corals extend to only about 20º present south latitude promoting theories that the earth rotated on an axis far different than today. Using the same techniques, corals from the Ordovician times, roughly 500 million years ago, indicate that the North Pole may have been near Hawaii and the South Pole off the coast of West Africa!

While these prototypical coral communities lie sequestered in the annals of extinction, many of today’s reefs originated in relatively ancient times. Some atolls of the Central Pacific are built on coral foundations borne of the Eocene era some 50 million years back. The Great Barrier Reef, at least the northern section, had its birthday in the Miocene over 25 million years ago. While terrestrial creatures underwent massive extinction due to extreme changes of climate and terrain, cradled within the relatively stable environment of the sea's embrace, life prospered.

The sea’s most powerful geologic arbiter of evolution was changes in sea level, which can be measured in hundreds of feet. Concurrent with water level dynamics were dramatic tectonic and volcanic movements, shoving land masses both up and down. While these presented serious challenges to early corals, they were not insurmountable.

To better understand their survival over the millennia, it is necessary to examine reproductive strategies. Corals, along with numerous other sea creatures, are mass spawners, shedding gametes directly into open water. At the whim of oceanic currents, larval forms are carried far and wide in search of suitable areas for colonization. As old habitats declined, new ones emerged. And larval corals were there and waiting.

Corals have an added advantage. By secreting massive amounts of limestone, they create additional usable substrate, a type of organic landfill. To their heirs, deceased corals leave a precious inheritance: a place to live. Thus new and ongoing colonies are literally supported by their ancestors. The physiological prosperity of corals is crucial--where there are reefs, other creatures follow. Millions of them. Coral reefs harbor more species of plants and animals than any other ecosystem on earth, arguably a consequence of having had more time.

The wealth of marine life on reefs today is the living legacy of longevity, a lifespan borne of relatively uniform environmental factors in tandem with evolutionary opportunity. Although opinions differ on the nature of evolution, no one debates the result. Competition for extensive, but limited resources, has given rise to an unparalleled variety of creatures successfully adapted to their environment.

Whether by chance or plan, some animals have acquired highly specialized capabilities while others have developed more generalized opportunistic talents. Snorkelers can easily observe and compare feeding habits, habitat, and social structure. While some fish limit their diet to a particular food source, others are happy to devour whatever comes their way. Clown fish, live only in the protective tentacles of anemones, while other species range more freely about the reef. Groupers are solitary, butterfly fish survive with lifetime mates, and grunts, surgeonfish, and silversides exist in schools. One strategy is no more "right" than the other: specialization and generalization both work. Combined, they weave an elastic evolutionary fabric that compounds the reef’s stability.

Unlike a chain, defeated by one broken link, the tightly intertwined coral reef community maintains integrity, even if a few strands are severed along the way. For example, ninety per cent of the long-spined sea urchins in the Caribbean were recently decimated by a lethal virus. This was devastating to them, but not to the reef. Parrotfish and other algal browsers took advantage of the opportunity. Individual species may die, or suffer great losses, but the community endures and the reef’s stability remains intact .... as it has for millions of years.

Coral reefs inspire profound appreciation: cooperative yet competitive, complex, yet stable, successfully mixing ancient organisms with modern ones. All of these elements, when integrated, endow the overall ecology with a resilience and durability to withstand the inevitable pressures of time. On your next snorkeling sojourn above a coral reef, celebrate this miraculous living chronicle. In fact, have the time of your life.