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 planets
most prolific and intricately interrelated ecosystems. They are
also the planets 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 seas 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.
Todays 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.
corals were thought to be plants. They have no heads, tails, or
feet; no eyes, no ears. They dont 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.
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 Australias
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 earths
equator -- a setting scientists would not have expected to host
such robust communities.
are nearly devoid of nutrients, which is one reason the water is
usually clear. Yet reefs which thrive within them are among the
planets 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 corals metabolism (basically fertilizer)
and carbon dioxide from the corals 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.
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.
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 todays 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.
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 earths 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!
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!
prototypical coral communities lie sequestered in the annals of
extinction, many of todays 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.
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.
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 reefs
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 reefs stability remains intact .... as it has for
millions of years.
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