$ Every ecosystem on Earth is unique, but the coral reef is perhaps the most unusual of all because it’s the only ecosystem made by and made of-animals. All coral reefs are constructed by coral polyps, which are generally small, about the size of this pencil eraser. But, the structures themselves are, well, enormous. Astronauts have been able to identify the Great Barrier Reef in Australia from space. Can you believe that? And the diversity of species in large coral reefs is second only to the rainforest habitats. In fact, we estimate that for every species we’ve identified on a coral reef, there are probably a hundred times that number that remain to be classified and studied.
But how do these little polyps build such impressive reefs? Well, hard coral secrete a shell of calcium carbonate around their bodies. The polyp isn’t hard, you see, but the shell is. And these shells are the material that forms a coral reef. So a coral reef is just a colony-millions and millions of coral animals whose shells are connected. And reproduction is really the basis for the construction of a large reef. You see, as each polyp matures, it converts the calcium and other minerals in ocean water to a hard limestone exoskeleton called a corallite. And this is fascinating. Although the polyps themselves don’t appreciably increase in size, they continue to build new shells periodically, um, connecting them with . . . with partitions.
Now coral can reproduce sexually through an activity called mass spawning. During one night in the spring when the moon is full, coral polyps release egg bundles that contain both eggs and sperm. Most polyps have both male and female reproductive cells. The egg bundles are round, about half the size of marbles, I would say. They’re brightly colored in orange or red or pink, and they float up to the surface to form a thick layer of, uh . . . well think of them as rather fragrant beads. So with the water so saturated with them, predators will only be able to devour a small number compared with the huge number that will survive and break open. The sperm cells swim away to fertilize the eggs from another bundle. So . . . once fertilized, the little egg begins to mature from a coral larva to a planulae, which can swim for a few hours, days, or even a few weeks. Ultimately it locates a hard surface on which to attach itself and from which it will not move for the rest of its life, except for the movement involved in the process of building a new, neighboring shell as . . . as it continues to mature.
But actually sexual reproduction isn’t the way that coral reefs are really constructed. When a polyp matures on the site it’s selected, the habitat is identified as being conducive to reef building. So the mature polyp doesn’t just grow bigger, it actually replicates itself in a process called budding. After the genetic material is duplicated, then the polyp divides itself in half, and each half becomes a completely mature polyp. This budding process repeats itself, eventually producing thousands of asexually budded coral polyps connected by a tissue that grows over the limestone shells between the polyps. So, as you can imagine, budding will produce a large number of individual polyps, but they’ll all have exactly the same genetic code as the first polyp. And this creates the beginning of a coral reef, but without the diversity that eventually populates the habitat. Wherever a coral reef is constructed, abundant sea life congregates. In fact, it’s been estimated that about 25 percent of all ocean species can be found within the coral reefs.
Now most coral polyps eat plankton-single-celled microscopic organisms that float or swim very slowly in the ocean water in their habitat. But, um, a coral reef has such a high concentration of polyps, they can’t rely solely on plankton to survive. So coral polyps have developed a symbiotic relationship with a single-celled algae called zooxanthella. Remember that to qualify as symbiotic, a relationship must be, um, mutually beneficial. So the zooxanthella produces food for the coral through the by-products of photosynthesis, and the coral provides a safe home for the zooxanthella, because it’s hidden from predators that inhabit the coral reef.
Every species of coral grows at a different rate, some as much as six inches a year. But faster growing colonies are more prone to breaking apart either from their own weight or from the continuous force of the ocean waves. Some species tend to grow more slowly, but they may live as long as a thousand years. Even so, only the top portion of any reef is actually alive and growing and the lower structure is comprised of the skeletal remains . . . that’s limestone corallite from coral that has died.
And what I find incredibly interesting about coral reefs is that each is a unique structure. But, of course, scientists need to classify, and so there’s a classification system for coral reefs. A fringing reef grows around islands and the shorelines of continents and extends out from the shore. In order to flourish, fringing reefs must have clean water, lots of sunshine, and a moderately high concentration of salt. Some good examples of fringing reefs can be found around the Hawaiian Islands. Oh, yes, these are the most common and also the most recently formed class of coral reefs. Here’s a drawing of a fringing reef. I think this is actually one of the Hawaiian reefs.
Now, barrier reefs-they’re found further from shore, and they’re usually separated from the shoreline by a shallow body of water, maybe a lagoon. As in the case of the Great Barrier Reef off the shore of Australia, the body of water can be miles wide, so the reef is miles away from the shoreline. And there may actually be a collection of coral reefs fused together. This is a drawing of a reef in the Great Barrier chain.
As I recall there are about twenty-five, or maybe even more individual coral reefs connected to form the Great Barrier Reef. As a general rule, barrier reefs are larger and older than fringing reefs.
But the oldest class of coral reef is the atoll, which is a ring-shaped reef with a lagoon in the middle and deep water surrounding the ring. These are scattered throughout the South Pacific, kind of like oasis settlements in the desert. And they abound with a diversity of sea life. This is one of the South Pacific atolls.
So, as we reflect on everything we’ve said about coral, we know that it’s a relatively simple organism with a body ending in a mouth and tentacles. It reproduces both sexually and asexually by budding, and, um . . . it survives by forming a symbiotic relationship with zooxanthella. But none of this is very extraordinary. What is unique about coral in the animal kingdom is its ability to construct a variety of reefs, creating habitats that are absolutely unlike any others on Earth.