Coral reefs are tropical underwater wonders that have been on earth for millions of years. Not only are they the most complex, productive, and genetically diverse ecosystems on the planet, they are also among the most fragile, and therefore among the most easily degraded or imperiled.
A reef is made up of colonies of tiny coral animals related to jellyfish and sea anemones. The polyps live in extremely large colonies, and their interconnected calcium carbonate skeletons build layer upon layer over the remains of their ancestors, forming hardened limestone of the reef. While some reefs grow to over 6,000 feet thick, only the top fraction-of-an-inch7—thinner than the thickness of your fingernail—are actually living.
Tropical reefs—like the ones we visit on our voyages—are built by hermatypic corals that occur mainly between the warmer latitudes of 30A* N and 30A* S on either side of the equator. These coral reefs not only need warm, silt-free water but also plenty of light and microscopic food for the polyps. However, the clear water that corals grow in do not have enough planktonic food to allow fast-enough growth to produce a reef. The secret nutrient supply that allows such rapid growth is the work of one-celled algae called zooxanthelae that live by the millions inside the coral polyp's tissue. In fact, the brilliant color palette of coral comes from the tiny zooxanthellae, which not only nourish the translucent polyps, but produce the wide variety of pigments.
Coral reefs cover about 110,000 square miles and are found in more than 100 countries, worldwide. There are four distinct types of reef: Fringing reefs, separated from coastal shores and islands by shallow lagoons; Barrier reefs, split from the shoreline by deep, wide lagoons; Atolls, circular or semi-circular reefs formed by the sunken peaks of volcanoes; and Patch reefs, which are isolated growths rising from an island floor.
Combined, the reefs of the world comprise over 700 coral species sheltering more than 4,000 species of fish and hundreds of thousands of marine invertebrates. The shape and rate of growth of coral depends upon its physical environment. The rounded corals grow where there is significant wave action, adding only about an inch per year. The more fragile, branch corals proliferate in sheltered waters, extending their arms up to an additional eight inches in the same time period.
Sunlight, clear water, and clean water are among the most important elements necessary for the growth and survival of coral reefs. Today while steadily increasing pollution from urban and agricultural areas take a swift toll on the health of the reefs, careless tourism, destructive fishing practices, and especially global warming (and therefore ocean surface warming), top the list of progressive destruction of coral reefs globally.
On several Zegrahm expeditions, we explore reef sites around the world. Most of them are rarely visited and many of them are near deep, nutrient-rich trenches and oceanic islands where the coral reefs are healthy and spectacular. We hope that in viewing these magnificent organisms in their naturally pristine state, it will inspire us all to preserve and protect these amazing wonderlands.