Coral reefs are the most biologically diverse marine eco-systems on earth, rivaled only by the tropical rainforests on land. Corals grow over geologic time and have been in existence about 200 million years. Corals reached their current level of diversity 50 million years ago.The delicately balanced marine environment of the coral reef relies on the interaction of hard and soft corals, sponges, anemones, snails, rays, crabs, lobsters, turtles, dolphins and other sea life.
A coral reef is composed of thin plates or layers of calcium carbonate secreted over thousands of years by billions of tiny soft bodied animals called coral polyps. It takes years for some corals to grow an inch and they range in size from a pinhead to a foot in length. Each polyp excretes a calcereous exo-skeleton and lives in a symbiotic relationship with a host algae, zooxanthellae, that gives the coral its color. Zooxanthellae takes in carbon dioxide, process is through photosynthesis, and then gives off oxygen as a by-product that is used by the host polyp. Millions of polyps grow on top of the limestone remains of former colonies to create the massive reefs. Yet these tiny animals form the only natural formation visible from outer space.
The ecology of coral reefs in Hawai'i is unique compared to reefs in other tropical areas. Hawaiian reefs are unique for several reasons:
1. They are geologically young and therefore not as well developed as other reefs. Most reefs in the windward islands of Hawai'i, Maui, O'ahu and Kaua'i are small and occur relatively close to shore. As these reefs grow and develop they are called fringing reefs, which eventually develop into barrier reefs, which occur in Kaneohe Bay on O'ahu and on the south shore of Moloka'i. There is a general westward trend towards greater reef development which coincides with the geologic ages of the islands. Superimposed upon this pattern, however, is the effects of wave exposure: in general more sheltered leeward coasts have reefs with greater coral cover than wave-pounded windward coasts (see Grigg, 1983).
2. As a consequence of their younger age and the general absence of barrier reefs in the windward islands, Hawaiian reefs are generally less productive than other reefs. With smaller reef areas and the absence of lagoons that trap and retain both coastal and terrestrial runoff, Hawaiian reefs are generally more nutrient poor and therefore lack a high abundance of filter feeding animals such as soft corals, sponges, tunicates, and bivalves. As a result, Hawaiian reefs are more clearly dominated by corals.
3. Due to the geographic isolation of the Hawaiian Islands they are less diverse that other reefs, especially those in the nearby Indo-Pacific province to which they belong. For example, in Hawai'i there are about 40 species of reef building corals; in the western Pacific island of Palua there are over 300! One consequence of lower diversity is that reef-building corals in Hawai'i are less specialized and thus more generalized in their distribution than species elsewhere. Furthermore, a relatively few number of corals dominant Hawaiian reefs. There is a unique advantage to science here, however, because Hawaiian reefs are far easier to study!
B. Coral Reef Zonation in Hawai'i
In response to variation in environmental conditions near the shoreline, coral reefs exhibit zonation where the abundance and composition of the coral community varies according to distance and depth from shore. Wave exposure is the primary factor causing zonation in Hawai'i but gradients in sedimentation, salinity, and temperature are also important (see Dollar, 1982). The following is a summary of zonation patterns typical of Hawai'i. Common names are linked to descriptions in the Marine Life of Hawai'i section.
In response to variation in environmental conditions near the shoreline, coral reefs exhibit zonation where the abundance and composition of the coral community varies according to distance and depth from shore. Wave exposure is the primary factor causing zonation in Hawai'i but gradients in sedimentation, salinity, and temperature are also important (see Dollar, 1982). The following is a summary of zonation patterns typical of Hawai'i. Common names are linked to descriptions in the Marine Life of Hawai'i section.
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