Geographic Distribution Of Reefs and Types of Reef
*There are two distinct biogeographic realms. Pacific and Atlantic.
As reef corals and their associated faunas evolved, most of the world’s tropical oceans were interconnected. The Atlantic and Pacific were connected by a broad seaway known as Tethys, or the Tethyan Sea, which dried up toward the end of the Miocene epoch, roughly 10 million years ago. Even at the beginning of the Miocene, divergence began between the Pacific and Atlantic, but by the end of the Miocene, faunas in the Atlantic had diverged from those of the Pacific, although there still was a connection through what is now Panama, because the Isthmus of Panama did not exist. More than 3 million year ago, the Isthmus of Panama did not exist, and many groups with Pacific affinities lived in the Caribbean Sea. After the rise of the isthmus, the Pacific forms in the Caribbean largely became extinct. In general, the number of living species is about twice as great in pacific reef as in the Atlantic reefs, with a maximum level in the southwest Pacific (Figure 15.15).15 Overall there is a great deal of difference in the species composition of Pacific and Atlantic reefs, now that there are essentially no connections. Maximum diversity seems to be associated with areas that have been historically subject to the least elimination of major regions of appropriate tropical open marine habitat, owing to climatic changes. The southwest Pacific has the least such environmental variation and has had the most continuous occupation of coral reefs over geological history. Paleogeographic maps demonstrate that fluctuations of sea level during the Pleistocene epoch caused major changes in the distribution of land and sea in the Indo-West Pacific. At times of glacial maxima, sea level was low and many areas between Australia and mainland southeast Asia were mostly dry land. At times of high sea level such as the present, most of this region is under water. Times of low sea level might have isolated coral populations and contributed to speciation.16