Tropical Region

Tropical Region 

a zoogeographic region of the world ocean that encompasses the tropical and subtropical zones, where the water temperature at the surface does not fall below 20°C. In the west the tropical region’s boundaries are situated at approximately 35° N and S lat., while in the east they are somewhat closer to the equator.

In the pelagic zone of the tropical region there are many sharks, tuna, sailfish, flying fish, and sea turtles. In the benthic zone there are coral reefs, with their characteristic communities, as well as coastal mangrove thickets and abundant crabs, spiny lobsters, and large gastropods and bivalves. In the pelagic zone, circumtropical species predominate over species found only in certain sectors. Therefore, the tropical region is uniform, but various parts of it are identified as provinces (Atlantic, Indo-West-Pacific, and East Pacific).

In the benthic zone—the littoral and the shelf—there are very few circumtropical species, and the tropical region is considered a realm, or superregion, which is broken down into four (sometimes three or five) regions: Indo-West-Pacific, Central American (which is sometimes divided into the West Atlantic and East Pacific), West African, and Mediterranean-Lusitanian (the last two are sometimes joined to form the East Atlantic). The regions identified by benthic fauna are separated by zoogeographic barriers (the land barriers of the continents of the New World and Old World and the ocean barriers of the Middle Atlantic and East Pacific). Less than 10 percent of the species of the fauna of neighboring regions penetrate these barriers, as the shelf fauna cannot cross the bottom of the ocean, whose length also exceeds the migration capability of the pelagic larvae of benthic animals.

The fauna of the Indo-West-Pacific region is the richest and most ancient. It is highly endemic and was apparently the original group for most of the groups of shelf fauna. The West Atlantic fauna is second in wealth and diversity of species. The East Pacific fauna is significantly poorer, and the East Atlantic fauna, containing many species of West Atlantic origin, is still poorer.

In the Central American region numerous unique genera are found in the shallow tropical waters of both the East Pacific and West Atlantic parts; crabs, sea urchins, and certain fishes are represented by different but closely related species. Such distribution developed because a strait existed in what is now Central America from the Miocene to Lower Pliocene. This region is sometimes divided into two independent regions on the basis of these differences. The fauna of the eastern tropical region is poorer as a result of sharp climatic changes of the Pleistocene that were slightly reflected in the western parts.

K. N. NESIS