West African Region

(also, the Guinean Region), a tropical zoogeographical region of the ocean.

As a result of the several ice sheets that covered Europe and North America during the Pleistocene epoch, the West African Region experienced an extreme cooling (the temperature of the surface water in the tropical zone of the Atlantic Ocean was 7° lower than it is at present). This cooling caused the region’s flourishing ancient fauna to die out. Northern forms migrated southward through the tropics to the southern hemisphere. The Mediterranean Sea and the western shelf of Africa became populated with boreal and subarctic species. During the postglacial period, the northern forms died out.

The greatly depleted tropical fauna of the West African Region comprises a small number of endemic families and genera. There is only one species of sea cucumbers of the family Rhopalodinidae. There are gastropods of the genus Pusionella, sea urchins of the genus Rotula, and two genera of crabs (Atlantotlos and Ilia). Forty percent of the decapod crayfish and approximately 60 percent of the echinoderms, ascidians, and mollusks are endemic species. Abundant river discharge flow and lengthy rainfall greatly dilute the salt content of the surface water and introduce suspended particles into the water. Therefore, madrepore corals, which flourish in pure seawater, are few in number in the West African Region and do not form reefs. In contrast, the mangrove vegetation forms heavy growths, which are inhabited by unique fauna.