Eastern Pacific region
The eastern Pacific region contains another radiation related to and only recently separated from the western Atlantic. The region contains only four to eight genera of zooxanthellate corals (Rosen 1988) and fewer species of fishes than are present in the western Atlantic. Some widespread taxa, such as the Bluefish (Pomatomus saltatrix) and Cobia (Rachycentron canadum), are absent. The eastern Pacific barrier, the huge expanse of open water between the central and south Pacific islands and the American mainland, acts as a distance barrier limiting the movement of 86% of shore species from the central Pacific (Briggs 1974).
Elevation of the Panamanian Isthmus approximately 3 million years ago separated the continuous distribution of species into eastern Pacific and western Atlantic populations. David Starr Jordan, pioneer American ichthyologist, referred to such pairs of species as geminate species, related species divided by the isthmus, such as the Spanish mackerels, Scomberomorus sierra, in the eastern Pacific, and S. brasiliensis, in the Caribbean Sea. Some geminate species have clearly differentiated into what can be called good species from a morphological and sometimes from a genetic point of view as well, such as Spanish mackerels and toadfishes of the genus Batrachoides. Others, such as the halfbeaks of the genus Hyporhamphus are less well differentiated morphologically, making molecular methods useful to reach decisions on the status of the populations on either side of the isthmus (see The Panama barrier).
The completeness of the eastern Pacific barrier is, however, not quite as distinct as Briggs (1974) implied (Lessios & Robertson 2006). Several Indo-West Pacific shore fishes actually cross the eastern Pacific barrier and are found at offshore islands such as the Revilligedos off the coast of Mexico, and Clipperton and Cocos off the coast of Costa Rica. Distributions of a species of mackerel (Scomber australasicus) and a needlefish (Tylosurus acus melanotus) extend from the western Pacific through the Hawaiian Islands to these islands, but these species are replaced by related forms (S. japonicus and T. Pacificus, respectively) along the eastern Pacific coast of Middle America. These exceptions to the completeness of the eastern Pacific barrier may be related to habitat differences between the offshore islands and the mainland.
The Panama Canal connects the eastern Pacific with the western Atlantic. However, unlike the Suez Canal, the Panama Canal is not at sea level. It contains a freshwater lake, Lake Gatun, in its middle, and fresh water is used to raise the water level in a series of locks to lift ships up to the lake and then down to the ocean on the other side. This freshwater barrier prevents marine species from moving between the two oceans, with the exception of a few species that tolerate a wide range of salinities (McCosker & Dawson 1975). A proposed sea-level canal would allow mixing of the two different faunas and might have grave effects on the fishes and marine invertebrates on both sides of the isthmus. Diseases, parasites, and aggressive Indo-West Pacific species that pose little current danger in the eastern Pacific, such as the crown-of-thorns starfish and a sea snake, might do severe damage to coral reefs and the fish fauna of the western Atlantic (Briggs 1974).