With more than 29,000 species, fishes are the most diverse group of vertebrates on the planet. Of that number, more than 12,000 species are found in freshwater ecosystems, which occupy less than 1 percent of the Earth's surface and contain only 2.4 percent of plant and animal species. But, on a hectare-for-hectare basis, freshwater ecosystems are richer in species than more extensive terrestrial and marine habitats. Examination of the distribution...