Freshwater Riches of the Amazon
Lundberg, John, Natural History
To find the reasons for a rivers abundance, a scientist goes fishing in deep time.
Home to more than two thousand freshwater fish species, the Amazon, ichthylogically speaking, is the hottest big river on the planet. Some of its inhabitants are familiar: cardinal tetras, discus, angel cichlids, and armored "pleco" catfish inhabit home aquariums; trophy-sized peacock bass and goliath catfishes beckon sporting anglers; giant-sized tambaqui and pirarucu are prime food fishes for residents of the region; and piranhas, electric eels, and river stingrays contribute to tales of tropical danger. But any notion that science has achieved a complete inventory of the Amazon's, or the world's, fishes is utterly dispelled by ongoing discoveries. Each year since 1960, more than 35 tropical American species of fishes, including catfishes, characins, electric fishes, killifishes, and cichlids, have been newly described and named. During the past decade the pace has quickened, with more than 50 fishes coming to scientific light annually. By contrast, the Congo River of Africa has about 700 fish species in total, and the well-studied Mississippi-- Missouri of North America, a relatively scant 375.
I had surveyed the fishes of the Orinoco River system of Venezuela and Colombia for many years before my first visit to the Amazon in 1990. For an ichthyologist, the Amazon represents a pinnacle of diversity, but I also wanted to investigate the history and origins of the river's many fish species. Biogeography is the science that seeks to document and explain patterns of diversity and regional differences in species abundance, so I delved into the ancient geography of South America to find out just why the Amazon has so many fishes.
Two broad, global patterns of biogeography help explain Amazon fish diversity, but they don't tell the whole story. Size matters, and the Amazon River basin is a large, watery place, covering more than 2.5 million square miles, or 30 percent of the South American continent. In terms of water volume, no other river on earth comes close to it. In the rainy season, the Amazon discharges 3-6 million cubic feet of water per second into the Atlantic and accounts for 20 percent of the worldwide flow of freshwater into the oceans. So we might expect the vast basin, or watershed-consisting of the main stem of the Amazon and its thousands of tributaries-to contain many fish species. Yet the Orinoco, with a watershed area of less than half a million square miles, boasts at least 1,000 species, and even smaller rivers of the Guianas teem with hundreds of species. Size, then, is at best a partial explanation for the fish-rich Amazon.
A second general pattern, which holds true for tropical America's birds, reptiles, amphibians, mammals, insects, marine fishes, invertebrates, and plants-that is, most organisms-is known as the latitudinal diversity gradient. Long recognized but never fully explained, this is the trend for life to be more diverse in tropical, low-latitude regions. It is a fact of geography that near the equator, the earth receives more energy from the sun. Temperature and day length are more seasonally stable (although in many cases rainfall is not). Under these conditions, vegetation abounds and, in turn, can support many animals of many species, including some that have extremely specialized lifestyles and exist only in small populations. The Amazon is full of gastronomic specialists. Hypophthalmus, for example, eats tiny zooplankton; other fish eat snails (Megalodoras uranoscopus), the scales of other fish (Catoprion), the tails of other fish (Magosternarchus), chunks of flesh and fins (piranhas), fruits and seeds (tambaqui), wood (Panaque), or blood and gills (candiru catfish). (One species of candiru catfish, which is attracted to the flow of water from the gills of its prey, has become notorious for its unfortunate tendency to mistake a stream of urine for gill flow and to enter human urethras. …