Freshwater Riches of the Amazon

September 2001

To find the reasons for a river’s abundance,
a scientist goes fishing in deep time.

Story by John Lundberg ~ Paintings by Ray Troll

Click for high resolution image

ABOUT THE ARTIST— In 1997 artist and fish aficionado Ray Troll traveled 1,000 miles of the Amazon. The river’s diversity inspired a seven-by-fifteen-foot painting of 120 species. (Click painting above to bring up a high-resolution image with species labels.)
Home to more than two thousand freshwater fish species, the Amazon, ichthyologically 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 pirarucú 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.
Size is at best a partial explanation for the Amazon’s bounty.
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 (candirú catfish). (One species of candirú 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.)

Amazon River Dolphin

Adapted for life in turbid waters, the Amazon river dolphin has extremely small eyes. It uses its finely tuned echolocation skills to detect prey.
While river systems both large and small in the northeastern two-thirds of the continent appear to exemplify the tendency of low latitudes to be species rich, other South American rivers do not. Fish diversity falls off precipitously in the watersheds north and west of the Andes. These rivers and tributaries are flanked by some of the richest land and marine communities, but by South American standards, they have relatively few freshwater fishes. For example, the Magdalena River and Lake Maracaibo in the northwest each host only about 150 species.

While heat, sunlight, and ample food can help maintain tropical diversity, they do not explain how great diversity arose in the first place. The bounty of the Amazon can be accounted for only if we consider time as well as space. Biogeographical patterns, particularly those of fishes, are rooted deep in the past. As denizens of rivers, streams, and lakes, the largest groups of endemic, or native, freshwater fishes in South America are relatively confined. Unlike many other creatures, they can neither move through coastal saltwater nor traverse land barriers. Where they are and how they move about depend on changes of continental proportions that have taken place at a geologically slow pace.

One controversial theory that seeks to explain tropical America’s biodiversity holds that climatic shifts starting about 2 million years ago and continuing through the Pleistocene Epoch, or until about ten thousand years ago, caused the repeated fragmentation and merging of tropical rainforests. Such shifts provided multiple opportunities for birds, butterflies, and plants to diverge and eventually become new species. But in the case of fishes, a time frame of 2 million years is just yesterday. Long before the Pleistocene, the Amazon teemed with species that are closely related to fishes alive today in the river.

For fishes, 2 million years ago is just yesterday.
We need, then, to consider phenomena much older than 2 million years as we search for clues to the diversification and biogeography of Amazon fishes.

As early as the middle Miocene, about 15 million years ago, the tropical American fish fauna was essentially like today’s. It included many living groups: stingrays, lungfish, pirarucú, piranha, goliath catfishes, some electric fishes, and cichlids. A primatologist friend and colleague once showed me a fossil of a 13-million-year-old fish he had excavated in Colombia. He had no idea what kind of fish he had found and was struck by its particularly large teeth. He was surprised when I was able to identify the fossil; the specimen looked just like a living fish I knew well, the tambaqui. Although no longer found in the rivers of central Colombia, the tambaqui, which feeds on seeds and fruits that fall from trees in the seasonally flooded forests, thrives today in the Amazon. Similarities between ancient and modern fishes crop up even further back in time. One well-preserved fossil fish about 59 million years old looks like, and is indeed related to, the Corydoras catfish popular today in the aquarium trade. And some fossil catfishes, lungfishes, and characins date back to the Late Cretaceous, 70 million years ago.

Today, 93 percent of South American freshwater drains into the Atlantic, but this was not always the case. The stage on which the present pattern of rivers took shape was the continent itself. The players, South America’s major physical features, included two huge slabs of stable continental crust, known as the Guyana and Brazilian Shields; the dynamic Andes mountain range; and the vast lowlands that lay between the shields and the mountains. About 90 million years ago, South America and Africa (both of which had earlier been part of the southern supercontinent of Gondwana) separated from each other. As South America, which comprised a continental plate, moved west, it crashed against an oceanic plate moving east. The collision caused the land along the west coast of South America to rise up, and the first stage of Andean mountain building began. During their long history, the Andes underwent several phases of uplift and subsidence. As the mountains grew, adjacent land to the east and north buckled, forming a troughlike basin running north and south. At times the basin was filled with sediment; at wetter intervals it contained large lakes, was periodically inundated by shallow ocean waters, and channeled major rivers in a north–south direction. Taken together, the formation of Andean and other land barriers, the shifting courses of rivers, and sporadic incursions of ocean waters would have produced many opportunities for fish species to evolve as some populations were isolated from others and began to adapt to new conditions.

Long before the end of the Miocene, a great river, consisting of what are today the western Amazon and the Orinoco, flowed north through the lowland basin and into the Caribbean. About 10 million years ago, the rise of the eastern Andes created the Magdalena River basin in Colombia. Uplift continued until, by 8 million years ago, the mouth of the north-flowing Amazon-Orinoco was dammed. The river could no longer reach the Caribbean Sea. This major event in the geographical history of South America split the Orinoco and Amazon and reoriented their course from north–south to west–east. The Amazon began to flow into the Atlantic, as it does today. Over time, new rivers appeared in the north, and 3.5 million years ago the elevation of the Isthmus of Panama formed a bridge between the American continents.

As rivers appeared, disappeared, and changed course, their communities of fishes went along for the ride. The ranges of various species expanded, merged, or were disrupted. The geological upheavals that divided rivers and river basins provided opportunities for speciation when fish populations were isolated. The results are the species that today are found only in those systems. However, fossils from the regions now occupied by the Magdalena River and Lake Maracaibo show they once contained fishes no longer found there but that still inhabit the Amazon. These include lungfish, Arapaima, tambaqui, piranha, Hydrolycus, and goliath and pirarara catfishes. Although the fossil record documents such localized extinctions of living fish groups, we have no evidence of widespread extinctions and only two or three cases in the past 65 million years in which an Amazon fish species was completely extinguished.

The biodiversity equation is balanced by speciation on one side and extinction on the other. Low extinction rates are part of the formula for fish-species richness in the Amazon basin and eastern South America. But in other regions of the world, freshwater fishes also provide textbook examples of rapid species formation. Chief among these examples are the African cichlids, which, in a geologically brief period (perhaps less than 100,000 years), evolved into hundreds of species that now live in the same body of water. In contrast, we have no evidence for any unusually high rates of speciation among fishes in South America. Given enough time and low extinction rates, a normal tempo of speciation gave rise to the Amazon’s array.

Since the day I recognized my colleague’s fossil as almost identical to a modern fish, I have discovered several other examples of this phenomenon, and I am still struck by the great antiquity of many of the living tropical American fish species. By comparison, the history of other groups, such as mammals, has been fast paced and marked by high turnover. Tropical American mammal species have immigrated, emigrated, evolved, and gone extinct with a rapidity that makes Amazon fishes look frozen in time. Fish like the tambaqui—whose fossil and modern jaws and teeth reveal unmistakable similarities—have, except in the restless edges of their ranges, changed little over the past 15 million years. As continents separated and connected, as the Andes grew and rivers took right-angle turns, the tambaqui persisted in its specialized diet of fruits and seeds. This fish and many modern-day finned icons of the Amazon recount a long and enlightening chapter of the story of life on earth.


Dark New World

The Amazon and its tributaries are not only vast but also fast moving, deep, and dark. In the clearer tributaries an observer may peer down to a depth of several yards, but in most of this silt-laden river system, light does not penetrate even three feet. One of the main challenges faced by ichthyologists hoping to discover and document the fish species in very big South American rivers is to devise ways to survey less accessible parts of the channels. My colleagues and I found that from large boats or even motorized canoes, we could trawl depths of a few to more than 150 feet by casting a weighted net with a wide mouth and very fine mesh. In the process, we lost plenty of equipment as our gear snagged on submerged trees and rocks in water that was moving six or more feet per second.

So far on our Amazon collecting trips, we have netted some 240 species of fish, some of them with surpassingly strange lifestyles. Especially well adapted to low or nonexistent light levels are several new species of electric fishes and catfishes. Many, such as the electric fish Orthosternarchus and the catfish Bathycetopsis, are blind or have only tiny vestigial eyes; some lack pigment. One transparent catfish, Micromyzon, has thick bones and armor plating but is among the tiniest of freshwater fish: an adult specimen carrying eggs measured just one-third of an inch long. Electric fishes of deep waters are able to navigate and locate prey using electric impulses. Each species has its own distinct patterns of electric discharges that it uses to communicate, much like birds use song. One electric fish bears a fleshy protrusion above its chin; we can only guess what that feature might be used for.

Some catfishes provide abundant food for the people living in the Amazon River basin and are often caught as adults in shallow water near river banks. But they appear to spend their juvenile stage floating and feeding near the surface of the river’s central channels. Understanding such lifestyles is important for managing the harvest of food fish in the Amazon.

Two of the strangest creatures in this new realm are a pair of related electric fish species that we named Magosternarchus raptor and M. duccis. About ten inches long, these fishes make a living by feeding on the tails of other species of electric fish. We found this out when their stomachs proved to contain nothing but fish tails. Their victims can conveniently regenerate the nibbled appendage, renewing their own posteriors and the food source of the tail eaters. (We suspect that Magosternarchus may also dine on the tails of its own kind.) While we expect the Amazon and other big tropical rivers around the world to reveal more secrets in the future, some spots in these rivers run forbiddingly deep—up to 300 feet in the Amazon near Manaus, Brazil—and this may be deep enough to preserve some riverine mysteries.

—J.L.  


Ichthyologist John Lundberg began studying the fishes of South America’s Orinoco River in 1974 and those of the Amazon River in 1990. His pioneering surveys of the fish life of deep river channels have turned up many new species, including catfishes and electric fishes with unusual lifestyles. Lundberg, left, says his “innate attraction to fossils” has led him to broaden his focus; he uses insights from paleontology, geology, and biogeography in his work on tropical fish diversity. Formerly a professor at Duke University and the University of Arizona, Lundberg is now curator of ichthyology at the Academy of Natural Sciences in Philadelphia and holds adjunct professorships at the University of Pennsylvania and Drexel University.

     After an autumn 1997 trip down the Amazon with a boatful of friends and scientists, artist Ray Troll was inspired to produce his most ambitious work to date: a mural-sized canvas that took a year to complete and that portrays about 120 species—furred, feathered, but mostly finned. Troll lives in Ketchikan, Alaska, but travels frequently in the service of art and fish. He and writer Brad Matsen have produced various books on their adventures with fish, both fossil and modern. Troll is currently completing a book on sharks. For an out-of-the-ordinary fish-viewing experience, see www.trollart.com.

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