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CHAPTER 1
GENERAL INTRODUCTION
Amazonia is a vast and complex landscape, with a biodiversity unrivaled on the surface of the Earth. The Amazon River is the largest in the world by any measure, including maximum length from mouth to most distant headwater tributary (6,712 km or 4,195 mi), total catchment area (7.05 million km2 or 2.72 million mi2), area of seasonally flooded wetlands (250,000 km2 or 96,530 mi2), average annual water discharge (219,000 m3 second-1), and proportion of global river surface area (25-28%) (Goulding et al. 2003). Near its mouth as it approaches the Atlantic Ocean, the Amazon is so wide that one cannot see across it from one bank to the other. Here the Amazon flows inexorably like an inland freshwater sea, discharging a volume of water into the Atlantic so immense that it accounts for about one-sixth to one-fifth of all the Earth's river water, depending on the year.
Many Amazonian headwaters arise as glacier and snow melt high in the Andes (>5,000 m or >16,400 ft), eroding the steep mountain slopes as they fall, and carrying with them a high sediment load. Other headwaters arise from clayey and sandy soils deep in the rainforest, where they are stained red by acidic plant compounds. Yet others originate on the crystalline granites of the Brazilian and Guiana shields, where the waters run clear. Some of these black- and clearwater headwaters of the Amazon basin are connected by permanent rivers (e.g., Casiquiare Canal), seasonally flooded swamps (e.g., Rupununi savanna), or occasional stream capture events, to headwaters of the adjacent Orinoco River and coastal rivers of the Guianas. Altogether these river basins constitute a biodiversity province known as Greater Amazonia (figs. 1, 2).
Greater Amazonia extends over more than 8.4 million square kilometers (3.2 million mi2) of northern South America. This enormous region is drained by hundreds of thousands of kilometers of terra firme (nonfloodplain) streams and small rivers that flow under a closed forest canopy, and tens of thousands of kilometers of larger lowland rivers that meander across broad and sunlit floodplains. At the start of the twenty-first century, most of Greater Amazonia remains covered with dense tropical rainforests. The region also includes other distinct ecosystems, such as the cloud forests in the Andean piedmont and the tabletop mountains (tepuis) of the Guiana Shield, seasonally flooded wetlands (Llanos) in the central Orinoco basin, seasonally burned tropical savannas (Cerrado) in central Brazil or Lavrado (also called Gran Sabana) in the western Guiana Shield, and coastal estuaries at the mouths of the Amazon (Marajó) and Orinoco (Amacuro) rivers.
The ecosystems of Greater Amazonia are home to the greatest concentration of species on Earth. This region is the global center of highest species richness for many groups of organisms, including flowering plants, insects, birds, mammals, reptiles, and amphibians. Greater Amazonia is also the center of diversity for continental (freshwater) fishes. From the torrential headwaters cascading off the Andes, to the murky waters of the large lowland river channels and floodplains, the fishes of Greater Amazonia thrive in astonishing abundance and diversity. To date more than 3,000 fish species have been described from Greater Amazonia, and dozens of new species are described every year.
This field guide provides descriptions and identification keys for all the known genera of fishes that inhabit Greater Amazonia. It summarizes our current state of knowledge on the taxonomy, species richness, and ecology of these fish groups, and provides references to relevant literature for species-level identifications. It is our sincere hope that the Field Guide to the Fishes of the Amazon, Orinoco, and Guianas will be useful to anyone interested in quickly and accurately identifying Amazonian fishes, including aquarists, aquatic biologists, ecotourists, environmental engineers, sport fishers, and fish taxonomists.
Evolutionary History of Amazonian Fishes
Paleogene Origins of Major Groups
Amazonian fishes trace their evolutionary origins to the super-greenhouse world of the Late Cretaceous and Early Cenozoic (120-50 million years ago or mya; Albert and Reis 2011). This was a time of Earth history without polar ice sheets, when tropical climates extended to high latitudes, and tropically adapted organisms like palm trees and crocodilians lived in the lands we call Greenland and Alaska today. Neotropical fishes diversified in concert with the major groups of plants and animals that dominate modern tropical rainforest ecosystems (Lundberg et al. 1998).
Over these immense time periods, different groups of Amazonian fishes diversified under a wildly diverse set of environmental conditions. Some of the most important general influences were the relatively stable, warm, and wet climates that prevailed globally at low latitudes for most of the Paleogene (66-23 mya), regional hydrological and climatic changes associated with rise of the Northern Andes and formation of the modern river basins during the Miocene (22-5 mya), and global cooling and eustatic sea-level changes during the Pliocene (5-2.6 mya) and Pleistocene (2.5-0 mya) (Albert and Reis 2011).
In terms of species richness, total abundances, and fish biomass, the Amazonian fish fauna is dominated by three major groups: Characiformes (including piranhas, tetras, and relatives), Siluriformes (catfishes of diverse sizes, shapes, and natural histories), and Cichlidae (including peacock basses, freshwater angel fishes, oscars, and relatives). Fossils from each of these groups ascribed to modern families and genera (Tremembichthys, Corydoras,Gymnogeophagus) have been discovered in Paleogene sediments (López-Fernández and Albert 2011). These fossils are direct evidence that at least some Neotropical fishes had diversified to modern forms more than 40 million years ago.
The formation of Amazonian fish fauna was thus a long, long time in the making. These myriad forms accumulated over the course of tens of millions of years, and across a geographical arena that included the whole continent of South America. In other words, Amazonian fishes did not arise as the result of a recent or rapid adaptive radiation. The great antiquity of Amazonian fish lineages is perhaps surprising, as they are much older than the rivers and drainage basins in which they live. The modern Amazon and Orinoco basins are in fact relatively young features of the South American landscape, having achieved their modern configurations from tectonic and erosional processes only in the past 10 million years or so, in association with the rise of the Northern Andes (Hoorn et al. 2010).
Formation of Megadiverse Fish Species Assemblages
The ecological and evolutionary reasons for the incredible diversity of Amazonian fishes have been debated for more than a century. Among the most influential hypotheses are the great age and size of the drainage system (Lovejoy et al. 2011), habitat succession and niche diversity (Lowe-McConnell 1987), the high proportion of lowlands with stable environmental conditions (capable of supporting a large abundance of fishes; Henderson and Crampton 1997), and a long and diverse history of river capture events (see section below).
As with most species-rich ecosystems worldwide, the Amazon is both a museum and a cradle of biodiversity. From the perspective of biodiversity, a museum is a place where species accumulate through dispersal and persist by resisting extinction, and a cradle a place where species are born through the process of speciation. The principal evolutionary forces that affected the formation of fish species assemblages are speciation and dispersal, which in combination serve to increase the number of species in a region or a basin, and local extinction, which serves to reduce the number of species. These three processes are interrelated: all else being equal, a species with a higher dispersal rate has more gene flow and therefore a lower chance of becoming genetically fragmented. That is to say, dispersal acts to reduce the chances for a population near the periphery of a species' geographic range to become genetically isolated and eventually form a new (daughter) species. By the same token, dispersal and gene flow reduce the chances that a species will become geographically restricted, thereby reducing its risk of extinction. By this reasoning, species with higher dispersal capacities are expected to have lower rates of both speciation and extinction, or in other words, lower species turnover. Contrariwise, species with low dispersal abilities should have higher rates of species turnover.
The effect of dispersal capacity on evolutionary diversification is well illustrated by comparing the catfish families Loricariidae and Pimelodidae. Loricariids attain relatively small adult body sizes (avg. 16.2 cm SL; size data in Albert et al. 2009), and most loricariid species have relatively low dispersal capacities, increasing the chances for geographic isolation and speciation. Indeed, more than 400 loricariid species are known in Greater Amazonia. These numbers stand in strong contrast to the large-bodied migratory catfishes in the family Pimelodidae (59 species with an avg. size of 60.5 cm SL), many of which are widely distributed over much of the Amazon or Orinoco basins. Reduced dispersal capacity has in fact been implicated in the diversification of all three of the most species-rich AOG fish families: Loricariidae, Characidae, and Cichlidae. Species in these families generally have relatively small adult body sizes (<20 cm SL) and restricted geographic distributions (Albert et al. 2011a). Of course many other factors may contribute to differences in the diversity of fish groups. For example, species can differ in their inherent capacity to diversify morphologically. The exceptional species richness of the Loricariidae may also be related to adaptations of the oral jaws, facilitating specializations in feeding ecology (Schaefer and Lauder 1996, Lujan and Armbruster 2012).
River and Stream Capture
How do fish populations get isolated and form new species, in one of the wettest continental regions on Earth? For freshwater organisms, landscapes are divided naturally into discrete drainage basins by watersheds, boundaries that separate different river basins. Watersheds are not, however, constant landscape features but rather change through time. River or stream capture occurs when an upstream portion of one river drainage is diverted into the downstream portion of an adjacent basin, shifting the location of the watershed divide. River capture can occur from multiple geophysical causes, including headward or lateral erosion, differential geophysical uplift or subsidence, or natural damming from landslides or mudflows.
River capture by erosion is a perennial Earth history process that episodically isolates and reunites portions of adjacent waterways and their resident biotas. By merging portions of adjacent basins, river capture allows species ranges to expand and increases gene flow among populations, thereby lowering extinction risk. By isolating other portions of the same basins, river capture also divides species ranges and reduces gene flow, thereby increasing genetic isolation and the chance for speciation. These twin effects of river capture on biotic diversification are especially pronounced in regions like the Amazon and Orinoco basins, with low (flat) topographic relief and readily eroded sediments, and therefore higher rates of river capture. Over the course of millions of years, river capture likely made a strong contribution to the assembly of basinwide fish faunas, and the accumulation of astonishing fish species diversity.
Amazonian Fish Diversity
Major Classes of Fishes
Continental freshwater fishes worldwide may be classified into three categories based on ecological and physiological criteria (Myers 1966, Matamoros et al. 2015). Primary (obligatory) freshwater fishes have little or no tolerance to salt or brackish water, inhabiting water with less than 0.5 grams total dissolved mineral salts per liter (i.e., <0.5 ppt). As a result, marine water is an important barrier to dispersal in primary freshwater fishes. Primary freshwater fishes of Greater Amazonia include the Ostariophysi and four other families that originated and diversified in freshwater habitats and were isolated in South America on its final separation from the remaining portions of the ancient supercontinent Gondwana. The Ostariophysi comprise approximately 75% of all freshwater fishes on Earth, and in South America this group includes the Characiformes (>1,700 species), Siluriformes (>2,000 species), and Gymnotiformes (>200 species). Primary freshwater fishes of Amazonia also include the paiche or pirarucu Arapaima gigas (Arapaimidae), the largest freshwater fish with scales in the world, two species of arowanasOsteoglossum (Osteoglossidae), three species of leaf-fishes (Polycentridae), and the South American lungfish Lepidosiren paradoxa (Lepidosirenidae).
Secondary freshwater fishes have greater tolerance to brackish waters but normally occur in continental aquatic systems rather than in the sea, and they are capable of occasionally crossing narrow marine barriers. Secondary freshwater fishes of Greater Amazonia include the Cichlinae (>500 species), Rivulidae (>270 species), Cyprinodontidae (>60 species), and livebearers of the families Anablepidae (about 17 species) and Poeciliidae (>250 species).
Peripheral freshwater fishes are members of otherwise marine groups and exhibit high salt tolerance. Peripheral freshwater fishes of Greater Amazonia include representatives of many otherwise marine families that invaded and specialized for life in freshwaters at different times during the history of the continent. Most of these groups include one or a few species. Peripheral freshwater fishes of Greater Amazonia include freshwater anchovies (Engraulidae), drums (Sciaenidae), flatfishes (Achiridae), gobies (Gobiidae), needlefishes (Belonidae), puffers (Tetraodontidae), and stingrays (Potamotrygonidae).
Composition of the Amazonian Fish Fauna
As with most faunas, Neotropical fish species are not distributed equally among higher taxa. More than 90% of the fish species in Greater Amazonia belong to only four taxonomic orders (fig. 3): Siluriformes (catfishes), Characiformes (tetras, piranhas, and relatives), Perciformes (e.g., cichlids), and Cyprinodontiformes (killifishes and relatives). Such an unbalanced pattern of species richness is also observed at other levels of the taxonomic hierarchy (table 1; fig. 4). For example, a single characiform family, Characidae, includes 581 species in the AOG region, or about 53% of all characiform species in the AOG region. Similarly, the siluriform family Loricariidae includes 403 species in the AOG region, or about 36% of all siluriform species in the AOG region. At the other end of the diversity spectrum, 24 fish families in the AOG region are represented by fewer than 10 species each, containing a total of just 99 species in all (3% of the AOG fauna). These species-poor families include eight families known from only one or two species, like Arapaimidae (2 species), Batrachoididae (2 species), and Lepidosirenidae (1 species).
Of course many new species are described each year, and additional species may eventually be described in these species-poor families as well. Yet the larger pattern is inescapable: just a few families in the AOG region are very species rich, while most of the families are not. As in most parts of the Tree of Life on Earth, species-rich higher taxa are rare, and species-poor higher taxa are common.
Comparisons with Other Faunas
The AOG fish fauna listed in this Field Guide comprises 3,000 species that inhabit an area of about 8.4 million km2, or an area slightly larger than that of the contiguous United States (8.08 million km2), or slightly smaller than that of Europe west of the Ural Mountains (10.18 million km2). These numbers correspond to a fish species density (species/km2) in the AOG region about 7.1 times greater than that of the contiguous United States, or 6.1 times greater than Europe's (fig. 5). This spectacular diversity is reflected in the large number of genera (564) to which these species have been assigned, compared with other biogeographic provinces.
Despite the high diversity of fish species and genera in the AOG region, this diversity represents relatively few distinct evolutionary lineages, as represented by higher taxonomic categories like orders and classes. This overall pattern reflects the geographic isolation of South America throughout most of the Cenozoic, a condition referred to as "splendid isolation" (Simpson 1983).
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Excerpted from "Field Guide to the Fishers of the Amazon, Orinoco & Guianas"
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