ECOLOGY OF FISHES IN THE AMAZON AND CONGO BASINS TYSON R. ROBERTS Abstract. Some relationships between the great a single group, the Ostariophysi. In con- diversity of fishes and physical and biological trast to tropical marine shore fishes, which aspects of the environment in the Amazon and have had only one principal center of radi- Congo basins are discussed. Aspects of physical ation—the Indo-Pacific— in environment considered are rainfall regimes; ostariophysans stabilizing influence of forests; seasonal fluctu- Africa, South America, and Eurasia have ations in water white le\el; water, clear water, radiated largely in isolation from one an- and black water; relative of Ama- accessibility other. Asia and Africa, of course, share zon and Congo basins to marine fishes; tidal elements. There is conditions in the lower Amazon; shoreline and many ostariophysan islands; river anastomoses and connections with some indication that the South American adjacent basins; stream captures; unique or and older x\frican ostariophysan elements peculiar causes of massi\e mortalitv' of biotypes: had a common origin. The fonnation of fishes. The main adaptive significance of parental the Amazon basin provided the opportunity care in Amazon and Congo fishes is e\identh for a remarkable radiation of related to reproduction in deoxygenated waters. Ostariophysi. The following topics are considered under bio- The Amazon and Congo basins have more logical interactions: fishes with brilliant coloration kinds of fishes than any other river basins or association of similarh' conspicuous markings; in the world, and both exhibit a high de- colored species of fishes in mixed schools; fishes gree of endemism. of minute size: responses to predation; and par- One of the reasons fish titioning of food resources. Aliout one third of principal tropical the paper is devoted to discussion of trophic faunas are richer than temperate ones is adaptations and feeding habits. The topics con- that the\- ha\e not been as adversely af- sidered are predatory fishes; scale-eating chara- fected by glaciation. Pleistocene glaciation coids; fin-eating characoids; feeding habits of probably caused extinction or withdrawal "parasitic" trichomycterid catfishes; parallelism in of marine fishes in lati- feeding habits of "weakl\- electrogenic" fishes in shore the higher the Amazon and Congo basins; plankton-feeding tudes, especially in the North Atlantic and habits of fishes; deposit feeders; feeding (Briggs, 1970). It is unlikely that fi.shes characins and other fishes in Amazonian rain- live more than a few miles inward imder forest streams. the ice caps. Freshwater fishes in most of North America and northern Eurasia were INTRODUCTION wiped out by Pleistocene glaciation. West- An overwhelming proportion of the spe- ern Europe now has a depauperate fish cies of fishes in continental fresh waters fauna of only about 60 species, all or al- are primarv' freshwater forms. This means most all derived from stocks that popu- that they are unable to live in salt water, lated the area in postglacial times. and have had a long history separate from It may be well to express here my suspi- that of marine fishes. The great majorit\' cion that the richness of the Congo and of these freshwater fishes are members of Amazon fish faunas is not necessarilv an- Bull. Mus. Comp. Zoo)., 143(2) : 117-147, February, 1972 117 118 Bulletin Museum of Comparative Zoology, Vol. 143, No. 2 cient. Fishes have undergone considerable studied Mississippi, but the numbers diversification, with great increase in the known from the Amazon and Congo will number of species, in lakes less than five undoubtedly increase considerably as syste- million years old. I do not believe that matic studies continue. lakes represent the only ecological situation The limnology, seasonal changes and in which explosive adaptive radiations of some biological aspects of the rivers, lagos, fishes have occurred. It is conceivable that flooded forest (igapo) and streams the present Amazonian fish fauna, with its (igarapes) that constitute the main habi- large number of species, is the product of tats of Amazon fishes are described by only a few million years of evolution from Sioli (1964, 1967) and Fittkau (1964, an original stock of two or three hundred 1967). The ecology of fish habitats in the founder species. Some of the founders, of cuvette centrale of the Congo basin, similar course, would represent groups of con- in many respects to that in the Amazon, is siderable antiquity. described by Gosse (1963) and Matthes Draining two and a quarter million (1964). McConnell (1964) described the square miles, the Amazon basin is the ecological groupings of fishes and effects largest river basin in the world. Its mouth of the seasonal cycle on the fishes in the discharges an average of three to four Rupununi savanna of British Guiana. Many million cubic feet of water per second. The of her remarks are applicable to the Ama- Congo, discharging 1.4 X 10" cubic feet zon basin, especially to the savanna parts of it. if in per second from sliglitly over one and a Most, not all, of the species the half million square miles, is the second Rupununi are present in the Amazon. Mc- Connell 1969 reviewed factors con- largest. The vast area of these basins, with ( ) some abundant water and varied habitats, un- tributing to speciation in tropical fresh- water of doubtedly contributes to the large number fishes, and many her remarks of fish species in them. Habitats such as apply indirectly or directly to fishes in the and basins. streams with high gradients or streams Amazon Congo Knoppel (1970) in detail on the contents draining dry ground (igarapes de terra reported stomach of a number of fish from finua) are sometimes separated by hun- large species dreds of miles. Meandering creates a small rain forest streams near Manaus. regular succession of habitats in the main Myers (1947, 1949a) gave a general ac- count of Amazonian fishes and their courses of the big rivers. The high per- centage of the basins at base level, along ecology. Tlie main of this is divided with the relatively stable existence of so part paper into two sections. first section deals much aquatic habitat, favors the existence The with the interactions of fishes and of very large numbers of individuals, which physical in turn is conducive to the existence of aspects of environment in the Amazon and the second section with large numbers of species (Preston, 1962). Congo basins, interactions fishes in the As of 1967 (the last year for which the biological among Amazon and basins. The rest of Zoological Record has been issued) ap- Congo this introduction a brief sketch of proximately 1300 species of fishes had been provides recorded from the Amazon and 560 from the main groups of ostariophysans and the Congo (including the Lualaba River other fishes under consideration. Readers but not lakes Bangweolu and Moeru). The familiar with these groups may turn di- Mississippi basin, in comparison, with an rectly to the main part of the paper if they as wish. In drafts of this I tended to area ( 1,244,000 square miles ) almost paper large as that of the Congo basin, has only include extraneous notes about species oc- 250 species. It is unlikely that many species currence, etc., which might distract readers remain unrecorded in the relatively well- interested mainly in the discussion of ecol- Amazon- and Conc.o Fish Ecology • Roberts 119 it seems the ogy and adaptations. These notes, indicated ception, highl\ likely Weberian has increased the by arabic numerals in the text, are given apparatus op- fish at the end ot the paper. They are of con- portunitic\s for interactions between zoo- cern primarilx' to systematists and species. Characoids. Characoids or characins are geographers. mostly laterally-compressed, open-water fishes, active in the da\'time, many of them Ostariophysi—the Predominant or iridescent. Tlie\ usually have Fishes in Both Basins silvery jaw teeth, often of a highly complex nature In the Amazon 43 percent of tlie fishes (Roberts, 1967), and invariably lack bar- 3 are characoids, 39 percent siluroids, and bels. \\'\\\\ few exceptions, they are not 15 percent gymnotoids. In the Congo per- known to produce biologically significant cent are characoids, 23 percent sihu'oids, sounds (almost all fishes produce noises and 16 percent c>prinoids. All of these incidental to feeding and locomotion). Of fishes belong to the order Ostariophysi, the large groups of fishes inhabiting the which thus comprises 85 percent of the earth's fresh waters, characoids (as a Amazon's and 54 percent of the Congo's group) exhibit the least tolerance for salt fish fauna. Ostariophysi differ from all or brackish water. They occur onl\' in other fishes in the manner in wliich some of Africa and Central and South America. the neural arches and ribs of the first four Tlieir presence in Central America un- vertebrae are modified into an apparatus, doubtedly is the result of recent invasion, the Weberian apparatus, which conducts and it is unreasonable to believe that they vibrations from the swim bladder to the could have reached Africa and South inner ear. There is no precise understand- America by w^ay of Europe and North ing of how the Weberian apparatus affects America without leaving the least trace of sound (and pressure?) perception, nor is their passage (Myers, 1966). The only much known about the effects of sound on fossils identified with certainty as characins the behavior of ostariophysans in nature. It are African, or South or Central American the\' is however, that are 1960 . In the of exidence generally agreed, ( Weitzman, ) light "acoustic specialists," and that their world- for Continental Drift and for characoid wide in fresh waters is some- et al., Roberts, predominance antiquity ( Greenwood 1966; how linked with the Weberian apparatus.