Central amazonia and its fishes

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C E N T R A L A M A Z O N I A A N D ITS FISHES*

WILLIAM L. FINK and SARA V. FINK Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, U.S.A.

(Received 5 April 1978)

Abstract--l. The Amazon river system is characterized by its great size (it drains an area of about 6.5 million km 2 ), its great depth (to 90 m or more in some places), the flat topography of its drainage basra, the annual cycles of high and low water periods, and the geological structure of the drainage area.

2. The central Amazonian ecosystem is complex and the types of habitat within it are numerous, with broad overlap and intergradation of categories. An especially important factor in the aquatic habitat is water type ("white", "black" or "clear"), a characteristic which is related to the geological character and the flora of the drainage area.

3. Cyclical changes in water level and associated fluctuations in the availability of oxygen are two factors which exert considerable influence over the biology of central Amazonian fishes.

4. The collection sites of Alpha Helix Phase IV are described briefly. 5. The systematics and evolution of most Amazonian fish groups are poorly known; the inabihty

to identify all fishes examined and lack of well-tested evolutionary hypotheses of relationship on which to base comparisons pose problems for the comparative biologist.

6. A brief survey of general aspects of phylogeny and of the natural history of fishes examined by Phase IV is presented, including, when possible, information about habitat ~references, food and other aspects of the biology of the fishes.

As a prelude to the following papers on comparative biochemistry and physiology of the hemoglobins of some Amazonian fishes, an attempt is made below to sketch briefly some basic facts about the Amazon river system and the lowland Amazonian ecosystem, and about the biology and evolutionary relationships of Amazonian fishes. The aim is to put the biochemical and physiological aspects of these fishes into the context of both the organisms of which they are a part and the ecosystem to which they represent an adaptive response. Special reference will be made to the environment and the fishes encountered by the Alpha Helix Phase IV expedition to central Amazonia (see Fig. I).

PHYSICAL CHARACTERISTICS OF THE RIVER SYSTEM

While size is not the only reason for the impressive- ness and unique character of this river system, it is certainly a major contributing factor. The Amazon River is by far the largest freshwater drainage in the world. The outflow of the Amazon constitutes 15-20~ of the fresh water entering the oceans in a year; on the average, 218 m 3 of water pour into the Atlantic every second (Sioli, 1967). "With a flow five times that of the Congo and 12 times that of the Mississippi, the Amazon. . . disgorges as much water into the Atlantic every 24 hours as the Thames carries past London in a year" (Meggers, 1971). The Amazon is navigable to ocean liners as far upriver as Iquitos. Peru, more than three-quarters of the way across the continent. These figures are perhaps more compre-

* A Portuguese translation of this work will appear in Acta Amazonica.

13

hensible when one considers that the Amazon and its tributaries drain a very large portion of the continent of South America--approx 6.5 million km 2 (including the Tocantins drainage)---and over most of that drainage area, the annual rainfall is high, ranging from 1500 to 3000mm annually (Haffer, 1974).

The depth of the Amazon itself and of several of the major tributaries, at least in their lower reaches, constitutes another unusual feature of the Amazonian river system. The depth of the lower course of the Amazon (east of the confluence with the Rio Negro) averages 25-30m and increases to two and three times that in some places (Lowe-McConnell, 1975; Sioli, 1967; Meggers, 1971); a U.S. Geological Survey team found 10 places in the lower Amazon with a depth of 90m or more (Sioli, 1967). Most major affluents to the Amazon form very deep "mouth- lakes" where they enter the Amazon; the depth of the Rio Negro "'mouth-lake" has been recorded at 93 m, far below sea level. These "mouth-lakes" appear to be fluvial erosion valleys, cut as a result of the drop in sea level during the last glaciation and inundated by the subsequent rise in sea level (Marlier, 1973). In central Amazonia, this unusual depth in- sures that major river channels will be of considerable size even during the periods of lowest water levels.

A third major factor contributing to the unique character of this river system is the remarkably fiat topography of the majority of the basin area. Except- ing only the Andes at the western rim of the continent, there are no major mountain ranges in the Amazonian drainage. A Tertiary freshwater lake bed covers a large, wedge-shaped area immediately east of the Andes and extendsin a band along the course of the lower Amazon; most of the remainder of the drainage is formed of the Precambrian and Paleozoic

14 WILLIAM L. FINK and SARA V. FINK

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igneous and metamorphic rock of the Brazilian and Guayanan shields and is gently rolling, punctuated at intervals by low ridges representing the roots of ancient mountains. As a result of this topography, there are few extensive rapids in the basin outside of the Andes, and the Amazon itself drops only 65 m between the eastern border of Peru and the Atlantic Ocean, a distance of nearly 3000 km (Meggers, 1971).

Another major factor shaping the character of this river system is the annual cycle of high and low water periods. Precipitation in the Amazon drainage is seasonal in intensity, creating fluctuations in water level of from 6m (at Iquitos, Peru), to 17m (in the upper reaches of the northwest tributary Japurfi), to 4 m at the very broad mouth of the Amazon. In central Amazonia, a crest of from 10 to 15m, occasionally more, occurs on the main river channels around June (Marlier, 1973; Lowe-McConnell, 1965).

A final factor shaping the character of the Amazon river system, and Amazonia as a whole, consists of the geochemical attributes of the drainage area. Neither the ancient Precambrian and Paleozoic rocks of the Brazilian and Guayanan shields nor the extensive Tertiary lake-bed deposits provide appreciable amounts of the minerals and salts necessary for life support. With the exception of restricted outcrop- pings of Carboniferous rocks north and south of the lower Amazon valley, localized in their impact, only the young rock of the Andes yields such nutrients in significant amounts. As a result, a sharp distinction exists between the water and soil chemistry of the Amazon floodplain, where the waters are comparatively rich and the soil receives annual renewal in the seasonal floods, and that of most of the remainder of Amazonia, where the waters and soils are very low in minerals and almost lacking in salts (Sioli, 1968).

CENTRAL AMAZONIAN AQUATIC HABITATS

The physical characteristics sketched above set the stage for the apparently complex and as yet little known Amazonian ecosystem. Some aspects of this ecosystem, particularly those which affect the fish fauna, are discussed in brief form below. An attempt is made to characterize discrete types of habitat, but a great deal of overlap and intergradation between these types must occur, due in part to the fluctuations in water level and resultant changing and intermin- gling of habitat types.

Certainly, the aquatic formations in central Ama- zonia defy easy description in the language of temperate zone limnology. The three factors of seasonal fluc- tuation in water level, a perennially extensive riverine system, and low topographical relief combine to produce a complex and extensive aquatic "landscape" in which riverine and lacustrine conditions merge. Formations on the floodplain of the main river (the "v~rzea") include not only the main river channel, but a network of lakes, sidearms, and anastomosing streams and channels (a small sample can be seen in Fig. 2a). Flooded forest Cigap6") and swampy grassland ("campos") also occur, sometimes merging into open water formations (Fig. 2b). The area over which such v~irzea formations occur is extensive, since the vfirzea averages 48 km wide (Lowe-McConnell, 1975) and is much wider, to 100 km, in some areas

(Junk, 1970). The amount of lake-like habitat on the v~irzea is much more extensive during high water than low water periods, since the various formations peripheral to the main river channel increase their volume by many cubic meters and in many cases remain largely dammed for a period of several months (Mar- lier, 1967).

Aquatic formations outside the v~irzea are also of considerable variety and undergo similar kinds of transformations during periods of high water. These formations include streams ("igarap6s") and rivers ("rios") which flow over high ground outside of the Amazon floodplain ("terra firme"); lake-like formations in the middle and lower courses of such rivers and streams; and flooded or swampy high forest, low forest, and grassland. As on the v~.rzea, the lacustrine characteristics of the broad, lake-like portions of the rivers and streams increase during periods of high water, when the inundation zone (igap6) along the watercourses is flooded and high waters downstream stem the flow of effluents (Marlier, 1967). Swampy or flooded forest, shrub and grassland may occur in valleys relatively isolated from nearby rivers and streams or form the igap6 along flooded water- cour se s .

Within many of these aquatic formations, waters of varying characteristics can occur. In central Ama- zonia, the waters can be classified into three general types, although it must be emphasized that inter- mediates do occur. These three types are "white", "black" and "clear". The differences in water quality appear to be associated with differences in the geological characteristics, and, in the case of "clear" and "black" waters, the type of soil and vegetation present in the catchment area of each watercourse (Sioli, 1964). The "white" water rivers flow out of the Andes and are given their pale muddy color by the large load of suspended silt; the Amazon itself remains essentially a white-water river all the way to its mouth in spite of the major contribution it receives from rivers of other water types. Deriving its chemical characteristics from the young Andean rock, this water is comparatively rich in minerals and salts and has a higher pH value--about neutral--than most other Amazonian waters. "White" water constitutes an important, but not the only, ingredient in most v~irzea formations peripheral to the main river channel; small streams composed only of clearer, ground runoff waters also occur. Depending on the time of the year, lakes, sidearms and channels receive "white" water and water from rain and ground runoff in varying proportions (Schmidt, 1973; Marlier, 1967).

Over most of Amazonia, the remaining two water types, "black" and "clear", derive their mineralogical content from the ancient rock of the Brazilian and Guayanan shields or from the Tertiary fluvial and lake sediments eroded from those ancient shield deposits. As a result, they are very poor in minerals and salts and more acid than "white" water (Klinge & Ohle, 1964; Sioli, 1968) with pH values varying from about 4 or 5 in the small streams to just slightly acid in the lower courses of major rivers (Sioli, 1967). The differences between the two water types seem to be associated with differences in ground water level, soil type and vegetation type. However, it must be stated that although "clear" and "black" water types are


distinct from each other in their most pronounced forms, they appear to grade into each other in their less pronounced forms. In the rain forest stream, for example, are found waters which range in color value from yellowish to olive to brown or red-brown (Sioli, 1964; Fittkau, 1967), and typing the waters of some vfirzea formations is equally problematic. In general terms, however, the differences between these two water types can be characterized as follows.

"'Clear" waters, according to Sioli (1964), arise in localities where the soil is loamy, the vegetation consists of typical high rain forest ("mata'), and the land is not subject to extended inundation or leaching. The clayey soils from which "clear" waters flow, although poor, tend to be slightly higher in mineral content and lower in humic content than the soils from which "black" waters flow (Stark, 1970), making "clear" waters slightly higher in minerals and less acid than "black" water (Klinge & Ohle, 1964; Sioli, 1957). The predominance of"clear" water igarap6s in the catchment areas of the lower Amazon tributaries Rio Xingu and Rio Tapaj6s makes these the major ~'clear" water rivers in Amazonia.

"'Black" waters, in their pronounced form, appear black in situ and yellowish or tea-colored in a glass. According to current hypotheses, these waters ori- ginate in flat land that is highly leached and often poorly drained, remaining swampy for many months each year (Marlier, 1973). In central Amazonia, these waters seem to be associated with a type of leached, porous, sandy soil (podzol), a soil which tends to be even lower in mineral content than the clayey tropical soils, and with a certain vegetation consisting either of a type of low rain forest ("caatinga" of the Rio Negro) or a related mixed tree and shrub vegetation ("campinas") (Sioli, 19671. The cause of the dark col- oration of the water is thought to be the presence of"humic acids", and perhaps other secondary plant compounds, leached from the superficial layer of fallen litter and soil humus of tropical podzols (Klinge & Ohle, 1964: Janzen, 1974). The most extensive area of podzols with caatinga or campinas in Amazonia occurs in the upper drainage area of the Rio Negro, and the Rio Negro is the major "black" water river in Amazonia; however, similar conditions have a spotty distribution through much of central Ama- zonia and many igarap6s and rios of the terra firme consist of this water type.

The characteristics of these water types have important consequences for the quality of the various habitat types in central Amazonia and for the modes of life of the fishes that live within them. The greater richness of "'white" waters supports a substantially greater aquatic plant development, including phytoplankton, swampy grasses ("campos"), and "'floating meadows", small islands of floating aquatic plants in- habited by a diverse fauna (Marfier, 1967; Junk, 1970). Particularly in many vfirzea formations peripheral to the main river channel, enriched by "'white'" waters but with lowered turbidity, phytoplankton development is relatively extensive (Fisher, this volume). This plant life in turn supports a rich and diverse animal community. As might be expected, the shoreline habitat appears to support both more biomass and greater numbers of species, in invertebrates as well as in fishes, than the open waters of lakes and

sidearms (experience of the senior author; Marlier, 1967); how the ichthyofauna of these two habitats compares with the fauna of the open waters of the main river channel is uncertain.

"'Black" and "'clear" water habitats of the terra firme seem fairly clearly to support less aquatic plant growth than "white" waters, with algae largely of the blue-green variety and only limited growth of a few kinds of aquatic higher plants (Fittkau, 1967; Marlier, 1967). The small amount of insolation of most rain forest igarap6s must be a factor in these low levels of primary production. In the relatively "clear" waters of the mata rain forest, however, these low levels are apparently no deterrent to a large and diverse ichthyofauna, which Fittkau (1967) found to number between 30 and 50 species. An indication of the importance of terrestrial food sources to fishes in rain forest habitats can be seen in Saul's (1975) study in Ecuador, where he found terrestrial insects, particularly ants, to comprise a large proportion of the diet of many fishes. In large "clear" water rivers, algal growth and "floating meadows" apparently occur (Sioli, 1964), although to what extent is unclear.

The amount and diversity of the fishes, as well as other organisms, to be found in "black" water habitats is an unsettled question. Legend has it that "'black" water rivers are poor in both numbers of species and absolute numbers of organisms, and a hypothesis about the "toxicity" of "black" waters due to leachates of plant defensive chemicals has been put forward (Janzen, 1974); but the extent to which the fauna of "'black" water habitats really differs from that of other, similar terra firme habitats has not been investigated. The "mouth-lake" of the Rio Negro apparently does not support primary production to the extent found in the Tapaj6s (Sioli, 1964), but whether this is due to compounds present in the water or to low levels of light penetration is not clear. Many species appear to be restricted to the "black" waters of the Rio Negro-Rio Orinoco (Venezuela) drainage, but some species occur in both "white" and "black" waters.

In summary, various combinations of aquatic for- mation, water quality, and associated primary plant production and food chains can occur. Although both the degree of faunal separation of the fishes among the resulting habitats and the amount of differentia- tion of modes of living within these habitats is unclear, an awareness of the profuse possibilities seems important for explorations in the biology of Ama- zonian fishes. For that reason an outline of the general habitat types, possible for each of the three water types, is given below; within each of these general types some niche separation on the basis of current and substrate might occur, particularly as indicated. Certainly for nearly all categories some niche separation on the basis of depth in the water column (benthic, midwater and surface layers) does occur. The niches within the igarap6 habitat are based on Fittkau (1967).

1. Open waters of rivers. 2. Open waters of lakes, sidearms and channels. 3. Shore areas of both of the above.

(a) Grassy (campo)---perhaps including the "floating meadow" habitat.

Central Amazoma and its fishes 19

(b) Forested. (c) Mud beach. (d) Sand beach.

4. Swampy areas. (a) Grassy (campos, campinas). (b) Forested (igapo, caatinga).

5. Streams (igarap6s). (a) Fast-running waters over hard substrate (sand,

roots or fallen wood, rock). (b) Slow waters and quiet pools over mud and lit-

tered substrate.

Two environmental parameters which exert considerable influence over the biology of central Ama- zonian fishes are the cyclical changes in water level and associated fluctuations in the availability of oxygen. Because of this considerable influence, and because their influence is felt across all habitats, these two variables merit special attention.

Since little is known of the life histories of most Amazonian fishes, the precise ways in which the cycle of wet and dry seasons affects the fishes can only be guessed at. However, the vast alterations in extent and quality of habitat have some general consequences that can be delineated. When waters are high, fishes are dispersed into the tributaries of the main rivers or over the savanna floor, and food is abun- dant. During the dry season, as water levels drop, many fishes are forced downstream or into pools and must accommodate to increasingly restricted living space. Food resources for most fishes (except fish pre- dators) become extremely scarce, to the extent that in many species feeding ceases altogether and stored fat reserves are utilized.

Oxygen tensions, although consistently lower than is usual in the fresh waters of colder climates, are especially low under the conditions of stagnation that occur during peaks of both high water and low water periods. In medium to large lacustrine formations, stable thermoclines can develop during periods of high water and result in hypoxic to anoxic conditions in lower water layers. Such hypoxia, apparently extending over several months, has been observed in lacustrine formations of the vfirzea but may be less pronounced in lakes of the terra firme (Schmidt, 1973; Marlier, 1967). Ponds and small lakes are more likely to experience periodic turnover due to wind-induced mixing, but when mixing stops or is prevented by surrounding forest, hypoxia may result, again particularly in lower water layers. Stagnation and hypoxia also occur in waters of the flooded forest, due primarily to lack of both wind mixing and photosynthesis under the shady canopy (Carter, 1934; Kramer et al., 1978). During the dry season, small ponds left by receding waters can develop extreme hypoxia when the biological oxygen demand per cubic meter is so high that even the surface water layers become de- pleted.

The various strategies Amazonian fishes have evolved to deal with oxygen scarcity and with the disappearance of vast areas of habitat during low water periods are some of the main themes of their evolutionary history. As might be expected, those fishes which inhabit benthic habitats or shallow, shoreline waters show the most striking adaptations. These adaptations include the ability to breath atmos-

pheric air (in Arapaima, Hoplerythrimls, Hoploster- num, Electrophorus, Synhranchus): to skim the oxygenated surface layer (Osteoglossum, some cyprinodonts); to estivate (the lungfish, Lepidosiren) or remain in damp mud for long periods (Synhranchus); and to produce eggs which can withstand desiccation and hatch when the water level rises again (some cyprinodonts). Some fishes, such as Hoplosternum and Synbranchus, can maneuver quite well on dry or nearly dry surfaces and are thus able to move from a dried pond to wetter areas. In addition, some fishes are able to utilize anaerobic metabolism to a large extent (Hochachka & Somero, 1971~ Hochachka, personal communication). Moreover, most Amazonian fishes are able to survive in water of much lower oxygen content and higher acidity than would support most temperate zone fishes.

Air-breathing fishes were of particular interest to Phase IV members. The obvious question to consider is whether these fishes have specialized hemoglobins to deal with this mode of life. The Amazon fauna presents a particularly good experimental model in this regard because of the presence of air-breathing in genera of phylogenetically distant lineages. These phylogenetically distant genera may be compared with each other and also with their non-air-breathing relatives. The most closely related air-breathing, non- air-breathing pair is Hoplias malabaricus (non-air- breathing) and Hoplerythrinus unitaeniatus (air-breathing). Other such pairs, though less closely related, are Osteoglossum bicirrhosum (non-air-breather) and Arapaima 9igas (air-breather), and some of the Ioricar- i~d catfishes.

THE PHASE IV COLLECTING SITES

The Phase IV collecting sites were in the area of the confluence of the Rio Solim6es (called on many non-Brazilian maps simply the Amazon River) and the Rio Negro (Fig. 1). Most collecting was done on the Rio Solim6es in an area about 50 km upriver from the confluence with the Rio Negro. In that area and during that season (November and December) the river level reaches its lowest point and begins to rise again. The ship was anchored on the south shore of the river near a channel which connected to a large lake, Lago Janauacfi. The channel itself (Fig. 2b, c) at this time averaged 20 m wide and was flowing from lake to river; during rising water levels on the river, the flow was reversed and channel width steadily in- creased. Most collections were made on this channel [called Paran~i de Janauac/t by Schmidt (1973)], but some collecting was also done both in the main river and in the lake.

About a week was spent at a site on the Rio Negro about 60km upstream from the confluence with the Solim6es. The ship was anchored on the eastern shore, near the mouth of the eastern tributary Rio Cuieiras. Most collecting was done on the Rio Cuieiras and the small affluents which entered it along its lower course (Fig. 2d).

The Rio Solim6es at the mouth of the Lago Janauacfi channel is characterized by silty, light brown water flowing at 6-8 km/hr. When the water was at its lowest, the bank was about 4 m high, with an angle of about 30-45 ° . Tall grasses covered the

20 WILLIAM L. FINK and SARA V. FmK

bank from its top to the low water mark; trees were not numerous nor were they densely grouped. Along the channel between Lago Janauac~ and the Soli- m6es, the bank ranged from very steep to almost fiat; during high water much of the low bank areas would be inundated completely. There were patches of dense, second-growth forest on areas above high water (Fig. 2c). Many small streams flowed into the Janauacfi channel (Fig. 2b) and these were fruitful collecting areas, partly because fishes congregated there early in the morning and partly because the gently sloped banks allowed relatively easy seining.

In the immediate area of the ship's anchorage there were no dwellings, but there were about 20 houses along the Lago Janauaca channel and a good many more on the lake itself. Agriculture was evident but not extensive m the area and a small brick factory was situated on the Janauac~i channel. The influence of the local people on the lake, river and outflow appeared to be minimal, limited for the most part to sewage disposal and, of course, fishing pressure.

The habitat on the Rio Negro and Rio Cuieiras (Fig. 2d) was quite different from that of the Rio Soli- m6es. The Rio Negro has water which is "'black" and of much slower flow (2-4 km/hr) than that of the Soli- m6es. The river banks were less steep than the Soli- m6es banks and a sandy or rocky beach was usually present. The forest in this area was less disturbed by man and much more dense than in the Solim6es area. The Rio Cuieiras had many small streams (igarap6s) feeding into it from its mouth to the limits of our survey about 5 km upriver. These small streams had a much slower water flow and rather low fish densities.

Human population densities on the Rio Negro/Rio Cuieiras were much lower than on the Rio Solim6es: agriculture was limited to the slash-and-burn type.

THE FISHES

Having described some basic elements of the aquatic ecosystem m central Amazonia, we would like now to give an overview of Amazonian fishes. This region presents an excellent experimental site for the comparative biologist and biochemist because of the nature of its ichthyofauna: it has a representative of the ancient lungfishes (Lepidosiren). several members of a very old teleost group (Osteoglossidae), a well- circumscribed, highly successful teleost group which has evolved in an "'explosive" manner (Ostariophysi), plus some highly evolved, advanced fishes such as cichlids and Synbranchus (Acanthopterygii).

The aim of this section is to give some idea of the size and diversity of the ichthyofauna as well as some general information about evolutionary relationships among the major groups of Amazonian fishes. Primary attention will be paid to discussion of the families of fishes with representatives in the Alpha Helix collections. This information must be preceded, however, by a brief caveat, in the form of a discussion of the current level of knowledge about the identification of species and hypotheses of the evolutionary relationships of most Neotropical fishes, and the resultant complications for the comparative biologist.

The technical ability to probe the biochemistry, hemoglobin, respiratory chemistry, and other physiological and biochemical aspects of Neotropical fishes far outstrips our current ability to present reasonable hypotheses of phylogenetic relationship or even to give names to the fishes involved. In fact, according to recent authors (B6hlke et al.: in press) the level of knowledge about the identity and relationships of South American fishes is about equivalent to knowledge of the North American ichthyofauna a century ago. When one realizes that, even today, nearly 10% of the North American fresh-water fish fauna remains undescribed (Jenkins, 1976), it becomes clear that an enormous amount of work must be done before systematic biologists will be able to provide comparative biologists, ecologists and other researchers with the kinds of information taken for granted in North America, i.e. the names of the fishes, reasonably good phylogenies and classifications, and at least some knowledge of natural history.

Every member of the Phase IV expedition felt the frustration of working with fishes to which no specific name could be applied, and at times even generic names were uncertain or of little meaning phylogenetically. Although the frustration must remain, perhaps its intensity may be alleviated if the reasons for the difficulties of identification are understood. As will be evident from the discussion below, the Amazonian fish fauna is immense, numbering into the thousands of species (many times the size of the North American fauna). Many of these species, perhaps as many as 30°~, are undescribed, and many more are known with certainty only from the original specimens described. A great number of species were described as new before modern systematic methods were in use and before investigators from North America and Europe realized the immensity of the tropical fauna. As a result, it is virtually impossible to identify many South American fishes from the literature or even from the few museum reference collections. Instead, original type specimens must be examined (when they exist), often a task of enormous cost and labor. Unfor- tunately, the difficulty and expense of making collections and the limited support for long-term storage and curation handicaps the rate of progress in in- creasing our knowledge of the fish fauna.

Other factors complicating species identification need mention [see Btihlke et al. (in press) for further information]. These include the explosive evolution in many groups of these fishes (e.g. several genera include 100 or more species), the complex evolution of many morphological characteristics leading to con- fusion over delineation of generic limits, and much convergent and reductive evolution. Thus, even the most fundamental and perhaps simplest task of systematic biology, the naming and cataloguing of the fauna, is a long-term and serious problem.

The other aspect of modern systematic biology, as important to comparative biologists as identification of species, is the task of hypothesizing evolutionary (phylogenetic) relationships and the production of classifications which represents those relationships. Because of the evolutionary complexities mentioned above, the creation and testing of hypotheses of phylogeny in South American fishes is in its infancy. Most classifications of these fishes, at the generic and


even the family levels, are entirely typological and reflect general evolutionary grades rather than genealogical relationships. Higher categories of fishes are less typological, at least among the groups referred to in this paper.

The importance of an awareness of genealogical lineages can perhaps be best emphasized by mention of the enormous time periods between the origins of the major fish groups studied by Phase IV scientists. For example, the lungfish Lepidosiren (Subclass Sar- copterygii) shared a common ancestor with the other fishes studied by Phase IV over 350 million years ago and has had an independent history since that time. Indeed, in terms of genealogical relationships, the lungfishes are more closely related to Homo sapiens, as descendents of sarcopterygian fishes, than to any other living fishes with the exception of Latimeria, the coelacanth. The arapaima and arowana (Super- order Osteoglossomorpha) shared a common ancestor with the characoids and catfishes (Superorder Ostariophysi) and with the cichlids and halt'beaks (Superorder Acanthopterygil) about 2130 million years ago, during the age of the dinosaurs. The common ancestor of the ostariophysans and acanthoptery- glans, in turn lived about 160-190 million years ago and these two groups have been evolving indepen- dently ever since. In view of these long time periods, the resemblances among these fish groups may indeed be more surprising than the differences. Presumably the aquatic habitat has placed certain limitations on body form and function, selecting for similar mechan- isms to deal with a similar environment. In spite of general similarities in appearance, it is important for the comparauve biologist to be aware of this evolutionary history and to examine research results from its perspective. When. for example, one compares the blood of Lepidosiren. the lungfish, with that of the air-breathing characoid Hoplerythrinus, one must keep in mind that 350 million years of independent evolution separate these two forms, a far greater expanse of independent history than that which separ- ates man from lizards.

A word about the phylogeny and classification used in this paper is in order. The past decade has been a time of great change m concepts of classification and phylogeny, with regard both to methods and to philosophy. Current concepts of the relationships of higher groups of fishes are at variance with those of a decade ago, but we are now in a period of flux and opinions regarding these relationships are widely varied. What we adopt here, for the teleosts at least, is the classification of Greenwood et al. (1966).

In the descriptions below, major groups--superorder and above--are discussed in approximate order of phylogenetic origin ; within these groups, no phylogenetic ranking is to be inferred unless explicit state- ments about relationships are made. In some in- stances, an entire family can be dealt with in a few sentences; in others, genera or species are discussed in more detail. The families, genera, and, when possible, species represented in the Alpha Helix Phase IV collections are listed in the classification in Table 1.

The oldest fish group represented in Amazonian waters is that of the cartilaginous fishes, class Chon- drichthyes, including sharks, rays and skates. One family of sharks and two families of rays, including

one family of sawfishes, occur in Amazonia. The only elasmobranchs collected by Phase IV were freshwater stingrays, genus Potamotryyon (Potamotrygomdae, Fig. 3a). These fishes were common in the channel

Table 1

Class Chondrichthyes Subclass Elasmobranchll

Order Ra31formes Family Potamotrygonidae

Potamotry@on sp. (4 specles)

Class Ostelchthyes Subclass Sarcopterygll

Order Dlpterlformes Family Lepldoslrenldae

Lepldoslren paradoxa

Subclass Actlnopterygll Superorder Clupeomorpha

Order Clupexformes Suborder Clupeoldel

Famlly Clupeldae Illsha amaTon*ca

Superorder Osteoglossomorpha Order Osteoqlosslformes

Family Osteoglossldae Osteoglossum bzclrrhosum ~ 9 1 9 a s

Superorder Ostarlophysl Order Cyprlnlformes

Suborder Characoldel Family Characldae

Chalceus sp. Charax sp. Colossoma sp. Mylossoma sp. Serrasalmus sp. (4 specles examlned) Tetra@onopterus sp. Trlportheus sp.

Family Erythrznldae Hopllas malabarlcus Hoplerythrlnus unltaenlatus

Fa/nlly Cynodontidae Cynodon ~Ibbus ;Lhaphlodon vulpxnus

Famlly Proch~lodontldae Prochxlodus sp.

Family Curlmatldae Curlmatus sp.

Family Anostomzdae Le orlnus sp.

sp. Schlzodon sp.

Family hemlodontldae Hemlodus sp. (3 specles examlned) M1cromlschodus su@lllatus

Suborder Gymnotoldel Family Gymnotldae

Gymnotus carapo Family Electrophorldae

Electrophorus electrlcus Family Apteronotldae

Apteronotus sp. Family Rhamphichthyldae

Elgenmannla sp. Rhamphichthzs sp. (2 specles ?) Sternopjgus sp.

Order Sllurlformes Family Doradldae

Acanthodoras sp. Anadoras sp. Astrodoras sp. Doras sp. Hassar sp. Hemidoras sp. Opsodoras sp. Pseudodoras sp. Trach~doras sp.

Family Auchenipterldae Auchenlpterus sp. Trach~cor~stes sp.

Family Pimelodloae Brachyplatystoma sp. (2 species examlned) Hemisorublm sp. Lelarlus sp. Phractocephalus hem111opterus Pimelodella sp. Pimelodus sp. Pinirampus sp. Pseudoplatystoma sp. Rhamdia sp. Sorubim lima

Fam~ne-~dae Ageneiosus sp.

Family Hypophthalmidae Hypophthalmus sp.

Family Cetopsidae Ceto~sls coecutlens

Family Calllchthyldae Hoplosternum littorale

( ' l l ,p 62 I a - - .

22 WILLIAM L. FINK and SAaA V. FINK

Table I (contimwd)

Famlly Lorlcarlldae* Anclstrus sp. Chaetostomus sp. H~postomus sp. Lorlcaria sp. Loricaria cf. clav1~inna Loricariichth~s acutus Loricariichth~s cf. maculatus Loricariichth~s sp. (undescribed) Parahemiodon sp. Pseudoloricarla sp. Pterygoplichthys sp. (2 specles examlned} Spatuloricarla sp. Sturisoma sp. Xenocara sp.

Superorder Atherxnomorpha Order Atherlnlformes

Famlly Belonldae Potamorrhaphxs sp.

Superorder Acanthopterygll Order Synbranchtformes

Famlly Synbranchldae Synbranchus marmoratus

Order Perclformes Suborder Percozdel

Famlly Sclaenldae Plaglosclon sp.

Famlly C1chlldae Acarlchthys heckelll Acaronla nassa Aequldens tetramerus Astronotus ocellatus Biotodoma cupido Chaetobranchopsls orblcularls Chaetobranchus flavescens Clchla ocellarxs Cichla temensls Clchlasoma blmaculatum Cichlasoma festivum Clchlasoma severum Geopha@us ~urupari Geopha@us surinamensls Pterophyllum sp.

Order Pleuronectlformes Famlly SoleJ~ae

Achxrus sp. Order Tetraodontiformes

Famlly Tetraodontldae Colomesus pslttacus

* One loricanid unidentifiable to genus. Xenocara sp.. tentatively ldent~fied here, is the same fish referred to as "Ancestrinae, unidentified species" by Fyhn et al. (1978) and Powers et al. (1978)

between Lago Janauaca and the Rm Solim6es and about four species were caught (the genus is in need of revision and identification to species cannot be done with confidence; the family is currently under study by Dr. Hugo P. Castello). The largest specimen captured by Phase IV weighed about 5 kg and was somewhat less than half a meter in disc diameter. Most specimens ranged from 20 to 30 cm in disc diameter. These are bottom-dwelling fishes which bury themselves in soft mud sediments, presenting a danger to waders. The sting is quite painful and slow to heal [see Halstead (1970) for a review of pertinent literature]. Presumably these fish, like many of their salt- water relatives, feed on invertebrates which live in the mud or sand substrates.

The lungfish, Lepidosiren paradoxa (Lepldosireni- dae; Fig. 3b), is a member of an ancient group of bony fishes, the subclass Sarcopterygii, or lobe-finned fishes. The lungfishes were widespread and successful from the Devoman to the close of the Triassic about 136 million years ago, but only three genera occur today. One genus and species, Neoceratodus forsteri, is found today in Australia, and several species of Protopterus occur in Africa. Lepidosiren and Protop- terus are both reported to dig burrows and estivate during low water periods (Carter & Beadle, 1930).

Lepidosiren is found in Amazonia in swamps, ponds, and also in lakes, although usually only near shore areas where swampy conditions exist. These fish are rather large (up to about 1.2 m), slow-moving and, according to Carter & Beadle (1930), probably feed primarily on aquatic plants when adult.

The Osteoglossidae, or "bony-tongued" fishes, is also an ancient group with few extant species. It and all groups to follow are part of the subclass Actinop- terygii, or "'ray-finned" fishes. Two genera and three species of Osteoglossidae occur in Amazonia--Ara- paima gigas, Osteoglossum bicirrhosum and O. ferreirai. All three species exhibit parental care. These fishes were of special interest to Phase IV, since Arapaima is an air-breather and Osteoglossum is not. Arapaima (Fig. 3c) is one of the largest freshwater fishes in the world, reported to approach 4 m in length and 200kg in weight [Gudger (1943), but see Migdalski (1957)]. Specimens 2.5 m in length are fairly common and support a commercial fishery (although evidence of overfishing is becoming apparent in Brazil and Peru). Arapaima is primarily a piscivore which cruises slowly or lies in wait for its prey; it is found most commonly in lake-like formations. Osteoglossum hicirrhosum (Fig. 3d), which reaches a meter m length, is an extremely graceful, slow swimmer and feeds on insects, fishes and other small vertebrates. Phase IV collectors found O. bicirrhosum in large shoals near small stream mouths on the Lago Janauac~i channel, particularly in early morning. When approached, the fishes attempted escape by making enormous leaps into the air. Osteoglossum is a mouth brooder. Like Arapaima, it is an important market fish.

The Clupeidae (herrings) ts represented in Alpha Helix findings by llisha amazomca, llisha is distri- buted in the tropical regions of the Atlantic, Pacific and Indian oceans and in the major rivers of the Amazon basin. Specimens of I. amazonica caught by Phase IV were rather large, approaching 35-40 cm in length. Typical of the family in form and color, this species is a silvery, strongly compressed fish, appearing much like an oversized sardine. Roberts (1972) reports that Amazonian clupeids tend to feed on fishes, rather than on plankton as do most members of the family.

The Ostariophysi (currently placed at the superorder level) is the dominant freshwater fish group in South America, with 12 families of characoids (chara- cins, tetras), four families of gymnotoids (electric fishes), and 14 families of siluroids (catfishes). It is probable that the ostariophysans in South America evolved from very few common ancestors in a relatively little utilized ecosystem, with a resultant "'explosive" radiation into almost every imaginable freshwater niche. This evolutionary phenomenon equals or surpasses any other such radiation of vertebrates-- characoid evolution alone has been considered far more spectacular than that of the marsupials of Aus- tralia (Weitzman, 1962). The Ostariophysi is in fact the dominant freshwater fish group of the world. The factors contributing to their success are not well understood, but the single outstanding morphological specialization held in common by all members is the Weberian apparatus, a complex structure consisting primarily of modified ribs connecting the gas bladder to the inner ear. The Weberian apparatus appears

Central Amazoma and its fishes

f

%

g

C h

d !

e l Fig. 3. Selected examples of Amazonian fishes: (a) Potamotrygon sp., (b) Leptdosiren paradoxa, (c) Ara- paima 9igas, (d) Osteoglossmn bicirrhosum, (el Serrasahnus sp., (f) Hoplias malabarzcus, (gl Hoplerythrmus unitaeniatus, (h) Cynodon 9ibbus, (i) Curimatus sp., 0) Leporinus sp. Line = 10 cm.

23

to provide ostariophysans with greater hearing abili- ties than other fishes.

Characoids (suborder Characoidei) are considered the most generalized of the Neotropical Ostariophysi since they are less modified than catfishes or gymnotoids in gross body morphology. Nevertheless, the range of specializations of characoids in terms of trophic structures and in terms of ecological and reproductive strategies is truly spectacular. Characoids now occupy niches similar to those of pikes, trouts, min- nows, gars, herrings and killifishes, and also fill niches not occupied by other fishes. Most characoid evolu-

tlon has centered around trophic specializations, and most families are defined based on feeding structures. While no research has been able to show just how or when these specializations are actually used, workers on characoids have hypothesized that a few early ancestors of modern characoids became adapted to feed in ways which minimized competition with their relatives, and that subsequent evolution has accentuated those specializations. This phenomenon of ecological separation (habitat partitioning) is poorly understood.

The largest characoid family, Characidae, consists

24 WILUAr~ L. FINK and SARA V. FINK

for the most part of generalized fishes of relatively small size. Many are familiar as the colorful tetras of the aquarium trade. Since Phase IV investigators were primarily interested in large fishes, only eight characid genera were examined, most of them more specialized than is usual for the family. Chalceus is an active, shoaling characid; it is an elongate fish and reaches about 25 cm in length. The most distinctive feature of this fish is that the scales above the lateral line are considerably larger than those below it. Charax species are strong-swimming, shoaling fishes which reach about 15 cm in length. Charax tend to swim in a head-down position, feeding on bottom dwelling invertebrates, particulary aquatic insect larvae. Charax specimens sampled by Phase IV were almost always caught along sandy beaches. Colossoma and Mylossoma are closely related fishes; both are deep-bodied and are predominantly vegetarian in diet. Colossoma specimens reach up to about 70cm in length while Mylossoma reach less than half that size: both fishes, especially the former, are a major local source of food. Mylossoma apparently travel in large shoals; the senior author observed local fishermen net about a thousand in two seine hauls, about 25 m from the shore of the Rio Negro near Manaus. Colossoma seem to occur in smaller groups. Both of these fishes are related rather closely to the flesh- eating p~ranhas.

Piranhas (Serrasalmus; Fig. 3e) are common fishes in central Amazoma and were well represented in Phase IV collections. Larger specimens were collected by Phase IV in the open waters of both main river channels and smaller formations: smaller specimens were caught closer to shore, in grassy areas. These fishes travel in schools, feeding on other fishes, invertebrates, or such swimming terrestrial vertebrates as may attract their attention. Little of documented fact is known about these fishes, but legends are numerous and impressive. Specimens of the largest piranhas are said to reach a length of about 45-60cm (Myers, 1972). Because of the quantity sampled by Phase IV, it should be stated here that our knowledge of piranha systematics is very poor. A number of genera have been described but all piranha species should probably be placed in the genus Serrasahn,~s. Most previous surveys of the group were limited by lack of large collections of specimens in all stages of growth and by lack of live color information. Deter- mination of the number of species sampled by Phase IV has been difficult. Probably four or more species are involved, but criteria which were used to separate ~'species'" in the field were found untenable when enough specimens were examined--our "species" began to merge together.

Tetragonopterus species are very generalized chara- cids, deep-bodied in form, which reach about 15 cm in length. Strong-swimming, shoaling fish, Tetra- gonopterus were collected by Phase IV along a sandy shore in the Rio Solim6es. Triportheus is an elongate fish with a deep, keeled "chest" region and greatly enlarged pectoral fins; members of this genus reach about 20cm in length. Triportheus specimens were seen to swim very rapidly with the anterior region out of the water, using the pectoral fins as "wings". Triportheus was caught by Phase IV in the open waters of the Paranh de Janauac~.

The Erythrinidae is a small family of predatory characoid fishes which are considered to be very primitive (Roberts, 1969). Two closely related eryth- rinid species were of particular interest to Phase IV investigators since one, Hoplerythrinus unitaeniatus, is a facultative air-breather and the other, Hoplias malabaricus, is not. Hoplias (Fig. 3f}, an efficient fish preda- tor, is a widespread and common fish; it is found throughout much of tropical and subtropical South America and makes up a very high percentage of the biomass in many South American waters (Bonetto et al., 1969). Hoplias occurs in a variety of shallow water habitats, During low water periods and in ponds where oxygen content is low, individuals of Hoplias can be seen lying close to shore just under the water surface where they are able to utilize the oxygenated upper water layers. Hoplerythrinus (Fig. 3g), while neither as widespread nor as numerous as Hoplias, is nevertheless also a common fish. When the oxygen content of its aquatic habitat drops, Hoplerythrinus is able to rise to the surface and take atmospheric air into its highly vascularized gas bladder. Lowe- McConnell (1975) states that Hoplerythrinus has been reported to travel overland; certainly the fish are able to hold themselves upright and travel rapidly over a ship deck using the paired fins and sinusoidal movements (personal observation). Hoplias reaches about 60cm in length, Hoplerythrinus about half that size.

The Cynodontidae is another small characoid family of predaceous fishes, but, in this case, the fishes are highly specialized. Rhaphiodon and Cynodon (Fig. 3h) were examined during Phase IV. Members of the Cynodontidae are quite active and tend to stay near the surface in open areas of larger waterways. The most striking aspect of their morphology is the presence of two long, canine-like teeth in the lower jaw; when the jaw is closed these teeth insert into deep cavities in the skull. Some individuals of Rha- phiodon reach at least 69 cm (Schultz, 1950). Cynodon reaches about 40 cm.

The Prochilodontidae is a small family of specialized, mud-eating characoids. About 30 species of the major genus, Proehilodus, have been described and the systematics of the genus is in great need of revision. Prochilodus is an important commercial fish m Amazonia because of its frequent use as a food item. Prochilodus species reach about 50cm. As adults, these fishes usually shoal in the open waters of lakes and rivers; they are known to undertake extensive spawning migrations (Mago-Leccia, 1972). Prochi- Iodus have large, evertable mouths and small teeth and exploit an apparently unlimited food resource; they swim over the bottom eating mud and presumably digest its organic content for nourishment (Kn6ppel, 1970; Marlier, 1968).

The Curimatidae is a characoid family of detritus and sand-eating fishes, of which Curimatus (Fig. 3i) was examined by Phase IV investigators. Curimatus species are toothless; like Prochilodus, they are active swimmers which shoal near the bottom in open waters. Curimatus species reach a length of about 15 cm.

The Anostomatidae is a small family of rather specialized characoid fishes which presumably use their forceps-like teeth to select food items. Leporinus (Fig. 3j), Schizodon and Rhytiodus are the three genera


a

_ , f Z ' - -

b g

h

tl !

e

Fig. 4. Selected examples of Amazoman fishes: (a) Hemlodus sp., (b) Rhamphichthys sp., (c) Pseudodoras niger, (d} Agemosus sp., (e) Hypophthahnus sp., (f) Cetopsis coecutiens, (g) Hoplosternum littorale, (h)

Hypostomus sp., (i) S ynbranchus marmoratus, (j) Cichlasoma festivum. Line = 10 cm.

pertinent to the discussions here. All species in these genera are powerful swimmers and all specimens were caught in open waters, either in large streams or in the main river channels. Members of this family are common in fish markets in the area. Large specimens reach about 40 cm.

Hemiodontidae collected by Phase IV include species in the genera Hemiodus (Fig. 4a) and Mic- romischodus. Hemiodontids are shoaling fishes which are streamlined in form and extremely able swimmers. They are found in large channels and probably in

the main river. When entrapped by a seine, these fishes often undertake impressive leaps out of the water in small groups. Adults reach about 30cm in length. All adult Hemiodus have toothless lower jaws and pediculate, multicuspid teeth in the upper jaw; Micromischodus adults also bear small teeth in the lower jaw. Both genera have quite specialized mouths and are probably selective feeders which take their food from detritus or from' bottom sediments. At least one genus of the Hemiodontidae, Anodus, is a specialized filter-feeder.


A second m~uor South American ostariophysan assemblage consists of the fishes in the suborder Gym- notoidei, a group which is thought to share a common ancestor with characoids. The gymnotoids or electric fishes are elongate in form and are able to swim either backwards or forwards with equal ease using undulatory movements of their long anal fins. Gymnotoids transmit weak electrical s~gnals which are used m intraspecific communication [see, for example, Hopkins (1974)] and also, presumably, in food location: there is an extensive literature on elec- trogenesis and electroreception in these fishes (see Bennett, 1970, 1971 a, b). Most gymnotolds are crepuscular or nocturnal, leaving their daytime burrows or other hiding places to forage over stream bottoms and returning to the same cover at sunrise (Steinbach, 1970).

Four families of gymnotoids are recognized by ichthyologists and Phase IV examined representatives of each. Electrophorus electrlcus, the famed electric "'eel", is the sole member of the Electrophoridae and is the only gymnotoid capable of a powerful electric discharge. It and Gymnotus carapo (Gymnotidae) are both inhabitants of slow, often stagnant waters; both are also air-breathers. In these two families, the electric organ is derived from muscle tissue and discharge rates are slow, from about 200 to about 500 Hz. Elec- trophoru,~ is probably primarily piscivorous and reaches about I m in length. Gymnotus feeds mostly on insect larvae (Kn6ppel, 1970) and averages less than 30 cm in length.

A third gymnotoid family, the Apteronotidae, is represented in Phase IV collections by a single species of Apteronotus. Most members of this genus stay in moving, well-aerated waters and feed primarily on aquatic insect larvae. Some apteronotid species develop elongate snouts. Apteronotids emit high-frequency electrical discharges (10,000-12,000 Hz) from their neurally derived electric organs. Most species of Apteronotus are smaller than 30cm when adult.

In the fourth gymnotoid family. Rhamphichthyidae, the three genera examined were Eigenmannia, Sterno- pygus and Rhamphichthys (Fig. 4b). Members of this family have bodies which taper posteriorly, sometimes to a long caudal filament, but lack a caudal fin. The caudal area contains the main electric organ; access- ory electric organs are known in some species and are located on the head. EigenmamTia species are translucent in life, reach a maximum length of about 30 cm, and have relatively short or very short snouts. Sternopygus macrurus is dark brown, reaches nearly l m in length, and is also short-snouted. Rham- phichthys, striking in appearance, reaches over I m in length and has a long, tubular snout. Several species of th~s genus have been described, but most authors have usually recognized only one, morpholo- gically variable species (Ellis, 1913). The findings of Fyhn et aL (1979) indicate a correspondence between hemoglobin patterns, color pattern and snout length, suggesting the existence of more than one species and a need for critical reappraisal of the genus. Rham- phichthys has a very low frequency discharge. It is reputed to spend daylight hours buried in stream or river bottom sediments. Based on the small size of the mouth, a diet consisting primarily of aquatic insect larvae is probable. See Schwassman (1976) for

information about the systematics and ecology of closely related Gymnorhamphichthys.

Siluriformes, the catfishes, are the third ostariophysan group in South America and are among the most spectacular fishes found in Amazonia in form and living habits. Their size and habits range from the tiny trichomycterid candirus, about 2.5cm in length, which parasitize the gills of larger fishes, to the giant pimelodid Brachyplatystoma which reaches nearly 3 m in length, about 90 kg in weight, and has bottom- dwelling habits typical of the order (Gudger, 1943). Characteristic of the order Siluriformes and present in most, but not all, South American catfishes, are several distinctive features. The first ray of the dorsal fin and of each pectoral fin usually form fairly prominent spines; these can be erected and locked into place, forming a thorny, triangulate protective device. A heavily ossified skull usually affords further protec- tion. The doradid (Fig. 4c), callichthyid (Fig. 4g) and Ioricariid (Fig. 4h) catfish families are also armed with bony plates, m patterns distinctive to each family; presumably such armor, at least in the case of the latter two families, decreases susceptibility to desiccation as well as to predation. Also characteristic of most members of the Siluriformes, but not peculiar to them, are the possession of barbels, the presence of small but usually numerous teeth which allow a variety of feeding strategies, and bottom-dwelling life habits.

Many of the ecological niches occupied by the catfishes are essentially similar to those held by many characoids, but the catfishes are usually crepuscular and forage while their diurnal counterparts are inac- tive. Some catfishes, however, successfully occupy diurnal niches, e.g. the filter-feeder Hypophthabnus. The interactions between the "day fauna" and the "'night fauna" of tropical waters are complex and little understood.

Phylogenetic relationships among catfish families are poorly known and it should be expected that genera will be raised and lowered to and from family rank or shifted from family to family as our knowledge increases. Fourteen families of catfishes, as currently understood, occur in the freshwaters of tropical Amer- .ca, of which all but the largely marine Ariidae are endemic. Eight families were sampled by Phase IV.

Members of the Doradidae (Fig. 4c) are immediately distinguishable by the row of bony plates, which usually bear prominent hooks, extending along the sides of the fish; in addition, the spiny rays of the pectoral and dorsal fins are usually serrated. This diverse and successful family is represented in Phase IV by nine genera (see Table 1). Species of Pseudo- doras grow large enough to be market fishes--up to nearly 1 m m length and about 8 kg in weight--but most doradids are considerably smaller, averaging about 20 cm in length. Man3, of the smaller doradids (e.g. Anadoras) are more heavily armored than are the larger members of the family and have more prominent and heavily serrated pectoral- and dorsal- fin spines. Doradids are slow-swimming, bottom- dwelling fishes which feed primarily on aquatic insect larvae (Marlier, 1968). Larger species, such as members of Pseudodoras, were caught by local fishermen in the deeper waters of main river channels; smaller doradids were caught in shoreline areas.


The Auchenipterldae is a small family represented here by Auchenipterus and Traehycorystes. These are small, active fishes, about 15 cm in length, which hide in logs or stream debris during daylight hours and feed at night. Auchenipterids are usually shoaling species and, according to Saul (1975), prefer swift water flowing over sandy bottoms, particularly near shoreline sand deposits. He found that terrestrial insects predominated in the diet of a related genus, Cen- tromoehlus. During Alpha Helix Phase IV, numerous specimens of Trachycorystes were visible nearly every night, swimming in the 8 km/hr current of the Rio Solim6es under the fight at the stern of the ship.

The Pimelodidae is the second largest Neotropical catfish family in numbers of species, with about 250 currently described. A greater number of species of large fishes are included in the Pimelodidae than in any other South American fish family. Notable among these are members of the genera Braehyplaty-. stoma (mentioned above), Hemisorubim, and Pseudo- platystoma. Most pimelodids are fairly active fishes' and are largely piscivorous but able to utilize almost anything of food value, including plants and invertebrates. While most species probably take most of their food from river or stream bottoms, food is probably captured throughout the living space. This relatively unspecialized diet requirement may be one reason pimelodids are so widespread and numerous. Some of the more speciose genera, particularly Rham- dia and Pimelodus, are in serious need of revision. Ten genera of pimelodids were sampled by Phase IV (see Table 1).

The Ageneiosidae is a small family of only two genera, of which only Ageneiosus (Fig. 4d) was sampled. Ageneiosus species are strong, mid-water swimmers and reach about half a meter in length. Only two barbels are present and these can be placed in grooves, presumably to increase the body streamlin- ing. Aoeneiosus is probably a piscivore.

The Hypophthalmidae contains a single genus, Hypophthalmus (Fig. 4e), and specialists disagree as to the number of species. Hypophthalmus is quite distinctive, with a long sloping back, four elongate chin barbels, and eyes which are set below the lateral edge of a broad, flat head. These fishes reach about 60 cm and are common in the commercial fish trade in Amazonia. Hypophthabnus is a strong-swimming, pelagic fish which travels in large schools. Lack of jaw teeth and the presence of numerous, elongate gill rakers indicate a planktonic diet.

One species of the Cetopsidae, Cetopsis coecutiens (Fig. 4f) was examined by Phase IV. With a torpedo- shaped body, and grooves for barbels and pectoral fins, these fishes are well suited to a life in swift, open waters. Numerous specimens were caught at night by hook and line from the stern of the Alpha Helix while it was anchored in the Rio Solim6es, in a water current which averaged 8 km/hr. Little is known of the ecology of these fishes, but their diet probably consists primarily of small fishes and aquatic insect larvae. Large fishes which are incapacitated may also form a part of their diet. The largest specimens captured were about 23 cm in length.

Air-brcathing is present in many, if not all, members of the Callichthyidae, including the single species sampled by Phase IV, Hoplosternum littorale

(Fig. 4g). This extremely hardy fish is fully encased by heavy plates and attains a size of about 20cm. As would be expected in a fish so heavily armored, Hoplosternum is a bottom-dweller; its diet probably consists primarily of aquatic insect larvae. Lowe- McConnell (1975) reports that fishes of the closely related genus Callichthys, also heavily armored, are able to travel overland for many meters and withstand wide fluctuations in temperature. Hoplosternum was observed by Phase IV to travel quickly and with ease over a dry, hard surface (in this case, a ship's deck).

The final catfish family sampled by Phase IV was the Loricariidae, the "sucker-mouth" catfishes (Fig. 4h). Members of this family are also well suited to a life close to the substrate, with heavy armor and, in most cases, a shallow body form which is flat on the belly and slightly convex dorsally. Loricariids have disc- shaped mouths which can be used as sucking devices, allowing the fishes to cling to the surface of rocks, logs, or exposed roots and move slowly along scrap- ing aufwuchs. These fishes may be found adhering to wood in rapid waters but are equally suited to moving over mud bottoms in stagnant pools, where they eat mud and bottom debris. This suitability for such varied habitats may be the reason Ioricariids comprise the largest single family of South American catfishes, with about 50-60 genera and over 400 species. Eleven genera of loricariids were sampled by Phase IV. Particular attention was paid to fishes in the genera Pterygoplichthys, Loricaria and Hypos- tomus because of suspected or confirmed air-breathing capabilities. Most loricariids do not exceed about 30 cm in length, but some, e.g. Pterygoplichthys, reach nearly 60cm. At least one species of Loricarichthys sampled by Phase IV is undescribed (Nijssen, litt.). That such a relatively large, conspicuous fish from one of the most heavily sampled areas in South Amer- ica remains unknown emphasizes our lack of knowledge of this fauna.

The Order Atheriniformes (Superorder Atherino- morpha) is a diverse assemblage and includes the cyprinodonts, poeciliids (e.g. the guppy, Poecilia rett- culata), halfbeaks, needlefishes and marine "flying fishes", and a number of other forms less familiar to non-ichthyologists. The only member of this group which was sampled by Phase IV is the needlefish genus Potamorrhaphis (Belonidae), a fish with very' elongated jaws. This fish is an active swimmer which cruises just below the water surface, feeding on terrestrial or flying insects which fall into the water. The Potamorrhaphis specimens caught were schooling near the middle of a small stream.

The superorder Acanthopterygii is the most recent major group in terms of phylogenetic origin. The acanthopterygians, or "spiny-rayed" fishes, include an enormous number of families, genera and species, most of which look not unlike the sunfishes and perch caught by north temperate fishermen. Most spiny- rayed fishes are marine, and in the freshwaters of South America, this group is much overshadowed by the Ostariophysi. Most acanthopterygian species which occur in Amazonia are not "typical" in appearance and are of peripheral importance to the ecosystem. The remaining fish groups described below belong to this major group.

28 WILLIAM L. FINK and SARA V. FtNK

The Synbranchidac are very atypical of the Acan- thopterygfi. Elongate and eel-like in form, the syn- branchids are actually highly specialized members of their own order (Synbranchiformes) and not at all closely related to the eels. They can be immediately identified by the single gill chamber opening in the ventral surface of the head. Two species occur in South America of which only one, Synhranchus marmoratus (Fig. 4i) was sampled by Phase IV. S. marmoratus reaches a length of about 60cm and commonly inhabits burrows in a wide variety of habitats, including ponds, lakes and river banks. When the surrounding water is poorly aerated, these fish breathe surface air by use of a specialized branchial chamber. Little is known of the natural history of Synbranchus; it is usually sluggish and preys on fishes and frogs (Breder, 1927; Zaret & Rand, 1971). Synhrandlus was reported by Carter & Beadle (1930) to estivate in burrows, much like the lungfish, Lepidosiren.

Of the Order Perciformes, Phase 1V examined members of two families--Sciaenidae and Cichlidae. The Sciaenidae, or croakers, is a family of predomina- rely marine or estuarine species, but some species occur only in fresh water. Plagioscion is the only freshwater sciaenid genus found in Amazonia and occurs widely in deeper waters, both in the main river channels and in peripheral formations. It is a school- mg fish which swims slowly over the bottom feeding on aquatic insect larvae and other organic matter. Although the species in this genus reach over 60 cm, most specimens examined by Phase IV were 45 cm or less. Plaqioscion were caught both in the Paran~. de JanauacA and in the Rio Negro.

The most numerous acanthopterygians in South America are the cichhds (Cichlidae; Fig. 4j), a group of fishes characterized by a complex pharyngeal jaw apparatus (only Cichla among the South American genera lacks it). This apparatus enables cichlids to take mouthfuls of bottom debris or other mixed matter, sort out desirable food items, and eject non- edible matter. Cichlids are evolutionarily advanced fishes with complex behavioral patterns and, especially in Africa, very complex evolutionary histories.

Eleven cichlid genera were examined by Phase IV and in many ways they are the most interesting of the fishes sampled in terms of complexity of hemoglobins. Most of these species are "'hovering" fishes which swim slowly over the substrate and nibble at it when food is sensed. For example, members of the genus Geophagus move over the bottom taking mouthfuls of sand, separating out food items, and either spitting out the sand or passing it out through the gill chambers. Only members of the genus Cichla are predominantly piscivorous, and the strategy of food capture ~s probably one of lying in wait for or slowly stalking prey. Cichla is the largest cichlid in South America and although the systematics of the genus is in disorder, specimens considered to be C. ocellaris were reported by Lowe-McConnell (1969) to reach about 50cm in length and about 3.5kg in weight. Machado (1971) listed a specimen of C. ocellaris 61.7cm in standard length (no weight given). Most other species examined by Phase IV are considerably smaller, reaching about 14-20 cm in length.

The only member of the Soleidae (Order Pleur-

onectiformes) captured during Phase IV was Achirus, a small flatfish about 20cm in length. Achirus is a bottom dweller which lies covered by a thin layer of bottom silt and feeds on small fishes or invertebrates.

The Order Tetraodontiformes is represented throughout Amazonia by Colomesus psittacus (Tetraodontidae), a small, yellow- and brown-banded puffer of up to 12cm in length. This fish was found by Phase IV to occur in large numbers in the campos along the shore line of Lago Janauacfi. Like other puffers, it is a relatively slow-moving fish which greatly increases its size when attacked or molested by filling its belly with water. Although a common fish, little is known of its natural history; presumably, however, it feeds on aquatic insect larvae.

In the above text, we have attempted to supply the reader with some background in natural history, ecology and systematics in order to make the following papers in this issue more meaningful. The information presented here is necessarily sketchy, partly because of space limitations but also, particularly in the case of the biology and ecology of the fishes, because of the dearth of information. Especially frus- trating has been our attempt to supply Phase IV par- ticipants with such information as life history, swimming habits, habitat preferences and diet of the fishes collected. We have much oversimplified but have had little choice, and can only hope that the growing interest in the fauna and flora of th.s fas- cinating area will provide the impetus for research needed to obtain such knowledge.

Acknowledgements--The senior author would particularly hke to express his thanks to Dr Austen Riggs, who proffered the invitation to participate m Phase IV of the Alpha Helix cruise. Thanks are also due to the many members of the cruise who seined and gill-netted, and got soaked, muddy and bitten by piranhas in order to catch the numerous specimens examined. Special mention must go to Drs Hans and Unto Fyhn and B. Jeanne Davis for their considerable field efforts. Drs Warwick E. Kerr and Wolfgang Junk graciously extended the hospitality of INPA (Institute Nacional de Pesquisas de Amazonia, Manaus) and arranged for shipment of fishes to the U.S. for identification. Mr Robert Schoknecht, formerly of the Museum of Comparative Zoology and now at Cornell University, aided in the identification of specimens and compilation of the species hst. Dr Hart Nijssen and Isaac lsbrucker were kind enough to provide identifications of several loricariid catfishes. Funds were provided by NSF Grant PCM75-06451 and by the Museum of Comparative Zoology, Harvard University.

Both authors would like to thank Drs S. H. Weitzman and D. L. Kramer for reading drafts of the manuscript and providing numerous helpful comments.

Voucher specimens of species examined by Phase IV investigators are being returned to the Museu de Zoologla da Umversgdade de $5.o Paulo.

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Central amazonia and its fishes

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