Popular Science Monthly/Volume 68/June 1906/The Fresh-Water Fishes of South and Middle America
By Professor CARL H. EIGENMANN
A. The salient features of the fish fauna of the Americas south of the United States are:
1. Great variety of fish life in the area between the Caribbean Sea and the Argentine Republic.
2. Paucity of 'types' or families contributing to this variety.
3. Paucity of the middle American fauna and its essentially South American character except for
4. the isolation of the fauna of the Mexican plateau.
5. Paucity of the Pacific slope fauna and its essentially Atlantic slope character.
6. The 'marine' character of the Titicacan fauna.
7. The paucity of the Patagonian fauna and its essential difference from the Brazilian fauna.
8. The similarity of the tropical American to the tropical African fauna.
The first fresh-water fishes of South America were described by Marcgraw in 1648. Additional accounts were given by Gronow, 1754 to 1756; Scopoli, 1777; Bloch, 1794; Lacépède, 1802; Bloch and Schneider, 1807; Cuvier, 1817 to 1818.
In 1817 the king of Bavaria sent Spix and Martius on an extended trip to Brazil. Spix was working at the report on the fishes when he died. The collection was turned over by Martius to Louis Agassiz, a student of twenty-one at Munich. It had been nip and tuck between Agassiz's desire to study natural history and his father's desire to have his son study medicine. The commission to work up the Brazilian fishes was surreptitiously undertaken by Agassiz and the results published in a superb folio volume. This work, which tinctured the entire later life of Louis Agassiz, was by far the most important contribution to the fresh-water fishes of South America that had appeared. Agassiz's desire to visit Brazil himself was not fulfilled until forty years later, when, as the head of the Thayer expedition, he spent sixteen months in Brazil with twelve assistants, devoting his time mainly to fresh-water fishes.I was a student under Jordan when Mrs. Agassiz's 'Life and Letters of Louis Agassiz' appeared in 1885. Agassiz's account of his expedition to South America, coupled with the statement of Jordan that no
comprehensive account of his collections had ever been prepared, created the desire to examine this lavishly rich fauna. In the fall of 1887, with Mrs. E. S. Eigenmann, I began work on Agassiz's unrivaled collections, to which I had gained access through the courtesy of Mr. Alexander Agassiz, the director of the Museum of Comparative Zoology, and Mr. S. Garman, the curator of fishes. Financial and other reasons compelled me to abandon the work when but half finished. Occasional collections received from South America for identification have, however, kept up my interest. The entire problem presented by this fauna has been reviewed and will be published in one of the reports of the Hatcher expedition to Patagonia. I am permitted to publish this summary of results through the courtesy of the editor of the Hatcher volumes, Professor W. B. Scott, of Princeton University.
1. Variety of Fish Life.
On February 23, 1866, Louis Agassiz wrote to the emperor of Brazil:
I estimate the total number of species which I actually possess [from the Amazon] at eighteen hundred, and it may be two thousand.
To this is added a footnote in Agassiz's 'A Journey in Brazil,' p. 383.
To-day I can not give a more precise account of the final results of my survey. Though all my collections are safely stored in the museum, every practical zoologist understands that a critical examination o'f more than eighty thousand specimens can not be made in less than several years.
Agassiz secured more species from a small lake in the valley of the Amazon than there are in all the fresh waters of Europe.
The number of species he collected was overestimated by Agassiz. While about half of his Amazonian collections have not, after forty years, been examined, it is certain that the species not yet examined will not swell his list to 1,800 species. The total number of species recorded from the Amazon basin up to date is 674.
Although Agassiz's estimate of the number of species he collected is too high, the total number of species found in South America is very great. About ten per cent, of all the known species of fishes have been recorded from the freshwaters of South America. In 1892 I estimated that three fourths of the fauna was known. Now, after examining recent lists, and considering that collections have largely been made in easily accessible and great highways, and that from great river basins like the Purus, Tapajos, Xingu and the Uruguay and the greater part of the Madeira and the Tocantins we have nothing at all, and that even from the great Orinoco and Magdalena we know next to nothing; I doubt very much if we even yet know so much as three fourths of the fauna of the area between the Caribbean Sea and the Argentine Republic.
The tropical American fresh-water fauna, having its center of greatest diversity in the middle Amazon basin, is attenuated northward till it reaches the vanishing point just on the borders of the United States. Southward it extends to somewhere—no one knows where—south of Buenos Aires. The Patagonian fauna and North American fauna are entirely different from the tropical American fauna and from each other.The key to the great diversity of the tropical American fauna is to be found in the enormous single water system, extending from 10°
north to 35° south latitude, and from 50° to 79° west longitude, providing a continuous north and south waterway of more than three thousand miles, and an east and west course of over two thousand miles, and embracing the Orinoco basin, the Amazon basin and the La Plata basin, draining over 3,000,000 square miles of territory, or an area about equal to that of the United States, exclusive of appendages.
2. Paucity of Types.
There are found in the rivers of South America representatives of many marine fishes. Soles, swellfish, stingrays, needlefishes, mullets, sciænoids and the herring tribe give to the South American fauna a peculiarly marine flavor. But all these form but a small fraction of the entire fauna, and their elimination would make little inroad on the number of species. All are recent additions from the sea.
There are also in South America a number of undoubted relicts of former times, and if one should judge by the interest excited by the genera Synbranchus, Lepidosiren, Arapaima and Osteoglossum, it would seem that these genera formed an important element in the present fauna; but they are so few in number that they also might be eliminated without any appreciable depreciation in the variety of the South American fauna.
After eliminating these, then, we come to the reigning element in the present fauna, the element now in its prime and best suited to contribute to the elucidation of the methods and paths of divergent
and convergent evolution and the paths of dispersal. Chief of these elements is the superorder Ostaryphysiæ, composed of the Characinidæ with about 500 species; the Gymnotidæ with about 30 species, and the various families of catfishes with about 500 species. Pœciliidæ, dominant in middle America, contributed materially to the fauna—about 45 species.
The largest contribution, aside from the characins and the catfishes, is furnished by the Cichlidæ, with about 145 species, in south and middle America.
The great variety of South American fishes is due to the divergence in the types of characin, catfish, cichlid and pœecilid.
3. Paucity of Middle American Fauna.
4. The Isolation of the Fauna of the Mexican Plateau.
The Lerma, draining the Mexican plateau, harbors an old fauna. Only three of its fifty-four species have been taken in any other river basin. Eight of its twenty-one genera are peculiar to it.
5. Paucity of the Pacific Slope Fauna.
From the Pacific slope of South America, south of Panama and north of Chile, but 55 species of fresh-water fishes have been reported. This condition is not unlike the condition in North America, for more species may be obtained from a single favorable brook in the Mississippi valley than are found on our entire Pacific slope, from San Diego to Alaska.
Fifteen out of the 55 species are also found on the Atlantic slope, and the others, with three exceptions, belong to genera of wide distribution on the Atlantic slope. None of the species would cause any surprise if found on the Atlantic slope.
6. The Marine Character of the Titicacan Fauna.
The fish fauna of Lake Titicaca, about as large as Lake Erie, consists of a catfish, Pygidium rivulatum, and several species of Orestias, a Fundulus-like pœcilid. The former belongs to a genus of mountain catfishes universally distributed in mountain streams of South America. It has succeeded in crossing all sorts of barriers, and has undoubtedly migrated into Lake Titicaca from the streams surrounding it. The genus Orestias, on the other hand, is confined to Lake Titicaca and the streams and lakes immediately surrounding it. The latter have doubtless received theirs from Lake Titicaca, which, on its part, could only have received them at the time it was an arm of the sea, in which its nearest relatives flourish.
7. The Patagonian Fauna.
The interest in the Patagonian fresh-water fish fauna is entirely out of proportion to its diversity and centers largely in its origin. Only about twenty-five species of fishes are known to live or enter the fresh waters south of the line joining Valparaiso and Bahia Blanca. These few species fall, according to their origin, into four distinct groups.
1. Immigrants from the sea (a) are in the process of acclimatization (species of Menidia and Atherinichthys), or (b) may be looked upon as long established (species of Percichthys and Percilia). Members of (a) are found in all the rivers; members of (b) are found in the north chiefly, but reach the Santa Cruz.
2. Immigrants from the fresh waters on the north: a very small overflow from the extremely rich fauna to the north and still retaining their generic affinity with the northern forms. Here belong the species of the genera Cheirdon and Astyanax, which are very widely distributed in tropical South America and are not known to extend much south of the Rio Negro.
3. Autochthons or of doubtful origin. Here belongs the highly
interesting Diplomyste, which is found on the northern border of Patagonia, but is not a derivative of the tropical American fauna. It is a relict of the original catfishes, in which the maxillary is still functional as a tooth bearer. Here belongs also Nematogenys, a catfish related to Pygidium. Like the members of group 2, these species are confined to the northern fringe of the Patagonian area.
4. There remain unaccounted for the members of the Aplochitonidæ, Galaxiidæ and Petromyzontidæ, chiefly of southern Patagonia.
Of the Aplochitonidæ there are two genera, Aplochiton, with an undetermined number of species in the Patagonian region, and Prototroctes, with three species, one in Queensland, one in South Australia and one in New Zealand.
Of the Galaxiidæ there are two genera, Neochanna (apoda) from New Zealand, where it frequently burrows in damp clay away from water, and Galaxias, with about 30 species, from New Zealand, New South Wales, South Australia, Tasmania, Cape of Good Hope, southern South America and the Falkland Islands. The Petromyzontidæ are found in all temperate fresh waters and seas in both the northern and southern hemispheres.
The distribution of the two former families is of interest in connection with the theory of a former antarctic continent connecting the land masses in which they are found. In favor of a former land connection it may be argued, and with justice, that while these species descend to the sea, the probability that any pair of individuals should migrate from Cape Horn to New Zealand, or vice versa, is highly improbable. (This objection loses some weight if they spawn in the sea, as is reported.) There are no intermediate places that might be colonized and serve as new centers of distribution. It may further be urged that these species could readily have been distributed to their present homes by migration from stream to stream along a continuous coast line or on a land-wave moving from one place to another. An obvious objection comes from the paucity of the forms with this peculiar distribution. If there was a continental mass connecting South America with New Zealand and Australia fit to be inhabited by fishes, there must have been an abundant and diverse fish fauna which has disappeared. If the antarctic continent depended entirely for its existence on the evidence from the distribution of the fresh-water fishes, its existence would be very highly theoretical and precarious.
However, the evidence from other sources of a former land connection has become conclusive, and T am of the opinion that during the submergence of large parts of Patagonia during the late Pliocene the formerly abundant fresh-water fauna became exterminated, with the exception of those that were indifferently fresh-water or marine.
The Petromyzontidre offer still another difficulty. There is no place on the American continents between the Mexican plateau and Central Chile that harbors any species of the family. The northern and southern species belong to distinct genera. At least two of the South American genera are peculiar while two others are found in Australia and New Zealand.
8. The Similarity of the Tropical American to the Tropical African Fauna and the Necessity of and Evidence for a Former Land Connection between Africa and, South America.
North America has not contributed a single element to the freshwater fish fauna of South America. Two prominent South American families, the Characiniclæ and the Cichlidæ, have representatives as far north as the Rio Grande basin, and one of these has succeeded in crossing over into Cuba, evidently from Yucatan; on the other hand, several members of the North American fauna have representatives as far south as the Isthmus of Tehuantepec. The North American fauna is entirely distinct from the tropical American fauna.
But four genera of fresh-water fishes of South America north of Patagonia are found in any other continent than North America. These are Synbranchus, Agonostomus, Cotylopus and Fundulus. The first found also in brackish water, the second belonging to the marine family, Mugilidæ and the others to the Pœciliidæ. Synbranchus is found in India, Agonostomus in middle America, the West Indies, northern South America and New Zealand, Australia, Celebes, Mauritius and Comoro Islands. There is no reason why Agonostomus may not have been independently evolved in the South Sea and in America from marine Mugilids. Cotylopus is found in Central America and Reunion and Fundulus in America and Europe.
It is possible that Pimelodus is found in Africa and Pseudauchenipterus in Madagascar. Both are found in South America.
Africa and South America have two groups of families in common. The first group comprises the Serranidæ, Sciænidæ, Mugilidæ and Tetraodontidæ.
11. Adaptive Radiation in the Characinidæ. The central figure of Astyanax bimaclatus with notched teeth probably represents the more primitive condition. The lower left hand figure represents Anodus latior with no teeth and a very long alimentary canal, a mud eater. The lower right-hand figure represents the scissor-like jaw and teeth of Serrasatmo humeralis whose relatives have been repeatedly reported to have killed bathers before they could reach shore after being attacked. The upper figure represents Raphiodon vulpinus Spix which has reached the extreme in conical teeth, the large canines protruding above when the jaws are closed. The over 100 South American genera and 500 species of this family offer pretty complete series from the center to the extremes with many lateral branches.
These are all marine families, some of which have also developed fresh-water forms in Europe and North America as well as in South America. The fresh-water forms of South America and Africa are local adaptations of marine families that require no change in the present condition to account for their origin.
The second group comprises the Lepidosirenidæ, Osteoglossidæ, Siluridæ, Characinidæ, Pœciliidæ and Cichlidas. Of these the Lepidosirenida are relicts of a former widely distributed group, and it requires no land connection to satisfactorily account for their presence in Africa and South America. The Pœciliidæ live indifferently in marine, brackish water or fresh water. They reach their maximum development in the fresh waters of Mexico and Central America. The marine species are found along the shores, not at sea, and there is, therefore, at present, no known means of getting them from the American to the African shore. Nevertheless, Fundulus is found on both sides of the Atlantic, and there must have been an intermigration much more recent than the youngest possible land connection between Africa and South America, or else there has been a very long persistence of this type. A land connection, while not absolutely required for this family, would be very convenient.
The Siluridæ are in part marine. All of the South American forms of Siluridæ can be derived from the marine Tachisurinæ, and the same is probably true of the American members of the family. Furthermore, the catfishes are found in North America, Europe and Asia and have been recorded in North America from the Tertiary. A land connection between Africa and South America is, therefore, not absolutely required to account for their presence in both continents, though, as in the case of the Pœciliidæ, such a connection would be very convenient.The Cichlidæ and Characinidæ are abundant in tropical America and in Africa, a few species of Cichlidæ being also found in India. There is no known means by which these two forms could have crossed the existing gap between Africa and South America. There has been no exchange of species in recent times, for there is no species or genus common to the two continents. The South American and African elements of these two families must have been derived from some intermediate land mass or must have gone from one continent to the other over a land bridge. That this connection, whatever it was, must have been obliterated before the tertiary, is evidenced by the facts that the tertiary deposits of Taubaté and Parana show existing genera and that there are many South American types, as the Gymnotidæ, Electrophoridæ, Bunocephalidæ, Loricariidæ, Argiidæ, Pygidiidæ, Callichthyidæ, Hypophthalmidæ and others not found in Africa that have all
Similarly, other families found in Africa and not in South America have either arisen in Africa since that time or have immigrated from the east.
A land connection, whether a land bridge, intermediate continent or land wave, between the two continents is imperative. This land connection must have existed before the origin of existing genera and before many of the existing families.
B. Conclusions and Bird's-eye View of the Problems.
America, south of the Tropic of Cancer, contains four (or five) distinct faunas. These faunas are the Transition, the Mexican, the South American and the Patagonian.
The northernmost, or Transition fauna, is characterized largely byThe Mexican fauna is unique, and occupies a narrow strip including elements from both the north and the south. It is found on the Atlantic slope from the tropic to the Isthmus of Tehuantepec and on the Pacific slope of this Isthmus.
the valley of the City of Mexico and the Lerma basin draining to the west, and the Rio San Juan, a tributary of the Panuco, draining to the east. While containing intrusive elements from the north, it contains none from the south, and its fauna is so distinct from either that there is slight hesitation in considering it as equivalent to the American, South American and Patagonian faunas.
The third, the South American fauna, is sharply divisible into the Brazilian and Andean. The Brazilian occupies the rivers from Southern Mexico to Buenos Aires and from Para to Callao, exclusive of the high Andes. This fauna is the richest in species in the world. From this region about ten per cent, of all the known fishes have been recorded. The Andean, from 3,000 to 5,000 feet and over above sea level, while possessing some forms in common with the Brazilian, is quite distinct. The species inhabiting this region would cause surprise if found at; those of the Pacific slope would not.
The Patagonian fauna is among the poorest in the world. It occupies the Rio Negro basin, and everything lying south of it and a line joining it with Valparaiso. Its fauna has been considered in detail.
For convenience these faunal areas may be enumerated as the following 'regions' of unequal value. (1) Transition. (2) Mexican. (3) Brazilian. (4) Andean. (5) Patagonian. I am not sure but that the Titicacan basin constitutes a 'region' distinct from the Andean north of Titicaca.
The origin and distribution of the fresh-water fishes of tropical South America have come about as follows: In the earliest tertiary tropical America consisted of two land areas, Archiguiana and Archamazona, separated by the lower valley of the Amazon, which was still submerged. There was a land mass, Helenis, between Africa and South America, possibly in contact with Guiana in South America and some point in tropical Africa.
This land mass was inhabited, among other things, by Lepidosirenidæ, Pceciliidæ, Characinidæ, Cichlidæ and Siluridæ. This land-mass sank beneath the surface of the ocean, forcing the fauna in two directions, towards Africa and towards South America, exterminating all types not moved to the east or the west. From these two rudiments have developed the present diverse faunas of Africa and South America, each reinforced by intrusives from the ocean and neighboring land areas and by autochthonous development within its own border. The one fauna can not be said to have been derived directly from the other.
The connection between Africa and South America existed before the origin of present genera and even before the origin of some of the present subfamilies and families, some time before the earlier tertiary. There has never been any exchange between Africa and South America since that time. There must have been an intimate connection between these two continents, for there is no evidence such as identical species or genera on the two coasts to indicate an occasional or accidental exchange of types across the Atlantic since the formation of existing genera, therefore such an interchange across the ocean probably never took place. The east Brazilian land mass south of the Amazon (Archamazona) must have become stocked from the western end of Helenis, or Archiguiana, very early, for it contains many genera peculiar to the region, indicating a long separation, and tertiary freshwater deposits in this area contain existing genera of fresh-water fishes.
When, later, the Cordilleras arose out of the ocean at a distance from Archiguiana and Archamazona too great to be traversed by colonists from them, their developing streams and arms of the sea, connected with brackish, and, later, fresh-water lakes, all became populated with marine types. In the north where they later came into competition with immigrants from Archiguiana most of them were exterminated with the continued elevation of the land. On the south, which was not, or not so easily, reached by immigrants, Orestias, Gastropterus and Protistius remain in the high Cordilleras of southern Peru as relicts of these marine species. Later, as the distance between the Cordilleras and Archiguiana was reduced, these mountain streams, especially those of Ecuador and Colombia, became populated by stragglers or accidental visitors from the land areas to the east. These in their turn, with the elevation of the Andes, became modified and gave rise to the genera now peculiar to both slopes of the high Andes, Pygidium, Eremophilus, Chœtostomus, Arges, Cyclopium, Astroblepus, etc.
With the further elevation of the Cordilleras into a continuous barrier and the formation of the Orinoco, Amazon and La Plata valleys through elevation and the debris brought from the land masses and the development of the enormous fresh-water system occupying these valleys, this system, particularly the Amazon, became progressively colonized from the older land areas and became the center of unparalleled adaptive radiation and a new center for distribution which it has remained to the present time. The comparatively few types inhabiting the old eastern land masses found themselves in possession of a continent and diverged in every conceivable direction. I have hinted at this divergence in a recent article (Biol. Bull., VIII., pp. 59-66). It will be considered in detail in a forthcoming monograph of the characins of America.
From the Amazon species moved in all directions till they met barriers of one sort or another. The Pacific slope fauna is derived to a very large extent from this later divergent migration over the Isthmus of Panama and through the valley of the Atrato, between the western and coast Cordilleras of Colombia. Others possibly crossed over the Andes east of Guayaquil before the Andes reached their present height. The Pacific slope fauna is less different from the Amazon fauna than that of the coastwise streams of Minas, if the number of peculiar genera is used as a measure of difference. The origin of the fauna of the plateau of Mexico is a separate subject.
The points of strategic importance for ichthyic chorology in South America are, therefore: (a) western Colombia and Panama; (b) Guayaquil to the Amazon, across the Andes; (c) the tableland of Guiana, Archiguiana; (d) the Rio San Francisco, with the Ria Parahyba and the headwaters of the Tieté and Rio Grande, in Archamazona; and (e) the area between the Rio Negro and the La Plata. It is these regions which will yield the richest scientific harvest to any museum undertaking South American exploration.