Origin and Evolution of the Fish Fauna of Western Ecuador

Introduction: The fish fauna of western Ecuador is much less diverse than that of the eastern Amazon region. There are approximately 140 species regularly inhabiting freshwaters of the west, of which approximately 80 come from families that primarily occur in freshwater (Barriga, 1991). The lower number of species in the western region is probably due to a number of factors, including the much smaller size of the drainages and the drier conditions.

Origin: The primary freshwater fish species in western Ecuador are mostly a derived subset of the eastern fauna that split off millions of years ago with the rise of the Andes Mountain Range (Eigenmann, 1921a, b). The common origin of the fish fauna east and west of the Andes in Ecuador is evidenced by the similarity at higher taxonomic ranks. Of the 40 genera of western fishes from primary freshwater families, 30 have representatives both in western and eastern Ecuador. The fish fauna of western Ecuador has been evolving independently for millions of years, however, and is quite distinct from all other South American fish faunas (including western Colombia). It was described by Eigenmann (1921b) as "old and highly specialized" and as an assemblage extends from just north of the Esmeraldas drainage to northern Peru. The fish fauna of western Colombia is very different from that of western Ecuador, and Eigenmann (1921b) indicated that the fish fauna of the Magdalena drainage in Colmbia, for example, is more distinct from that of western Ecuador than from the fauna on the eastern side of the Andes in Colombia. These two faunas thus appear to have distinct origins and independent evolutionary histories. The delimitation of the Ecuadorian and Colombian faunas appears to occur somewhere between the Mira and Esmeraldas basins (Eigenmann, 1921b).

Evolution: The divergent nature of the fish fauna in western Ecuador relative to that in eastern Ecuador is evidenced by the existence of endemic genera, particularly in the order Characiformes. Western Ecuador harbors 10 genera (nine from the order Characiformes) that are not found in eastern Ecuador (Barriga, 1991; Table A). Of these, the five characid genera are only found in Ecuador and the other genera include species that occur in northwestern Peru, western Colombia, and Central America. Piabucina is the most widely distributed of these genera and includes species occurring in other parts of northern South America including Venezuela and Guyana.

Table A. Genera from western Ecuador that are absent in eastern Ecuador (From Barriga, 1991).

Order	Family	Genus	No. spp. W. Ecu.	Distribution of Genus
Characiformes	Characidae	Iotabrycon	1	Endemic to Vinces R., Guayas D.1
Characiformes	Characidae	Landonia	1	Endemic to Vinces R., Guayas D.1
Characiformes	Characidae	Phenacobryon	1	Endemic to Vinces R., Guayas D.1
Characiformes	Characidae	Pseudochalceus	2	Endemic to Western Ecuador1
Characiformes	Characidae	Rhoadsia	2	Endemic to Western Ecuador2
Characiformes	Paradontidae	Piabucina	2	W. Ecuador, NW Peru, W. Colombia, Venezuela, Guayana1
Characiformes	Paradontidae	Saccodon	1	W. Ecuador, NW. Peru, W. Colombia, C. America1
Characiformes	Prochilodontidae	Ichthyolephas	1	W. Ecuador, W. Colombia1
Characiformes	Curimatidae	Pseudocurimata	5	W. Ecuador, NW Peru, W. Colombia3
Silurifomres	Trychomycteridae	Eremophilus	1	W. slope Andes, Bogota D. in Colombia1

¹Fishbase, ²Gery (1977), ³Vari (1989)

These 10 genera probably evolved from eastern genera since the rise of the Andes and have diverged enough to warrant their placement in new genera. In addition, there are only two species listed by Barriga (1991) that occur both on the eastern and western sides of the Andes, Hoplias malabaricus (Characiformes, Erythrinidae) and Synbranchus marmoratus (Synbranchidae, Synbranchiformes). Thus, though the fauna of western Ecuador is derived from that of eastern Ecuador, the species composition in each region is completely distinct.

Questions/Future Research: Relatively little research has been carried out on the ecology and evolution of the fish fauna of western Ecuador, so there is much to learn about it. Perhaps the most basic question is how have fish communities in western Ecuador evolved since they were separated from the assemblages of eastern Ecuador? In addition, it would be interesting to examine the extent to which different taxa have responded in similar ways in terms of species richness, morphological disparity, morphological innovations, and physiological adaptations. There are many other questions that have yet to be answered as well. For example, how different are the community assemblages in the Santiago, Esmeraldas, and Guayas rivers? How much do species shared among drainages differ genetically from one drainage to the next? How much do the fish communities differ in the rivers of the Northern and Southern Coastal drainages? Is there significant endemism in any of the rivers of the Coastal drainage systems? Of course given the extent of human induced environmental degradation in western Ecuador, we must also begin to try to understand the status of species, especially endemics, but also rare and commercially important species. It is possible, unfortunately, that many fish species in the region are presently facing the threat of extinction.

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Last Updated June 6, 2008