THE CICHLID FISH OF CUATRO CIÉNEGAS, MEXICO: DIRECT EVIDENCE OF CONSPECIFICITY AMONG DISTINCT TROPHIC MORPHS.

Abstract

The idea that different individuals within a biological species may have radically different morphologies and distinct feeding modes has commanded attention because of its profound implications for evolutionary biology. If individuals from a panmictic population are recognized as species by standard taxonomic criteria, organismal diversity in nature may be more apparent than real. This concern is particularly germane to the great diversity seen within assemblages of tropical freshwater fishes such as cyprinids and cichlids, where endemic specializations are extreme (Myers, 1960; Greenwood, 1974; Reid, 1980). Several examples of substantial dichotomous morphological divergence have been reported among sympatric collections of closely related fishes (Roberts, 1974; Sage and Selander, 1975; Vrijenhoek, 1978; Turner and Grosse, 1980). The cichlids of Cuatro Cienegas, Mexico are particularly interesting and their study by Sage and Selander set a precedent which has had a significnat impact on subsequent systematic studies (Kirkpatrick and Selander, 1979; Graves and Rosenblatt, 1980). By any phenetic criteria, the extreme distinctiveness of sympatric cichlids in Cuatro Cienegas suggests the presence of two discrete species. A small tooth form with papilliform pharyngeal dentition feeds on plant material and possesses a relatively narrow head, slender pharyngeal jaw, and long intestine. A large tooth form with molariform pharyngeal teeth feeds on snails and has a wider head, relatively stouter jaw, and short intestine. The morphological and trophic differentiation between these two forms exceeds that observed among many closely related biological species and precisely mimics the differences which define numerous cichlids in the African Great Lakes (Fryer and Iles, 1972). Further, it is this type of variation that might be associated with incipient macroevolution (Greenwood, 1979). The magnitude of differences between these forms has thus historically caused them to be treated as two distinct species (Taylor and Minckley, 1966; Minckley, 1969; LaBounty, 1974; Kornfield and Koehn, 1975). Alternatively, on the basis of the segregation of both types within wild-caught broods, complete electrophoretic similarity, and concordant patterns of allozymic variation between forms, Sage and Selander (1975) suggested that both forms belong to a single polymorphic species. However, this evidence did not provide a definitive test of the two species hypothesis (Hutchinson, 1978). We present in this article the results of complementary studies of genetics and reproductive biology initiated to resolve the biological status of these fishes. Our allozyme data and observations of mating in natural populations verify the insight of Sage and Selander and attest to their accurate electrophoretic evaluation of conspecificity. This demonstration of substantial morphological variation within a single species has significant implications for ecology and conventional taxonomic characterization.