Morphometric analyses of 220 specimens of a characid, Bryconops sp. cf. melanurus, from the Brazilian Pantanal were used to describe allometric growth in that species and determine whether specimens from highland habitats were more streamlined than those from lowland habitats. Relative warp analysis of 14 landmarks and principal component analysis of 28 interlandmark distances returned complementary results. The increased streamlining of the highland specimens is highly consistent with known inductive effects of high water velocity on fish phenotypes. Genetic differentiation and inductive effects of temperature variation are also potential explanations of the observed phenotypic differentiation.Key words Ecomorphology · Morphometrics · Polymorphism · South America (Fig. 1). Bryconops sp. cf. melanurus from the Pantanal belongs to a monophyletic group including B. melanurus (Bloch), B. inpai (Knöppel et al.), and B. affinis (Günther) (Chernoff and Machado-Allison, 1999).Bryconops sp. cf. melanurus and Bryconops melanurus are distinguished from the other members of this group by the coloration of the caudal fin. The caudal fins of Bryconops sp. cf. melanurus and B. melanurus have a central dark stripe that is lacking in all other species of Bryconops. Bryconops melanurus has a clearly defined stripe that occupies the central rays of the caudal fin with clear areas above and below the stripe (Chernoff et al., 1994: fig. 2). In the species from the Pantanal, the caudal fin stripe extends well up onto the fin rays of the dorsal lobe, and in larger specimens almost the entire dorsal lobe of the caudal fin is darkened. Bryconops. sp. cf. melanurus and B. melanurus also differ in the thickness of the lateral stripe, the anteroposterior position of the pelvic-fin insertion, and the degree of denticulation of the gill rakers of the first pharyngeal arch. The two species also inhabit nonoverlapping geographic ranges, with B. melanurus occurring in the Guyanas (Chernoff et al., 1994).Because this sample of Bryconops sp. cf. melanurus was collected from a variety of highland and lowland stream habitats, it provided an excellent opportunity to study intraspecific phenotypic variation in the context of environmental and geographic variation. Our objectives in this study were (1) to quantify the phenotypic variation of
IchthyologicalResearch
Fossil fishes from the Miocene La Venta fauna of the Magdalena River Valley, Colombia, are identified as Colossoma macropomum (Characidae), a living species from the Orinoco and Amazon basins. The fossils document a long and conservative history for a species that is highly specialized for feeding on streamside plants. The phylogenetically advanced position of Colossoma in the subfamily Serrasalminae implies that six related genera and other higher characid taxa originated well before 15 million years ago. This discovery also corroborates neontological evidence for a vicariance event that contributed species from Miocene Orinoco-Amazon faunas to the original Magdalena region fauna. The fossils suggest a formerly diverse Magdalena fauna that has suffered local extinction, perhaps associated with late Cenozoic tectonism. This new evidence may help explain the depauperate nature of the modern Magdalena River.
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