Rapid morphological change in multiple cichlid ecotypes following the damming of a major clearwater river in Brazil.

While anthropogenic disturbances can have damaging effects on biodiversity, they also offer an opportunity to understand how species adapt to new environments and may even provide insights into the earliest stages of evolutionary diversification. With these topics in mind, we explored the morphological changes that have occurred across several cichlid species following the damming of the Tocantins River, Brazil. The Tocantins was once a large (2,450 km), contiguous river system; however, upon closure of the Tucuruí Hydroelectric Dam in 1984, a large (~2,850 km2), permanent reservoir was established. We used geometric morphometrics to evaluate changes in native cichlids, comparing historical museum specimens collected from the Tocantins to contemporary specimens collected from the Tucuruí reservoir. Six species across five genera were included to represent distinct ecomorphs, from large piscivores to relatively small opportunistic omnivores. Notably, statistically significant changes in shape and morphological disparity were observed in all species. Moreover, the documented changes tended to be associated with functionally relevant aspects of anatomy, including head, fin, and body shape. Our data offer insights into the ways cichlids have responded, morphologically, to a novel lake environment and provide a robust foundation for exploring the mechanisms through which these changes have occurred.

Cranial morphology and osteology of the sexually dimorphic electric fish, Compsaraia samueli Albert Crampton (Apteronotidae, Gymnotiformes), with comparisons to C. compsa (Mago-Leccia).

Sexual dimorphism of the snout has evolved independently in at least four separate clades of the gymnotiform family Apteronotidae. This phenomenon may help identify sex, except in the absence of mature individuals, and has led to confused taxonomy for several species. We examined a large collection of Compsaraia samueli collected during the breeding season from a remote stream in the Rio Negro drainage. This collection contains a wide range of sizes of both sexes, but most individuals were easily identified as mature. To quantify the sexual dimorphism of these specimens, 15 measurements were taken from the head and the body. In addition, some specimens were cleared-and-stained to study cranial osteology. We found that long-snouted males of C. samueli span a wide range of body sizes. As the snout length increases the distance between the eye and the occiput does not increase at the same rate, suggesting that it is only the anterior portion of the head that has an increased allometry. Skeletal anatomy differs between the sexes in that the lower jaw is more triangular in females and more linear in males. The coronomeckelian is small and round in females in contrast to being longer and pointed in males. There is strong interlacing of the dentary and anguloarticular bones in males, whereas this contact is not as extensive in females. We also discuss the implications of sexual dimorphism for identification of this species relative to its congener (C. compsa), and for the evolution of sexual dimorphism in the family.

Sex differences in the electrocommunication signals of the electric fish Apteronotus bonapartii.

The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior.

Dimorfismo sexual em Siluriformes e Gymnotiformes (Ostariophysi) da Amazônia / Sexual dimorfism in Amazonian Siluriformes and Gymnotiformes (Ostariophysi)

No presente trabalho é feito um levantamento dos casos de dimorfismo sexual em Siluriformes e Gymnotiformes. Este levantamento se restringe às formas Neotropicais, com ênfase mais especificamente, às da Amazônia. Peixes dessa região possuem uma grande diversidade, ainda que os limites intraespecíficos não estejam bem definidos. Embora preliminar, o mapeamento de características dimórficas em um cladograma referente a família Loricariidae (Siluriformes) auxilia na demonstração de que padrões de dimorfismo sexual são consistentes com hipóteses de monofiletismo. Em Apteronotidae (Gymnotiformes), com base no mapeamento dos caracteres tamanho e forma do focinho e presença de dentes diferenciada em árvores filogenéticas, podemos inferir que esses caracteres originaram-se como eventos independentes em vários táxons. Recentemente, em Gymnotiformes, foram detectados casos de erros taxonômicos atribuídos a diferenças extremas entre machos e fêmeas.

Amazonian ecology: tributaries enhance the diversity of electric fishes.

Neotropical rivers support a diverse array of endemic taxa, including electric fishes of the order Gymnotiformes. A comprehensive survey of the main channels of the Amazon River and its major tributaries (>2000-kilometer transect) yielded 43 electric fish species. Biogeographical analyses suggest that local mainstem electric fish diversity is enhanced by tributaries. Mainstem species richness tends to increase downstream of tributary confluences, and species composition is most similar between tributaries and adjacent downstream mainstem locations. These findings support a "nodal" or heterogeneous model of riverine community organization across a particularly extensive and diverse geographical region.