Cristina Cox Fernandes

Magosternarchus, a New Genus with Two New Species of Electric Fishes (Gymnotiformes: Apteronotidae) from the Amazon River Basin, South America

A new group of predaceous apteronotid electric fishes is described from recent trawl collections made in large, white- and black-water river channels of the Amazon Basin in Brazil. Magosternarchus, n. gen., is diagnosed by greatly enlarged jaws and teeth. The type species, Magosternarchus raptor, n. sp., possesses uniquely enlarged premaxillary bones and an associated hypertrophied mesethmoid. Magosternarchus duccis, n. sp., is unique in its strongly projecting lower jaw that includes and often extends dorsal to the upper jaw and snout. The only identifiable stomach contents in specimens of both species of Magosternarchus are tails of other gymnotiform fishes. Shared features of the ethmoid and jaw bones, mesopterygoid, opercle, gillrakers, basibranchials, and infraorbital laterosensory canal suggest that Magosternarchus is most closely related to Sternarchella. Um novo grupo de peixes eletricos predadores, capturados recentemente com redes de arrasto, e descrito proveniente de canais de rios da Bacia Amazonica Brasileira. Magosternarchus, gen. nov., e diagnosticado pela presenca de dentes e de mandibulas bem desenvolvidas. A especie tipo, Magosternarchus raptor, sp. nov., possui uma expans ao dos premaxilares associada ao mesetmoide hipertrofiada que e unica entre os peixes eletricos. Magosternarchus duccis, sp. nov., e diferenciado por apresentar a mandibula inferior prolongada, a qual se extende dorsalmente sobre a mandibula superior e o focinho. O unico material identificado no conteudo estomacal dos especimens das duas especies de Magosternarchus foram elementos das nadadeiras caudais e das vertebras de outros peixes eletricos. Caracteres comuns relativos aos ossos etmoide e das mandibulas, mesopterigoide, operculo, rastros branquiais, elementos do basibranquial e canal do infraorbital sugerem que Magosternarchus e filogeneticamente mais proximo do genero Sternarchella.

Dimorfismo sexual em Siluriformes e Gymnotiformes (Ostariophysi) da Amazônia

The present paper aims to summarize cases of sexual dimorphism in Siluriformes and Gymnotiformes. This summary focuses on Neotropical representatives, with emphasis on Amazonian fish. Fishes from this region exhibit high species-level diversity, although the intraspecific limits of many of these forms are unknown. In Loricariidae (Siluriformes), mapping of dimorphic traits on cladograms has helped to demonstrate that patterns of sex dimorphism are consistent with hypotheses of monophyly, thus reinforcing its role as an important evolutionary phenomenon. In Apteronotidae (Gymnotiformes), mapping of characters on phylogenetic trees suggests that sexual dimorphism (regarding snout size, form, and presence/absence of teeth) has evolved independently in multiple taxa. Recently in Gymnotiformes, detected taxonomic errors are attributed to extreme differences between males and females in their morphology.

Structure of Fish Assemblages in Amazonian Rain-Forest Streams: Effects of Habitats and Locality

Abstract We examined fish assemblage structure in three headwater rain-forest streams in the Urubu River Basin, Central Amazonia, as it relates to habitat heterogeneity. Riffles and pools, two types of stream habitat, were defined by current, depth, and substrate parameters, adjusted for Amazonian stream characteristics. We assessed species richness, composition, and abundance as they varied between riffles and pools across three similar streams. Four riffles and four pools were identified in each stream and sampled four times during 1996 and 1997. The samples yielded a total of 856 individuals of 22 species. The most abundant species were in the families Characidae, Lebiasinidae, Helogenidae, and Rivulidae. We found an interactive influence of riffle/pool habitat and stream factors on species richness, composition, and abundance. Rain-forest stream fish assemblages are structured by both habitat and stream locality.

Largest of All Electric-Fish Snouts: Hypermorphic Facial Growth in Male Apteronotus hasemani and the Identity of Apteronotus anas (Gymnotiformes: Apteronotidae)

Abstract South American gymnotiform electric fishes exhibit sexual dimorphism of shape within species, and divergence of shape among species. Recent collecting in floating vegetation mats near Manaus, Brazil, yielded a remarkable association of female and “normal” males of Apteronotus hasemani plus a series of sexually mature male specimens with greatly hypertrophied snouts and gapes. We argue that these fish represent a single species based on shared distinctive features of morphology and coloration, continuous variation of morphometric characters including allometric and dimorphic facial growth in males, ecological and possible reproductive association, and identity in 16S mt rDNA sequences. The degree of dimorphism shown by the large males greatly exceeds previously known limits of intraspecific variation for A. hasemani. The males with the most extreme snouts and gapes closely approach the holotype of Apteronotus anas that is also a mature male. We conclude that A. anas is based on a large male of A. hasemani; the older-named A. hasemani is the senior synonym.

Redescription of Orthosternarchus tamandua (Boulenger, 1898) (Gymnotiformes, Apteronotidae), with reviews of its ecology, electric organ discharges, external morphology, osteology, and phylogenetic affinities

ABSTRACT In this paper, we redescribe the apteronotid fish Orthosternarchus tamandua, the only species of its genus. Historically, O. tamandua was rare in collections and only limited anatomical data was available for this species. However, numerous specimens have become available recently as a result of deep-water trawls in the main channel of the Amazon River, thereby making this redescription possible. In addition to describing the external anatomy and osteology of O. tamandua, we present data on their electronic organ discharges (EODs) recorded at time of capture, and review what is known of its distribution and ecology. We describe and illustrate the osteology of O. tamandua, and in doing so, clarify many aspects of its skeleton. For instance, contrary to previous reports, O. tamandua does possess a lateral ethmoid, although it is exceptionally small. We conclude by discussing the EODs of O. tamandua in comparison to those of other apteronotids and the anatomical characters used in the support of phylogenetic hypotheses of the genus within the family Apteronotidae.

Sex-Related Morphological Variation in Two Species of Apteronotid Fishes (Gymnotiformes) from the Amazon River Basin

with the classification of fishes is sexual dimorphism: pronounced dimorphism can be mistakenly interpreted as species differences (Darwin, 1871; Andersson, 1994). In this paper, I examine sexual dimorphism in two species of apteronotid Neotropical electric fishes. The detection of sexual dimorphism can help to clarify the species-level taxonomy and stresses the importance of considering sex differences. Gymnotiformes are, for the most part, weakly electric fishes, endemic to the Neotropics. The electric organ discharges of most gymnotiforms are weak (2-3 volts), except for Electrophorus electricus, a species capable of stunning or killing

Two new species and a new subgenus of toothed Brachyhypopomus electric knifefishes (Gymnotiformes, Hypopomidae) from the central Amazon and considerations pertaining to the evolution of a monophasic electric organ discharge

Abstract We describe two new, closely related species of toothed Brachyhypopomus (Hypopomidae: Gymnotiformes: Teleostei) from the central Amazon basin and create a new subgenus for them. Odontohypopomus, new subgenus of Brachyhypopomus, is diagnosed by (1) small teeth present on premaxillae; (2) medialmost two branchiostegal rays thin with blades oriented more vertically than remaining three rays; (3) background color in life (and to lesser extent in preservation) distinctly yellowish with head and sides peppered with small, widely spaced, very dark brown stellate chromatophores that greatly contrast with light background coloration; (4) a dark blotch or bar of subcutaneous pigment below the eye; (5) electric organ discharge waveform of very long duration (head-positive phase approx. 2 milliseconds or longer, head-negative phase shorter or absent) and slow pulse repetition rate (3–16 Hz). The type species of the new subgenus, Brachyhypopomus (Odontohypopomus) walteri sp. n., is diagnosed by the following additional character states: (1) subcutaneous dark pigment at base of orbit particularly prominent, (2) body semi-translucent and nearly bright yellow background coloration in life, (3) a biphasic electric organ discharge (EOD) waveform of very long duration (between 3.5 and 4 milliseconds at 25° C) with head-positive first phase significantly longer than second head-negative phase in both sexes. Brachyhypopomus (Odontohypopomus) bennetti sp. n. is diagnosed by two character states in addition to those used to diagnose the subgenus Odontohypopomus: (1) a deep electric organ, visible as large semi-transparent area, occupying approximately 14–17% body depth directly posterior to the abdominal cavity in combination with a short, but deep, caudal filament, and (2) a monophasic, head-positive EOD waveform, approximately 2.1 milliseconds in duration in both sexes. These are the only described rhamphichthyoid gymnotiforms with oral teeth, and Brachyhypopomus bennetti is the first Brachyhypopomus reported to have a monophasic (head-positive) EOD waveform. Unlike biphasic species, the waveform of its EOD is largely unaffected by tail damage from predators. Such injuries are common among specimens in our collections. This species’ preference for floating meadow habitat along the major channels of the Amazon River basin may put it at particularly high risk of predation and “tail grazing.”

Sexual Dimorphism in Apteronotus Bonapartii (Gymnotiformes: Apteronotidae)

Abstract Here we document sexual dimorphism in both external measurements of the head and discrete features of the skull in Apteronotus bonapartii. To quantify morphological variation in A. bonapartii, ten external morphometric measurements were taken from the head and body of 58 individuals. By using Discriminant Function Analysis, we found that the best predictors for distinguishing sexes are the body depth, snout length, and the distance from the tip of the lower jaw to rictus (i.e., the gape length). Two measurements showed significant differences between males and females: supraoccipital to base of dorsal thong and tip of the lower jaw to rictus. Most skeletal elements were found to be similar between males and females; however, in males some elements of the skull roof (e.g., frontal) and suspensorium (e.g., endopterygoid) are elongate relative to females, and the lateral ethmoid is more obliquely positioned. In females, the dentary and anguloarticular are relatively deep and come together in a sharp angle dorsally, giving the lower jaw an almost triangular shape; in males the dorsal margin of the lower jaw is smoothly arched. No presente trabalho nós documentamos o dimorfismo sexual em Apteronotus bonapartii utilizando medidas externas e características do crânio. Para quantificar a variação morfológica em A. bonapartii, dez medidas morfométricas foram obtidas da cabeça e corpo de 58 indivíduos. Através da Análise de Função Discriminante, os melhores preditores para distinguir machos e fêmeas foram a altura da cavidade abdominal, comprimento do focinho, e distância da mandíbula ao rictus. Na Análise de Covariância, duas medidas mostraram diferenças significativas entre machos e fêmeas: a altura do corpo na base do supraoccipital e mandíbula ao rictus. A maioria dos elementos do crânio dos machos e fêmeas são similares. Entretanto, em machos alguns elementos do crânio (e.g., frontal) e suspensório (e.g., endopterigóide) são alongados, em comparação com as fêmeas. Em machos, o etmóide lateral é posicionado mais oblíquamente do que nas fêmeas. Em fêmeas o dentário e o ânguloarticular têm maior altura e proporcionam uma forma quase triangular na porção anterior da mandíbula, onde em machos a margem dorsal da mandíbula é levemente arqueada.

Procerusternarchus pixuna, a new genus and species of electric knifefish (Gymnotiformes: Hypopomidae, Microsternarchini) from the Negro River, South America

ABSTRACT. Procerusternarchus pixuna, a new genus and species of hypopomid gymnotiform, is described from several tributaries of Negro River, Amazonas State, Brazil. The new genus is included in the tribe Microsternarchini with the genera Microsternarchus and Racenisia based on a phylogenetic analysis of 41 anatomical, morphometric, meristic, and electric organ discharge variable characters. The new taxon is distinguished from Microsternarchus and Racenisia by the apomorphic presence of a deep, enlarged myogenic electric organ, an electric organ discharge with a greatly reduced negative phase and the plesiomorphic condition of a fully-scaled body, whereas scales are absent from the upper back of Microsternarchus and Racenisia.

Hormonal and behavioral correlates of morphological variation in an Amazonian electric fish (Sternarchogiton nattereri: Apteronotidae)

The weakly electric fish from the main channel of the Amazon river, Sternarchogiton nattereri, offers a striking case of morphological variation. Females and most males are toothless, or present only few minute teeth on the mandible, whereas some males exhibit exaggerated, spike-like teeth that project externally from the snout and chin. Androgens are known to influence the expression of sexually dimorphic traits, and might be involved in tooth emergence. In this study we assess the relationship in S. nattereri between morphological variation, 11 ketotestosterone (11-KT) and testosterone (T). We also examine relationships of morphology and androgen levels with electric organ discharge (EOD) frequency, reproductive condition, and seasonality. Our main finding is that male morph categories differed significantly in plasma concentrations of 11-KT, with toothed males showing higher levels of 11-KT than toothless males. By contrast, we did not detect statistical differences in T levels among male morph categories. Reproductive condition, as measured by gonadosomatic indexes (GSI), differed across two sample years, increased as the season progressed, and was higher in toothed males than in non-toothed males. EOD frequency was higher in toothed males than in either toothless males or females. Taken together, our findings suggest that S. nattereri male sexual characters are regulated by 11-KT levels, and that both morphology and androgens interact with reproductive condition and EOD frequency in ways that vary within and across reproductive seasons.