Functional Analysis of Sexual Dimorphism in an Electric Fish, <i>Hypopomus pinnicaudatus, </i>Order Gymnotiformes

Abstract

Hypopomus pinnicaudatus, an electric fish, has a marked sexual dimorphism in its tail filament. Sexually mature males have long, 'feathered' tails as compared with females. The sexual dimorphism emerges when a fish reaches about 110 mm total length. Mature males have larger electrocytes which are more widely spaced and more numerous than those in mature females: The biphasic electric organ discharge (EOD) is longer in males than in females. The peak-to-peak amplitude of the male's EOD is weaker than a female's of the same total length. The weaker discharge is unexpected given the increase in size and number of electrocytes. It is suggested that the reduction in EOD amplitude is a consequence of the increase in EOD duration among males. Further, female choice probably played a role in the evolution of long duration EODs among males, and males may have secondarily grown long tails to compensate for the loss in active space that would otherwise accompany a weaker EOD.

Electrosensory prey detection in the African sharptooth catfish, Clarias gariepinus (Clariidae), of a weakly electric mormyrid fish, the bulldog ( Marcusenius macrolepidotus )

“…EOD amplitude increased with fish size (Fig. 6; as also observed in gymnotids : Knudsen 1975;Caputi et al 1989;Hopkins et al 1990). Even though the electromotive force of an electric organ is species specific and independent of fish length, measured field amplitude associated with an EOD increases with fish length because the internal resistance of the whole fish body is an inverse function of its length (Caputi and Budelli 1995;Caputi 1999).…”

Dimorfismo sexual em Siluriformes e Gymnotiformes (Ostariophysi) da Amazônia

“…Espécies da família Hypopomidae (como os demais Gymnotiformes, à exceção de Apteronotidae) não possuem nadadeira caudal, mas um apêndice terminal, cilíndrico, contendo colunas de eletrócitos. Machos maduros de algumas espécies, como por exemplo Brachyhypopomus pinnicaudatus, B. beebei, B. occidentalis e B. brevirostris possuem o filamento caudal mais longo e grosso do que as fêmeas (Hagedorn & Carr, 1985;Hopkins et al, 1990). O tamanho do filamento pode estar relacionado ao tamanho dos eletrócitos, sendo maiores nos machos (Hagedorn & Carr, 1985).…”

“…Em Brachyhypopomus occidentalis a amplitude da descarga dos machos tende a ser mais forte e de maior duração do que as das fêmeas (Hagedorn 1988;Hagedorn & Carr, 1985). Em Brachyhypopomus pinnicaudatus, machos grandes possuem mais eletrócitos no filamento caudal do que fêmeas grandes, e conseqüentemente uma descarga mais forte (Hopkins et al, 1990 que fêmeas, as quais geram descargas com maior freqüência, embora machos maduros sejam maiores que fêmeas e produzam descargas mais fortes (Hagedorn & Heiligenberg, 1985). O mesmo foi observado em machos de Sternopygus (Fleishman, 1992;Dunlap et.…”

“…Filamentos prolongados ocorrem nas nadadeiras dorsais em machos dimórficos (Garzon & Acero, 1988). Em peixes que não possuem nadadeira caudal, prolongamentos na porção posterior do corpo diferenciam machos e fêmeas (Hagedorn & Carr, 1985;Hopkins et al, 1990). Além destas, existe uma série de características nos peixes que são tipicamente diferenças sexuais dimórficas que desaparecem logo após a reprodução, tais como tubérculos nupciais (em ciclídeos) (Heins & Baker, 1989) e gibosidades (em salmões, poecilídeos e muitos outros) (Shepard & Burr, 1984;Quinn & Foote, 1994;Barlow & Siri, 1997;Moran et al, 1999).…”

Communication Behavior and Sensory Mechanisms in Weakly Electric Fishes

“…Both Hypopomus occidentalis and Hypopomus pinnicaudatus emit EODs resembling single-cycle sinusoids, with the male EODs being of longer duration compared to those of females ' (Hagedorn and Carr, 1985;Hopkins et al, 1990). When female H. occidentalis were stimulated with trains of single-cycle sinusoidal pulses of male duration they gave more discharge rate responses ("decrement bursts";Hagedorn, 1988) compared to under stimulation with pulses of shorter (female) duration (Shumway and Zelick, 1988).…”