Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 May 2004

Sternopygus branco: A New Species of Neotropical Electric Fish (Gymnotiformes: Sternopygidae) from the Lowland Amazon Basin, with Descriptions of Osteology, Ecology, and Electric Organ Discharges

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Page Range: 245 – 259
DOI: 10.1643/CI-03-105R1
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Abstract

Sternopygus branco n. sp. is described from the Amazon River between its confluences with the Rios Japurá and Negro, and from the lower 100 km of the Rio Negro. This species is described using features of external morphology, meristics, pigmentation, osteology, and electric organ discharges (EODs). The new species is diagnosed by a very low EOD repetition rate (24–35 Hz vs approximately 40–300 Hz in congeners); very pale pigmentation on the entire body surface in live specimens; the absence of a pale lateral stripe along dorsal margin of anal-fin pterygiophores (present in adults and/or juveniles in all congeners); a slender body (body depth 8.3–10.9% length to end of anal fin vs 10.3–16.1% in congeners); posterior margins of cleithrum and supracleithrum short and robust. Sternopygus branco is found in the main river channels of whitewater and blackwater rivers and in adjacent side channels. It is not known from seasonally flooded forests or terra firme stream systems. Sternopygus branco is able to rapidly modulate the amplitude of its EOD.

Resumen

Sternopygus branco é descrita como uma espécie nova proveniente do rio Amazonas entre sua confluência com o rios Japurá e com o rio Negro, e também do trecho baixo do rio Negro, até 100 km da sua foz. Esta espécie é descrita com base em características de morfologia externa, dados merísticos, pigmentação, osteologia e propriedades da descarga do órgão elétrico (DOE). Sternopygus branco diferencia-se de outros membros de Sternopygus pelas seguintes características: frequência muito baixa da DOE (24–35 Hz contra 40–300 Hz em outras espécies); pigmentação muito pálida na superfície inteira do corpo em espécimens vivos; ausência de listra lateral pálida ao longo da margem dorsal dos pterigióforos da nadadeira anal (ausente em adultos e/ou juvenis de outras espécies); corpo alongado (altura do corpo 8.3– 10.9% do comprimento até o limite posterior da nadadeira anal contra 10.3–16.1% em outras espécies); margem posterior do cleitro e supracleitro curta e robusta. O habitat de S. branco constitui-se de calhas principais e braços rasos adjacentes de rios de água branca e água preta. Esta espécie não ocorre em florestas sazonalmente alagáveis nem tampouco em riachos (igarapés) de terra firme. Sternopygus branco é capaz de modular o amplitude da DOE espontaneamente.

Copyright: The American Society of Ichthyologists and Herpetologists
 Fig. 1. 
 Fig. 1. 

Map of Tefé area, Brazil showing positions of the holotype (white triangle) and paratype localities (white circles) of Sternopygus branco n. sp. Some symbols represent more than one sample station. The base map is a NASDA JERS-1 SAR radar image from June 1995. Flooded forests appear white illustrating the maximum extent of the annual flood cycle. The area between the thick dotted lines is the Holocene alluvial floodplain (várzea) of high conductivity rivers. River systems draining the Quaternary peneplain to the north and south of the várzea contain low-conductivity blackwaters

 Fig. 2. 
 Fig. 2. 

Diagrammatic representation of the body (A) and head of Sternopygus in left lateral (B), dorsal (C), and ventral (D) views showing landmarks for morphometric measurements reported. Measurements with an asterisk taken with ruler to nearest millimeter. Other measurements taken point-to-point with needle-point calipers to nearest 0.01 mm. Abbreviations are (A): TL, total length; LEA, length to end of anal fin; AFL, anal fin base. (B): BD, body depth; HD, head depth; PR, preorbital head length; ED, eye diameter; HL, head length; PO, postorbital head length; P1, pectoral-fin length. (C): NN, internarial length; IO, interorbital distance; HW, head width. (D): MW, mouth width at rictus; PA, preanal distance; BW, body width

 Fig. 3. 
 Fig. 3. 

Photograph of head of holotype of Sternopygus branco n. sp., MCP 32451 (WGRC 02.070201) and body of paratype MCP 32246 (WGRC 14.070201). Scale bars = 10 mm. Both photographs from live specimens

 Fig. 7. 
 Fig. 7. 

Electric Organ Discharge (EOD) waveform (A) and Fourier Power Spectrum (B) of holotype of Sternopygus branco n. sp. EOD plotted with head-positive upwards. The Power Spectrum was computed from a 2048 point Fast-Fourier-Transform and the Peak-Power-Frequency scaled to the minimum attenuation of 0 dB

 Fig. 5. 
 Fig. 5. 

Lateral view of the neurocranium of Sternopygus branco n. sp. Gray indicates foramina or absence of bone. Abbreviations: MEt, mesethmoid; LEt, lateral ethmoid; Fro, frontal; SOC supraorbital canal bone; SpO, sphenotic; Par, parietal; Exs, extrascapular; EpO, epioccipital; SuO, supraoccipital; ExO, exoccipital; BaO, basioccipital; VEt, ventral ethmoid; Vom, vomer; PaS, parasphenoid; OrS, orbitosphenoid; PtS, pterosphenoid; PrO, prootic; PtO, pterotic; F, foramina for cranial nerves (V2–3. VII, IX, X) and lateral line nerves (LL). Scale bar = 5 mm

 Fig. 4. 
 Fig. 4. 

Head of the holotype of Sternopygus branco n. sp. illustrating organization of cephalic sensory canals and pores. Centerline of canals (ossified and unossified) indicated by dashed lines. Pores indicated by small circles. Eye and anterior and posterior nares shaded gray. Abbreviations: SO, supraorbital; IO, infraorbital; PL, posterior lateral-line; PM, preopercular- mandibular; ST, supratemporal; M, medial. Scale bar = 10 mm

 Fig. 6. 
 Fig. 6. 

Lateral view of the suspensorium and pectoral girdle of Sternopygus branco n. sp. Gray indicates cartilage. Abbreviations: Mes, mesopterygoid; Met, metapterygoid; Hyo, hyomandibula; Ope, opercle; Max, maxilla; Den, dentary; Ang, anguloarticular; Ret, retroarticular; Qua, quadrate; Sym, symplectic; Int, interopercle; Pre, preopercle; Sub, subopercle; Pos, posttemporal; Sup, supracleithrum; Cle, cleithrum; Cor, coracoid; Sca, scapula; Fpr, first pectoral ray; Prr, proximal radials. Scale bar = 5 mm

 Fig. 8. 
 Fig. 8. 

Electric Organ Discharge (EOD) waveform of holotype of Sternopygus branco n. sp. showing spontaneous changes in amplitude of discharge. The central dashed line represents 0 V. The EOD is truncated by amplifier overload at the dotted lines

 Fig. 9. 
 Fig. 9. 

Part of Northern South America showing collection records of Sternopygus branco n. sp. 1 = holotype locality. Some symbols represent more than one locality. Base map by M. Weitzman

Contributor Notes

(WGRC, JSA) Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611; and (KGH) Department of Zoology, University of Florida, Gainesville, Florida 32611. (WGRC) willc@flmnh.ufl.edu Send reprint requests to (WGRC).

Accepted: 24 Oct 2003
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