Gymnotus Linnaeus

James S. Albert & William G. R. Crampton, 2003, Seven new species of the Neotropical electric fish Gymnotus (Teleostei, Gymnotiformes) with a redescription of G. carapo (Linnaeus)., Zootaxa 287, pp. 1-54 : 46-49

[[ Genus Gymnotus Linnaeus ]]

Discussion

Species and populations. The new taxa described in this report are advanced as hypotheses of species or populations, to be tested with additional morphological, molecular sequence, and electric organ discharge data. Here we regard species as taxa representing separate evolutionary lineages, and populations as taxa representing geographic variants of a single evolutionary lineage (Ricklefs 1989; Moritz 1994; Da Silva & Patton 1998). Maintaining distinct phenotypes in sympatry is regarded as direct evidence that populations represent different species. Several morphological traits found useful in diagnosing sympatric species also provided information on the amount and quality of interspecific phenotypic differences observed in allopatry. In Gymnotus these are differences of four or more precaudal vertebrae, 3-5% head length (as percentage total length), and the appearance of dark bands (continuous vs. broken) above the lateral line at midbody. Discontinuities in the geographic variation of these meristic, morphometric, and color traits distinguish G. carapo from several allopatric species (i.e., G.bahianus , G. diamantinensis , G. inaequilabiatus , and G. sylvius ). None of these traits differ among the six allopatric populations of G. carapo s.s. recognized here. The six allopatric populations of G. carapo s.s. recognized here do not possess diagnostic characters, but they do differ in the mean values of several morphological traits exhibiting overlapping values (Tables 2 and 3).

Locality sampling density of G. carapo s.s. across the landscape to date is too poor to apply the criterion of continuity in morphological variation to distinguish species from subspecies (Fig. 2). In comparison to previous studies of gymnotiform fishes the materials examined for this study are relatively extensive, including 781 lots collected over the whole of tropical South America over a period of 137 years (1865 to 2002). Nevertheless, the geographic sampling density remains too sparse to assess the nature of phenotypic intergrades between allopatric populations of G. carapo s.s. These collections are concentrated near or on the main axis of the Amazon river and some of its major tributaries, the Western Amazon, the llanos of Venezuela, and the Atlantic coast of the Guyanas. There are large areas of little or no information in the interior basins of the Guyanas and Brazilian shields, the Rio Negro, Rio Juruá and Rio Purus basins. This distribution apparently reflects a bias in the distribution of gymnotiform collections. A search using the NEODAT online database for several widespread nominal gymnotiform species in South America returned 324 georeferenced records for “ Gymnotus carapo ”, 332 records for “ Sternopygus macrurus ”, 75 records for “ Apteronotus albifrons ”, and 82 records for “ Brachyhypopomus brevirostris ” or “ Hypopomus brevirostris ”. With very few exceptions the distributions of these nominal species are qualitatively similar to that of G. carapo .

Trans-Andean distributions. The biogeographic distributions of the G. carapo and G. pantherinus species-groups suggest relatively ancient origins for these taxa (Figs. 2, 4, 12). The G. carapo species-group is widespread and diverse on both slopes of the Andes; in all cis-Andean watersheds of Neotropical South America, except several isolated coastal basins in the extreme northeast of Brazil (e.g., Salgado, Piranhas), and several trans-Andean drainages on the Pacific slope of Ecuador and Colombia (e.g., Guayaquil, Esmeraldas, San Juan, Baudó) and the Caribbean (Atrato). Many species within the G. carapo species-group are sympatric; G. carapo , G. arapaima , G. mamiraua , and G. tigre exhibit zones of sympatry in the Amazon basin, G. choco and G. henni in the Chocó region of Colombia, and G. inaequilabiatus , G. sylvius and G. paraguensis in the Paraguay basin. The G. pantherinus species-group is also widespread and diverse in cis-Andean watersheds, and is also present in Panama. These observations suggest that the origins and early divergence of these two species-groups minimally predate the most recent Andean orogeny, c. 8-12 mya (Lundberg 1998).

Summary

The geographically widespread taxon G. carapo s.s. Linnaeus is redescribed and six allopatric populations are recognized. Gymnotus carapo s.s. differs from all congeners except G. arapaima in the following unique combination of characters: clear patch at caudal end of anal fin without oblique hyaline and dark stripes; two laterosensory canal pores in dorso-posterior corner of preopercle; 16-27 [mean 22] dark pigment band-pairs with irregular margins often broken into spots above lateral line on anterior half of body; no pale blotches on head; circular scales, about as long as wide above lateral line at midbody; deep body, its depth 9.7-12.1% total length (mean 10.2%); long head, its length 10.2-13.7% total length (mean 12.3%); 33-37 (mode 33) precaudal vertebrae; 6-27 ventral lateral line rami (mean 15); and no dorsal lateral line rami. Gymnotus carapo s.s. from the Western Amazon can be distinguished from sympatric G. arapaima by: 1, fewer pored lateral line scales to the first ventral lateral line ramus (44-54 [mode 48] vs. 53-64 [mode 57]); 2, fewer scales over the anal fin pterygiophores (8-10 [mode 9] vs. 10-13 [mode 12]); 3, smaller size (maximum total length 418 mm vs. 550 mm). Six allopatric populations of G. carapo s.s. are recognized from differences in the mean values of morphometric and meristic traits: 1, Eastern Amazon, including the lower-most Negro, and the Trombetas, Tapajos, and Tocantins basins; 2, Parnaíba and Itapicuru basins in the Brazilian state of Piauí; 3, Branco basin in the Brazilian sate of Roraima; 4; Guianas Shield, Orinoco basin and Island of Trinidad, 5, Madeira basin of Brazil, Bolivia and Peru; and 6, Western Amazon, including Tefé, Japurá and Javarí basins of Brazil, Napo basin of Ecuador, and Nanay, Pastaza and Ucayali basins of Peru.

Seven new species of Gymnotus are described from cis- and trans-Andean basins in South America on the basis of unique combinations of characters. 1. Gymnotus choco n. sp., from the Baudó and Atrato basins on the Pacific and Caribbean slopes of Colombia, respectively, differs from congeners by: posterior portion of anal fin without stripes and with clear patch; two canal pores in dorso-posterior preopercle; 18-22 (median 21) thin pale bands with wavy margins restricted to area below lateral line on anterior 2/3 body; 1- 3 dark bands divided ventrally on posterior portion of body; circular scales; deep body, its depth 8.8-11.3% total length (mean 10.8%); 32-35 (mode 35) precaudal vertebrae; 4-13 (median 8) ventral lateral line rami; and no dorsal lateral line rami. 2. Gymnotus esmeraldas n. sp., from Esmeraldas and Guayaquil basins on the Pacific Slope of Ecuador, differs from congeners by: anal fin striped posteriorly and with clear patch at caudal end; two canal pores in dorso-posterior preopercle; no bands on majority of body surface and no pale blotches on head; elongate scales, 2-3 times as long as deep above lateral line at midbody; slender body, depth 8.2-9.8% total length (mean 8.8%); 41-46 (mode 44) precaudal vertebrae; 14-18 (median 15) ventral lateral line rami; and no dorsal lateral line rami. 3. Gymnotus henni n. sp., from the Calima and Juradó basins on the Pacific Slope of Colombia, differs from congeners by: anal fin striped posteriorly and with clear patch at caudal end; two canal pores in dorso-posterior preopercle; irregularly shaped pale-yellow blotches on chin and gular regions, behind and under eyes, over opercle, and between eyes; 13-16 (median 15) dark band-pairs with pale yellow interbands are as broad or broader than dark bands on anterior half of body; elongate scales, 3-5 times as long as deep above lateral line at midbody; slender body, its depth 7.2-9.8% total length (mean 8.8%); 43-44 (mode 44) precaudal vertebrae; 16-22 (median 18) ventral lateral line rami; and no dorsal lateral line rami. 4. Gymnotus paraguensis n. sp., from the Paraguay basin, differs from congeners by: posterior portion of anal fin without stripes and with clear patch; two canal pores in dorso-posterior corner of preopercle; regularly arranged, 23-26 (median 24) unpaired dark bands with straight, high contrast margins; circular scales; deep body, its depth 9.8-10.3% total length (mean 10.1%); many (49-55, median 51), short ventral lateral line rami; and no dorsal lateral line rami. 5. Gymnotus tigre n. sp., from the Amazon basin of Ecuador, Peru and Brazil, differs from congeners by: anal fin striped posteriorly and with clear patch at caudal end; two canal pores in dorso-posterior preopercle; irregularly shaped pale-yellow blotches on chin and gular regions, behind and under eyes, over opercle, and between eyes; 16-23 (median 21) dark band-pairs with pale yellow interbands as broad or broader than dark bands on anterior half of body; elongate scales, 3-5 times as long as deep above lateral line at midbody; 46-48 (mode 47) precaudal vertebrae; 29-55 (median 51) ventral lateral line rami; and no dorsal lateral line rami. 6. Gymnotus javari n. sp., from the Amazonas, Napo, Javarí, and Ucayali basins, differs from congeners by: brown anal fin without stripes or clear patch posteriorly; one canal pore in dorso-posterior preopercle; 13-21 (mode 15) thin pale bands less than one-fourth width of dark bands; 5-8 dark H-shaped bands on the posterior part of the body; head depth 60-65% its length; large circular scales, 7-9 (mode 8) rows to dorsal midline at midbody; slender body, its depth 6.7-9.1% total length (mean 8.9%); 40-44 (mode 42) precaudal vertebrae; 1-15 (median 13) ventral lateral line rami; and no dorsal lateral line rami. 7. Gymnotus panamensis n. sp., from Atlantic slope of western Panama, differs from congeners by: anal fin mottled, without stripes or clear patch posteriorly; one canal pore in dorso-posterior preopercle; pale bands restricted to area below lateral line on anterior half body; dark interbands heavily mottled, obscuring banding pattern; scales large and circular, with 7-9 (mode 8) rows to dorsal midline at midbody; slender body, its depth 6.6-7.8% total length (mean 7.2%); 36 precaudal vertebrae; 11-15 (median 13) ventral lateral line rami; and no dorsal lateral line rami.

The existence of so many previously unrecognized and undescribed taxa is due in part to the cryptic nature of many Gymnotus species, and in part to a poor understanding of intraspecific variation and interspecific character-state diversity in the group.