Gymnotus pantanal, Flora M. C. Fernandes, James S. Albert, Maria de Fatima Z. Daniel-Silva, Carlos E. Lopes, William G. R. Crampton & Lurdes F. Almeida-Toledo, 2005

Flora M. C. Fernandes, James S. Albert, Maria de Fatima Z. Daniel-Silva, Carlos E. Lopes, William G. R. Crampton & Lurdes F. Almeida-Toledo, 2005, A new Gymnotus (Teleostei: Gymnotiformes: Gymnotidae) from the Pantanal Matogrossense of Brazil and adjacent drainages: continued documentation of a cryptic fauna., Zootaxa 933, pp. 1-14 : 4-11

Gymnotus pantanal new species

(Fig. 1)

Holotype: MZUSP 67874, female, Brazil, Mato Grosso do Sul State, Rio Miranda, 196 mm, 20 July 2000, near Miranda, 20°11' 78" S, 56°30' 13" W, F. M. C. Fernandes.

Paratypes: 2 spec. MZUSP 67875, 189 mm, and MZUSP 67876, 264 mm, Brazil, Mato Grosso do Sul State, Rio Paraguay, 22 July 2000, Corumb., 18°59' 81" S, 57°39' 24" W, F. M. C. Fernandes.

Nontypes: Bolivia: UF 82146 (1), Santa Cruz, near Concepcion, Rio Blanco, 153 mm, 1990.06.16. Brazil: MZUSP 67876 (1), Mato Grosso do Sul, Rio Paraguay, 18°59'81"S, 57°39'24"W, 251 mm, 22 July 2000. MZUSP 67875 (1), Mato Grosso do Sul, Rio Paraguay, 18°59'81"S, 57°39'24"W, 192 mm, 22 July 2000. Paraguay: NRM 42830 (1), Rio Parana, 240 mm, 1998.03.15. NRM 42397 (1), Rio Paraguay, 171 mm, 1998.03.25. UF 38173 (1), Dept Cochabamba, Province Chapare, Rio Espiritu, Chapare-Mamore drainage, brook at Villa Tunari, elevation 350 m., 192 mm, 1982.XII.12. UMMZ 206080 (21), Arroyo in Parque Nacional Ybycui, Rio Paraguay, 82-260 mm, 1979.VI.20.

Diagnosis: Gymnotus pantanal differs from other members of the G. pantherinus species-group (except G. anguillaris ) in possessing a color pattern composed of thin obliquely oriented pale pigment bands (about one third the width of the dark bands) with wavy margins restricted to the ventral portion of the body (rarely extending above the lateral line) on the anterior half of the body. Gymnotus pantanal further differs from other members of the G. pantherinus species-group in possessing a wider head (72-77 vs. 52-69% head length). Gymnotus pantanal further differs from G. anguillaris in possessing more narrowly set eyes (37-41 vs. 43-56% head length), a deeper head (66-74 vs. 60-65% head length), larger branchial openings (38-43 vs. 29-36% head length), longer pectoral fins (51-56 vs. 42-50% head length), more pectoral-fin rays (mode 17 vs. 16), and fewer pored posterior lateral-line scales (to first ventral ramus: 47-58 vs. 58-62; total: 102-114 vs. 124-130).

Description: Fig. 1 illustrates body shape and pigment patterns. Morphometric and meristic data for specimens in type series and additional lots in Table 1. Size up to 251 mm. No known sexual dimorphism. Adult body proportions attained at about 120 mm total length. Size at sexual maturity unknown. Adult body shape subcylindrical, ratio body width/depth 0.70-0.81. Body profile slender, body depth 8.0-9.5% total length. Head length moderate, 8.6-9.3% total length. Snout length moderate, 34-37% head length. Mouth width moderate, 44-48% head length. Preanal distance moderate, 82-93% head length. Anal-fin long, 78-82% total length. Scales present on entire post-cranial portion of body from nape to caudal appendage. Scales above lateral line large, (7-8, mode 8). Scales cycloid, ovoid. Scales over anal-fin pterygiophores large, (mode) 5-6 rows.

Gape size in mature specimens large, extending to or beyond posterior nares. Mouth position superior, rictus decurved. Eye position below horizontal with front of mouth. Anterior narial pore partially or entirely included within gape. Circumorbial series ovoid. Maxilla orientation vertical. Maxilla rod- or paddle-shaped with straight ventral margin. Dorsoposterior laterosensory ramus of preopercle with single superficial pore. Cranial fontanels closed in juveniles and adults. Anterior margin of frontal straight, continuous with margins of adjacent roofing bones. Frontal postorbital process narrow, less than two times width of supraorbital canal. Frontal broad, its width at the posterior articulation of the infraorbital series subequal to that of parietal. Pectoral fin broad, with 15-19 (mode 17) rays. Anterior limb of cleithrum long, more than 1.8 times ascending limb. Cleithrum anterior notch absent. Cleithrum without large facet for insertion of muscle from supracleithrum. Body cavity long, with 35-38 (mode 37) precaudal vertebrae. Rib 5 broad, with a large medial triangular shelf. Hemal spines present. Displaced hemal spines absent. Anal fin of moderate length, with 217-260 rays. Multiple anal-fin ray branching posterior to rays 10-17. Lateral-line ventral rami 7-12. Lateral-line dorsal rami absent in adults. Length anal-fin pterygiophores equal to or longer than hemal spines. Caudal appendage long, more than 0.5 time pectoral-fin length in undamaged and unregenerated specimens. Single hypaxial electric organ, extending along entire ventral margin of body. Two to three (mode 3) rows of electroplates near caudal insertion of anal fin. Electric organ discharge not known.

Color in alcohol: Ground color of body dark brown. All juveniles and some adults (TL more than 160 mm) with 21 to 26 obliquely oriented, thin pale-yellow bands with wavy irregular margins on ventrolateral surface, extending from tip of tail to pectoral-fin base. Some subadults and most adults with fewer pale bands. Band appearance variable in shape, width, arrangement, and number, both on and among individuals. Band-interband margins irregular and wavy. Dark bands evenly pigmented, or partially divided ventrally (inverted Y-shaped) at middle to posterior portions of body. Dark bands four times as broad as pale bands on anterior half of body. In adults pale bands rarely branched, and never extending above lateral line on anterior half of body. Interband contrast increases ventrally and caudally; more pronounced in smaller specimens. Anterior 80% of dorsum (anterior to anal-fin clear patch) without banding. Three bands from either side meet on ventral midline, between the anus and anal-fin origin. One band lies posterior to last analfin ray.

Head not banded or blotched; ground color dark brown dorsally grading to lighter brown ventrally, without freckles and with numerous speckles distributed over branchiostegal membranes and ventral surface of head. Pectoral-fin rays brown or gray, interradial membranes hyaline. Anal-fin membrane uniformly light brown to dusky gray.

Comparisons with other species. In terms of salient features of color pattern and general body proportions, G. pantanal most closely resembles G. anguillaris from which it differs by character states provided in the Diagnosis. Gymnotus anguillaris and G. pantanal can be differentiated from other members of the G. pantherinus species-group by the following unique combination of characters: no pale band at nape, pale bands not extending above lateral line on anterior half of body, adult body size greater than 200 mm total length, fewer ventral lateral-line rami (7-18 vs. 19-30), fewer anterior anal-fin rays without multiple branching (10-17 vs. 18-26), and a single row (vs. two rows) of conical teeth along outer margin of dentary.

On morphometric grounds, G. pantanal can be separated from sympatric congeners (i.e., G. carapo , G. inaequilabiatus , and G. sylvius ) of similar size (130-250 mm TL) by a shorter head (head length 8.6-10.1% vs. 10.5-14.0 total length) and a more slender body (BD 9.0-9.5 vs. 10.0-13.0% total length. Among species of Gymnotus , G. pantanal most closely resembles G. anguillaris from Surinam, in terms of morphometric, meristic, and color variables. Gymnotus pantanal shares several meristic counts with other species from southern Brazil, including the number of the pectoral-fin rays (16-18) and number of scales above the lateral line at midbody (7-8). Juvenile specimens of G. pantanal also share 21-26 oblique bands (or band pairs) with sympatric congeners and G. anguillaris from Surinam.

Karyological features: Diploid number of 2n=40 chromosomes, arranged in seven pairs of metacentric/submetacentric and 13 pairs of subtelocentric/acrocentric chromosomes. Chromosome pair 19 with a secondary constriction on the short arms, at the site of active NORs (evidenced after silver staining). A third active NOR is also present in one of the homologues of the larger pair of metacentric/ submetacentric on the terminal region of the long arm. The C-bands evidence the constitutive heterochromatin distribution in the pericentromeric region of all chromosome pairs. The karyological features observed in the holotype (MZUSP 67874) are illustrated in Fig. 2 and summarized in Table 2. According to data available in the literature, G. pantanal shares the diploid number of 2n=40 chromosomes with G. sylvius . However, the chromosome morphology differs greatly between these two species. Gymnotus pantanal possesses seven pairs of metacentric/submetacentric and 13 pairs of subtelocentric/acrocentric chromosomes, whereas G. sylvius possesses 14 pairs of metacentric, 5 pairs of submetacentric and 1 pair of subtelocentric/acrocentric chromosomes. The occurrence of constitutive heterochromatin in the pericentromeric region of the chromosomes is shared with the other species from southern Brazil (see Fernandes-Matioli et al. 1998). G. pantanal also possesses a unique cytological characteristic among the species chromosomally characterized; the presence of 3 active NORs in the metaphase plates (only two active NORs is observed in the other species).

Molecular characterization: Amplification of the microsatellite marker micro11 (Fernandes-Matioli et al. 2000) using the SPAR-PCR technique resulted in a unique pattern of two amplified bands (fragments) of relatively high molecular weight, 800 base pairs (bp) and 1,370 bp. This pattern was observed in ten individuals analyzed (Table 2, Fig. 3). The molecular marker micro11 exhibits species-specific patterns among members of the Gymnotidae (Fernandes-Matioli et al. 2000). The micro11 pattern observed in all G. pantanal individuals analyzed, with two bands of relatively high molecular size (800 bp and 1,370 bp, Table 2), differs conspicuously from its congeners (Fig. 3). Amplified fragments with high molecular weight, albeit with different size, are also observed in G. carapo (one of two bands with 1,476 bp), in G. inaequilabiatus (one of four bands with 1,968 bp) and in G. pantherinus (one of four bands with 1,150 bp). The molecular micro11 pattern observed in G. pantanal is unique among congeners in southern Brazil.

Distribution: Known from the Parana-Paraguay system of Brazil and Paraguay, and the Rio Chapare-Mamoré of Bolivia.

Etymology: The specific epithet pantanal from the Pantanal Matogrossense of Brazil, the hydrological region of the type locality. A noun in apposition.

Discussion

Among congeners, G. pantanal most closely resembles G. anguillaris from the area of the type locality in Surinam in aspects of coloration, and also in several morphometric and meristic features associated with a highly elongate body (i.e., body slender, body cavity long, and relative head length short). In combination, these particular features are known to have diagnostic value for differentiating other Gymnotus species living in sympatry, under conditions when species-identities are well established from additional electric signal, chromosomal, or molecular data (Albert et al. 1999; Albert & Crampton, 2001). Gymnotus pantanal also differs from G. anguillaris in several features known to have diagnostic value at the species level within Gymnotus : i.e., interorbital distance, size of the branchial opening, pectoral-fin length, number of fin pectoral-fin rays, and lateral-line scale counts.

At present it is not possible to determine whether phenotypic intergrades occur between G. pantanal and G. anguillaris . Specimens of the G. anguillaris species-complex are absent from collections from the central and eastern Amazon (Crampton, 1998; Albert & Crampton, 2001), including from the extensive INPA collections of these areas (JSA and WGRC, pers. obs.). Undescribed populations resembling G. anguillaris are known from tributaries of the Napo and Madeira rivers in the Amazon basin (Fig. 4), each possessing distinct phenotypes. The morphological, cytogenetic, and molecular data reported here indicate that the populations described here as G. pantanal represent a distinct species (i.e., a separately evolving evolutionary lineage) rather than a geographic variant of G. anguillaris . This hypothesis will be tested with future collections of the G. pantherinus species-group from across its range, and by comparisons of molecular sequence data.