Gymnotus sylvius Albert, de Campos Fernandes-Matioli and de Almeida-Toledo, 1999

Craig, Jack M., Malabarba, Luiz R., Crampton, William G. R. & Albert, James S., 2018, Revision of Banded Knifefishes of the Gymnotus carapo and G. tigre clades (Gymnotidae Gymnotiformes) from the Southern Neotropics, Zootaxa 4379 (1), pp. 47-73 : 65-66

Gymnotus sylvius Albert, de Campos Fernandes-Matioli and de Almeida-Toledo, 1999

Figure 14, Table 1

Material examined: Brazil, Paraná: MCP 47810, 81 mm, Cascavel, Iguaçu drainage, Rio do Oeste , between Rio do Salto and Castelo Branco (25°09’19”S, 053°19’41”W) . Brazil, São Paulo: LGP88 (P2336), 180 mm, Jacareí, Rio Paraíba do Sul ; LGP 93, 160 mm, same locality as LGP88; LGP94 (P2333), 230 mm , same locality as LGP88; LGP95 (P2352), 130 mm , same locality as LGP88; LGP 0 925.1 (holotype), 259 mm, Rio Ribeira de Iguape, near Miracatu (24°32’50”S, 047°26’13”W); LGP 0 925.2 (2) (paratypes), 251–307 mm , same locality as LGP 0925.1; LGP 0 931 (paratype), 157 mm, Rio Pardo drainage, São Simão, Rio Tamanduá (21°30’00”S, 047°31’11”W) ; UMMZ 234347 (2) (paratypes), 255–271 mm, same locality as LGP 0931; LGP96 (P2338), 170 mm , same locality as LGP88; LGP91 (P2353), 170 mm, Paraibuna, Rio Paraíba do Sul; LGP 92, 280 mm, Paraibuna, Rio Paraíba do Sul ; LGP98 (P2330), 160 mm , same locality as LGP92; UFRGS 14743 (4), 87–168 mm, Pindamonhangaba, downstream of the Bosque da Princesa, near Chacrinha (22°55’05”S, 045°27’45”W).

Diagnosis. Gymnotus sylvius differs from all other Gymnotus on its color pattern, characterized by 25–28, mode 25 chocolate-colored bands with wavy, irregular margins and pale interbands wider than dark bands.

Description: Morphometric and meristic data in Table 1. Sexually monomorphic, including in breeding condition. Total length to 291 mm. Morphological maturity at roughly 110 mm. Scales rounded to slightly ovoid, present on entire postcranial portion of body. Gape large in mature specimens, to or beyond posterior nares. Mouth superior with lower jaw longer than upper, rictus decurved. Chin fleshy and protuberant with fleshy pad of electrosensitive organs overlying tip of snout and oral jaws. Anterior narial pore partially to entirely included within gape in narial fold. Anterior nares large, subequal to eye diameter. Circumorbital series ovoid. Ethmoid region broad between anterior nares, with rounded anterior margin. Eye position lateral, lower margin of eye dorsal or horizontal to rictus. Premaxilla with 16 teeth disposed in single row along outer margin, arrow-head shaped anteriorly, conical posteriorly. Curved median margin of premaxilla. Maxilla-palatine articulation near anterior tip of endopterygoid. Maxilla vertical, rod-shaped, narrow distally with a straight ventral margin, length equal to roughly width of 4–6 dentary teeth. Dentary with one row of 28–30 (median 29) teeth, 5–6 arrow-head shaped anteriorly, all others conical posteriorly. Posterodorsal and posteroventral dentary processes abut at midlength. Dentary posteroventral process shorter than posterodorsal, narrow distally. Dentary ventral margin lamella narrow, depth less than posterior process. Opercle dorsal margin straight to slightly convex. Dorsal opercular process lamellar or rugose, crest absent or small, posterior margin entirely smooth, without spines or processes. Preopercle with anteroventral notch, posterodorsal laterosensory ramus with two superficial pores, margin of medial shelf entire, median shelf large, more than one-half width of symplectic. Metapterygoid superior and inferior portions approximately equal in size, ascending process robust, long, base shorter than length, curved, tip simple. Interopercle dorsal margin ascending process broad. Subopercle dorsal margin concave. Retroarticular with an arched lamella posteriorly forming a small canal, posterior margin square. Anguloarticular process long, extending beyond ventral margin of dentary. Mandible short, compressed, length greater than twice depth. Trigeminal nerve canals divided within the hyomandibula. Posterior lateral line fenestra contacting posterodorsal margin of hyomandibula. Cranial fontanels closed in juveniles and adults. Frontal shape narrow, width at fourth infraorbital less than that of parietal, anterior margin of straight, continuous with margins of adjacent roofing bones, postorbital process broad, more than two times width of supraorbital canal. Lateral ethmoid absent. Parietal rectangular, length less than width. Parasphenoid anteroventral portion gracile, extending ventral to lateral margin of parasphenoid, posterior processes narrow. Prootic foraminae separate for cranial nerves Vp and V2–3 +VII. Adductor mandibula muscle undivided at insertion, intermuscular bones absent. All basibranchials unossified. Gill rakers not contacting gill bar. Pectoral with 15–16 (mode 16) rays, medial radial large. Mesocoracoid elongate, length more than four times width.

Cleithrum broad, ventral margin curved, anterior limb long, more than 1.8 times ascending limb, deeply incised on its anteroventral margin, without large facet for insertion of muscle from supracleithrum. Postcleithrum thin, discoid or sickle shaped. Body cavity of moderate length, with 32–35 precaudal vertebrae. Rib five robust along its entire extent, less than three times width of rib six. Hemal spines present. Displaced hemal spines absent. Length anal-fin pterygiophores equal to or longer than hemal spines. Anal fin with 165–189 (median 175) rays. Lateral line ventral rami 19–25 (median 22). Caudal appendage short, less one-half length of pectoral fin. Single hypaxial electric organ along entire ventral margin of body. Rows of electroplates 3–4 (mode 3) near caudal insertion of anal fin.

Color in Alcohol: Bands irregular in shape, width and color, on specimens and among individuals. Ground color tan ventrally grading to darker brown dorsally in adults, pale yellow throughout in smaller specimens. As in G. c. australis , obliquely-oriented, chocolate-colored bands with wavy, irregular margins on lateral surface from nape and to tip of caudal appendage, occurring singly or as band-pairs, increasingly divided and irregular with size (25–28 mode 25). Uniquely, pale interbands wider than dark bands. Bands often divided dorsally or ventrally to form X - or inverted Y -shaped patterns. Band-interband contrast increases ventrally and caudally, fades with growth (juveniles> 150 mm with distinct margins, specimens 200– 200 mm faintly banded, and some specimens over 250 mm unbanded except very faintly in posterior one-third of body). Head never banded, spotted, or blotched, dark brown grading to lighter brown dorsally to ventrally. Numerous chromophores speckled over branchiostegal membranes and ventral surface of head. Pectoral-fin rays brown, interradial membranes hyaline. Anterior 80% of anal fin membrane dark brown, gray or black, posterior 20% translucent.

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