Gymnotus inaequilabiatus Valenciennes, 1847

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 : 67-69

Gymnotus inaequilabiatus Valenciennes, 1847

Figure 15 , Table 2

Material examined: Argentina: MACN 5979 , 250 mm, Rosario, Río La Plata (~ 32°56’S, 060°38’W) ; MLP 3353 , 250 mm, Misiones, Río Paraná (~ 26°49’S, 055°00’W) ; MNHN 4615 (neotype), 920 mm (600 mm according to the original description, 850 mm according to the label), Argentina, Buenos Aires, Rio de La Plata (~30°40’S, 058°30’W according to MNHN database or ~34°37’S, 058°23’W). Brazil, Mato Grosso: LBP 12616, ~ 650 mm, Corumbá, Rio Paraguai drainage, Rio Cuiabá (~ 18°09’S, 057°33’W) . Brazil, Mato Grosso do Sul: MCP 15818, 535 mm, Rio Paraguai near Cáceres ; USNM 1643 , 791 mm, Rio Paraguai drainage (~ 17°53’S, 057°29’W) . Brazil, Rio Grande do Sul: MCP 6059 (3), 173–355 mm, Iraí, Rio Uruguai drainage, Sanga das Águas Frias, 100 m from Rio Uruguai (27°12’00”S, 053°16’60”W) ; MCP 6956 , 602 mm, Rio Uruguai at Santana Velha, near Uruguaiana (~ 29°45’S, 057°50’W) ; MCP 19552 (3), 145–336 mm, Rosário do Sul, pool next to Olaria in the floodplain along Rio Santa Maria , North side of BR290 (30°14’16”, 054°53’33”W) ; MCP 31177, 560 mm, Roque Gonzales, Rio Uruguai drainage, Rio Ijuí near small hydroelectric power station ( PCH) Pirapó (~ 28°30’S, 055°10’W) ; MCP 39375, 550 mm, same locality as MCP 31177 ; UFRGS 14457, 498 mm, (~27°11’S, 053°06’W). Brazil, Santa Catarina: MCP 19044, 440 mm, Itá, Rio Uruguai, Rio do Engano , road between Itá and Seara (27°08’60”S, 52°13’00”W) ; MCP 20683, 410 mm, Itapiranga, Rio Uruguai, near Fortaleza (27°11’42”S, 053°38’34”W) . Brazil, São Paulo: MZUSP 22246 (3), 130–282 mm, Represa do Boa, Corumbataí, Rio Paraná drainage, Rio Tietê (~ 21°46’S, 048°58’W) ; MZUSP 22862 , 238 mm, Corumbataí, Rio Paraná drainage, Rio Tietê , Lagoa Ponte Seca (~ 21°46’S, 048°58’W) ; MZUSP 46001 , 998 mm, Rio Paraná drainage, Porto Primavera (~ 22°31’S, 053°10’W).

Diagnosis. Gymnotus inaequilabiatus differs from all sympatric members of the Gymnotus tigre clade ( G. paraguensis ) on the following characters: 1, more precaudal vertebrae (45 in all specimens vs. 32–35, median 34); 2, fewer ventral lateral-line rami (VLR 23–38, median 32 vs. 49–55, median 50). Gymnotus inaequilabiatus further differs from sympatric Gymnotus species on the following characters: 1, total length (max. 998 mm vs. max. 419 mm in G. c. australis and max. 305 mm in G. cuia the next two largest species); 2, a color pattern characterized by 21–29, mode 22 obliquely-oriented, chocolate-colored bands in juveniles (<400 mm), replaced in adults by large, irregular spots and blotches.

Description: Morphometric and meristic data in Table 1. Sexually monomorphic, including in breeding condition. Total length to 1 m. Morphological maturity at roughly 130 mm. Scales rounded to slightly ovoid on anterior two thirds of body, ovoid on posterior one third. Gape large in mature specimens, to or beyond posterior nares. Mouth superior with lower jaw longer than upper, rictus decurved. Chin especially 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 11 teeth disposed in single row along outer margin, arrow-head shaped anteriorly, conical posteriorly. Straight 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 12–15 teeth, none arrow-head shaped anteriorly. 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 short, extending beyond ventral margin of dentary. Mandible long, extended, length greater than twice depth. Trigeminal nerve canals divided within the hyomandibula. Posterior lateral line fenestra not contacting posterodorsal margin of hyomandibula. Cranial fontanels closed in juveniles and adults. Frontal shape broad, width at fourth infraorbital greater 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 robust, 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 13–21 (mode 19) 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 long, with 45 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 214–390 (median 370) rays. Lateral line ventral rami 23–38 (median 32). Caudal appendage short, less one-half length of pectoral fin. Single hypaxial electric organ along entire ventral margin of body. Four rows of electroplates near caudal insertion of anal fin.

Color in Alcohol: Bands, spots and blotches irregular in shape, width and color, on specimens and among individuals. Ground color tan to yellow. In juveniles (250–400 mm), 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 (21–29 mode 22). In adults, bands replaced by spots and blotches. Head never banded, spotted, or blotched, dark grey dorsally grading to lighter grey ventrally. Numerous chromophores speckled over branchiostegal membranes and ventral surface of head. Pectoral-fin rays brown, interradial membranes hyaline. Anal-fin membrane with irregular broken horizontal stripes on posterior 20%.

Figures

FIGURE 15. Guide to the differentiation of the Gymnotus tigre and G. carapo clades on the basis of readily-apparent morphologicaltraits (ventral lateral line ramus shape, shapeof scales on anterior portion of the bodyand coloration of anal fin). A. Gymnotus inaequilabiatus, MCP 6956 (683 mm). B. Posterior body region of G. inaequilabiatus and G. c. australis. Note shorter, straight VLRs in G. inaequilabiatus and longer, curved VLRs in G.c. australis. C. Scales removedfrom posterior body regions(at about two-thirds TL, first scalerow above lateral line) of G.inaequilabiatus and G.c. australis. Note the smaller and axially-elongate scales of G. inaequilabiatus. and larger, ovoid scales of G. c. australis. D. Caudal body regions of G. inaequilabiatus and G. c. australis. Note pigment speckles arranged in broken stripes on the anal-fin membrane of G. inaequilabiatus, and darkly pigmented anal-fin membrane of G.c. australis.