Gymnotus chimarrao Cognato, Richer-de-Forges, Crampton and Albert, 2007b

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 : 60

Gymnotus chimarrao Cognato, Richer-de-Forges, Crampton and Albert, 2007b

Figure 11, Table 1

Material examined: Brazil, Rio Grande do Sul: UFRGS 6774 (holotype), 185 mm, Rio Taquari Drainage, Arroio do Meio, Arroio Grande (29°21’09”S, 051°57’28”W); UFRGS 6770 (paratype), 190 mm, same locality as UFRGS 6774; UFRGS 6771 (paratype), 243 mm, same locality as UFRGS 6774; UFRGS 6772 (paratype), 177 mm, same locality as UFRGS 6774; UFRGS 6773 (paratype), 206 mm, same locality as UFRGS 6774; UFRGS 6776 (paratype), 124 mm, same locality as UFRGS 6774; UFRGS 11952, 223 mm, Nova Roma do Sul (28°57’18”S, 051°22’32”W); UFRGS 17607 (2), 111–222 mm, Canudos do Vale (29°24’22”S, 052°03’19”W).

Diagnosis. Gymnotus chimarrao differs from all sympatric congeners in possessing a color pattern characterized by singly-occurring dark pigment bands in juveniles (29–45, mode 32) and faint or absent bands in mature specimens (vs. distinct dark bands and pale interbands in all sympatric congeners except G. inaequilabiatus , which possesses bands as a juvenile, and irregular spots when mature).

Description: Morphometric and meristic data in Table 1. Sexually monomorphic, including in breeding condition. Total length to 243 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 11–13 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 12–17 teeth, 2–4 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 14–17 (mode 15) 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 29–32 (mode 30) 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 191–250 (median 210) rays. Lateral line ventral rami 13–24 (median 23). Caudal appendage short, less one-half length of pectoral fin. Single hypaxial electric organ along entire ventral margin of body. Three rows of electroplates near caudal insertion of anal fin.

Color in Alcohol: Bands barely visible in all specimens. Ground color olive green to yellow ventrally grading to darker brown dorsally in all specimens. Obliquely-oriented, darker brown bands narrow anteriorly, wider posteriorly, typically occurring singly (29–45 mode 32). Band-interband contrast increases ventrally and caudally, fades with growth (juveniles> 150 mm with distinct margins, specimens 150–200 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 grey dorsally grading to lighter grey ventrally. Numerous chromophores speckled over branchiostegal membranes and ventral surface of head. Pectoral-fin rays dark brown, interradial membranes hyaline. Anterior 80% of anal fin membrane dark brown, gray or black, posterior 20% translucent.