Gymnotus omarorum, Richer-de-Forges, Crampton & Albert, 2009

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 : 61-63

Gymnotus omarorum Richer-de-Forges, Crampton and Albert, 2009

Figure 12 , Table 1

Material examined: Uruguay, Artigas: ZVC-P 16, 185 mm, Río Uruguay drainage, Río Cuareim, del Yucutuja, Rancho El Ombu (31°40’12”S, 057°10’12”W); ZVC-P 404 (2), 160–190 mm, Río Uruguay drainage, Río Cuareim, Picada Tareira (34°25’12”S, 056°25’12”W); ZVC-P 2273, 278 mm, Canal de Riego; ZVC-P 2906 (3), 95–205 mm, Río Uruguay drainage, Río Cuareim, Rancho Pereira Reverbell (30°11’24”S, 057°09’36”W); ZVC-P 3407b (3), 195–240 mm, Río Uruguay drainage, Catalán Chico, Rancho Martine; ZVC-P 3634, 286 mm, Salto, Río Uruguay drainage, Salto City. Uruguay, Florida: ZVC-P 105 (2), 100–170 mm, Río Uruguay drainage, Laguna en Cañada Invernada (31°00’36”S, 056°00’36”W); ZVC-P 303 (2), 173–215 mm, Río Uruguay drainage, Río Santa Lucia (34°26’24”S, 056°23’60”W); ZVC-P 3423, 146 mm, Río Uruguay drainage, Cañada Milano; ZVC-P 5502, 227 mm, Río Uruguay drainage, Río Santa Lucia, Cañada Casupa. Uruguay, Lavalleja: ZVC-P 950 (2), 123–158 mm, Rio Cebollatí, Laguna Merín, Rancho Sosa Diaz, Canada Mariscala (33°26’24”S, 054°23’60”W); ZVC-P 1917, 82 mm, Laguna Merín drainage, Villa Serrana, Cañada los Chanchos (34°19’12”S, 054°57’36”W). Uruguay, Maldonado: ZVC-P 6480 (holotype), 250 mm, Río Cisne drainage, Laguna del Sauce (34°50’20”S, 055°06’52”W); AMNH 239656 (16) (paratypes), 32–222 mm, same locality as ZVC-P 6480; MCP 41266 (3) (paratypes), 150–190 mm, same locality as ZVC-P 6480; ZVC-P 6481 (33) (paratypes), 23–201 mm, same locality as ZVC-P 6480. Uruguay, Rivera: ZVC-P 1351, 245 mm, Río Negro, Cañada Cuñapirú (31°11’60”S, 055°36’00”W). Uruguay, Treinta y Tres: ZVC-P 7429 (3), 227–269 mm, Laguna El Tigre. Uruguay, Tacuarembó: ZVC-P 7430, 222 mm, Laguna Lavalle.

Diagnosis. Gymnotus omarorum differs from all sympatric members of the G. carapo clade on the following characters: 1, few anal-fin pterygiophore scales (APS 2–6, mode 6 vs. 6–13, mode 8); 2, few pored lateral-line scales anterior to the first ventral lateral-line ramus (PLR 23–30, median 27 vs. 30–50, median 38); 3, many ventral lateral-line rami (VLR 28–30, median 29 vs. 13–28, median 22).

Description: Morphometric and meristic data in Table 1. Sexually monomorphic, including in breeding condition. Total length to 262 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 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 20 teeth, 3 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 less 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 13–14 (mode 13) 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 34 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 130–184 (median 166) rays. Lateral line ventral rami 28–30 (median 29). 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 irregular in shape, width and color, on specimens and among individuals. Ground color light grey or tan ventrally grading to darker grey or brown dorsally in adults. As in G. cuia , many obliquelyoriented, 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 and broken into spots anterodorsally in large specimens (24–29 mode 29). Pale interbands less than one-third width of dark bands at midbody. Bands rarely 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 200–300 mm more faintly banded). 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. Pectoralfin rays brown, interradial membranes hyaline. Pectoral-fin rays brown, interradial membranes hyaline. Anterior 80% of anal fin membrane dark brown, gray or black, posterior 20% translucent.

Figures

FIGURE 12. Gymnotus omarorum with the position of first ventral lateral-line ramus illustrated. A. Gymnotus omarorum holotype, ZVC-P 6480 (254 mm). B. Gymnotus cuia holotype, UFRGS 23700 (193 mm). Note the posterior position of first ventral lateral-line ramus relative to G. omarorum. C. Gymnotus carapo australis (MLP 11222, 289 mm). Note, again, the posterior position of first ventral lateral-line ramus relative to G.omarorum.