Gymnotus riberalta, Craig & Correa-Roldán & Ortega & Crampton & Albert, 2018

Craig, Jack M., Correa-Roldán, Vanessa, Ortega, Hernán, Crampton, William G. R. & Albert, James S., 2018, Revision of Gymnotus (Gymnotiformes: Gymnotidae) from the Upper Madeira Basin of Bolivia and Peru, with descriptions of two new species, Zootaxa 4413 (1), pp. 111-132 : 125-127

Gymnotus riberalta n. sp.

Figure 6E , Table 3

Holotype: CBF 10248, 199 mm TL, Bolivia, Beni, Riberalta, Rio Beni, Rio Madeira drainage, Rio Amazonas drainage, Arroyo near Lago de San Jose (10°54’47”S, 065°59’49”W), coll. William Crampton, 06/27/2008.

Paratypes: CBF 10243, 193 mm TL, same date and locality as CBF 10248; CBF 10244, 135 mm TL, same locality as CBF 10248; CBF 10245, 220 mm TL, same locality as CBF 10248; CBF 10246, 142 mm TL, same locality as CBF 10248; CBF 10247, 196 mm TL, same locality as CBF 10248; CBF 10249, 173 mm TL, same locality as CBF 10248; UMSS 7008, 138 mm TL, same locality as CBF 10248; UMSS 7009, 153 mm TL, same locality as CBF 10248; UMSS 7010, 200 mm TL, same locality as CBF 10248; UMSS 7011, 152 mm TL, same locality as CBF 10248; UMSS 7012, 237 mm TL, same locality as CBF 10248; UMSS 7013, 209 mm TL, same locality as CBF 10248; UMSS 7014, 247 mm TL, same locality as CBF 10248.

Nontypes: UMMZ 82146 , 153 mm TL, Bolivia, Santa Cruz, Ñuflo de Chaves, Río Blanco near Concepción (~ 16°07’55”S, 062°01’34”W) ; UMSS 0 7015, 194 mm TL, Brazil, Amazonas, Rio Beni, Rio Madeira drainage, stream near Porto Hamburgo (10°54’7.75”S, 065°59’8.18”W) ; UMSS 0 7016, 216 mm TL, same locality as UMSS 07015 ; UMSS 0 7017 170 mm TL, same locality as UMSS 07015 ; UF 180238 (3), 187–267 mm TL, Bolivia, Beni, Riberalta, Rio Beni, Rio Madeira drainage, Rio Amazonas drainage, Small Stream near Puerto Hamburgo, Riberalta (11°01’52”S, 066°05’39”W, 147 m) .

Diagnosis: Gymnotus riberalta is morphologically similar to G. pantanal , but differs on the following characters: one, a narrow maximum body width (BW 53.3–72.0% TL, mean 61.2% TL vs. 71.5–76.7% TL, mean 73.7 TL in G. pantanal ); two, a narrow mouth (MW 32.7–45.7% HL, mean 41.2% vs. 42.0–50.7% HL, mean 46.0% in G. pantanal ); three, a narrower pectoral fin, possessing fewer rays (P1R 10–13, mode 13 vs. 14–18, mode 15 in G. pantanal ); four, more anal-fin pterygiophore scales (APS 10–11, mode 10 vs. seven to eight, mode seven in G. pantanal ). Gymnotus riberalta further differs from other members of the G. carapo clade throughout the Neotropics in possessing a color pattern (similar to G. pantanal ) consisting of narrow pale bands less than one quarter width of dark bands, wavy band margins, pale bands exclusively on ventral portion of body anteriorly vs. bands interrupted into patches anterodorsally, with silver, blue or green metallic countershading on dorsum of adults of G. arapaima , dark bands lost in large adults (> 250 mm TL) of G. ardilai , small, rounded dark spots over entire body except posterior 20% of some specimens of G. bahianus , bands faint or absent in all specimens of G. chimarrao , one to three inverted Y-shaped dark bands posteriorly and pale bands not reaching above lateral line in anterior two thirds of body of G. choco , bands broken into speckles throughout in G. diamantinensis , narrow pale bands (less than one fifth width of dark bands) with sharp margins, and pale bands extending fully to dorsal midline in G. mamiraua , pale bands wider than dark bands in G. sylvius , narrow pale interbands (less than one third width of dark bands) extending above lateral line and often to dorsal midline in G. ucamara .

Description: Sexually monomorphic. Size up to 302 mm TL with adult body proportions attained at about 150 mm TL. Adult body shape subcylindrical with a mean ratio of body width to depth of 65%. Body profile slender, body depth 81.1–102.1% total length. Head length moderate, 7.3–10.3% total length. Snout length moderate, 31.1– 37.3% head length. Mouth width narrow, 32.7–45.7% head length. Preanal distance long, 71.9–97.9% head length. Anal-fin long, 79.7–97.4% total length. Cycloid or ovoid scales present on entire post-cranial portion of body from nape to caudal appendage. Scales above lateral line intermediate, six to eight, mode six. Scales over anal-fin pterygiophores large, with 10–11, mode 10 rows. Gape large, extending to or beyond posterior nares. Mouth position superior, lower jaw longer than upper, rictus decurved. Chin round in lateral, dorsal profiles, fleshy and bulbous with mental electroreceptive organ overlying lower jaw. Anterior narial pore partially or entirely included within gape, in small narial fold. Anterior nares small, its diameter less than that of eye. Eye below horizontal with mouth. Circumorbital series ovoid. Premaxilla with 20 teeth disposed in two rows along outer margin, arrowhead 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 four to six dentary teeth. Dentary with one row of 18 teeth, four arrowhead shaped anteriorly, all others conical posteriorly. Posterodorsal and posteroventral dentary processes abut at midlength. Dentary posteroventral process nearly as long as posterodorsal, narrow distally. Dentary ventral margin lamella narrow, depth less than posterior process. Dentary anteroventral margin rounded in lateral view, with a hook. Mandible long, extended, length greater than twice depth. Anguloarticular process long, extending beyond ventral margin of dentary. Retroarticular with an arched lamella posteriorly forming a small canal, posterior margin square. 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. Hyomandibular trigeminal canals connected. Dorsal region of hyomandibula with four lateral foramenae, supraorbital and infraorbital nerves connected. Posterodorsal laterosensory ramus of preopercle with single superficial pore. Preopercle with anteroventral notch, posterodorsal laterosensory ramus with two superficial pores, margin of medial shelf entire, median shelf large, more than half width of symplectic. Opercle dorsal margin straight to slightly convex, its posterior margin smooth. Opercular posterior margin entirely smooth. Subopercle dorsal margin concave. Cranial fontanels closed in juveniles and adults. Frontal shape narrow, width at fourth infraorbital less than that of parietal, anterior margin straight, continuous with margins of adjacent roofing bones, postorbital process broad, more than two times width of supraorbital canal. Lateral ethmoid unossified. Parietal rectangular, length less than width. Parasphenoid anteroventral portion reduced, extending ventral to lateral margin of parasphenoid, posterior processes robust. Prootic foramen Vp combined with V2-3 +VII. Adductor mandibula undivided at insertion, intermusculars absent. All basibranchials unossified. Gill rakers not contacting gill bar.

Cleithrum narrow, ventral margin straight, anterior limb long, more than 1.8 times ascending limb, without large facet for insertion of muscle from supracleithrum. Postcleithrum thin, discoid or sickle-shaped. Body cavity of moderate length, with 35 precaudal vertebrae, mode=35, n=2. Rib 5 robust along its entire extent, less than three times width of rib 6. Displaced hemal spines absent. Length anal-fin pterygiophores equal to or longer than hemal spines. Pectoral fin narrow, with 10–13 rays, mode 13 rays. Anal fin of moderate length, with 169–239 rays, median 202. Lateral-line variable, with 10–16 ventral rami, mode 13. Lateral-line dorsal rami absent in adults. Single hypaxial electric organ, extending along entire ventral margin of body with three rows of electroplates near caudal insertion of anal fin.

Color in alcohol: Body ground color dark, chocolate brown. Bands variable in number among individuals and in shape and arrangement within individuals, uncorrelated with sex. Juveniles (TL <150 mm TL) and some adults (TL> 150 mm TL) with 22–27 (mode 24, n=9) obliquely-oriented, thin cream-colored bands with wavy, irregular margins ventrolaterally located between tail tip and pectoral-fin base. Pale bands infrequently branched, and very rarely extend above lateral line on anterior half of body. Dark bands chocolate-brown in color, four times as wide as pale anteriorly and forming inverted Y-shapes posteriorly. Interband contrast greatest in juveniles and ventrally to caudally in adults. Anterior 80% of dorsum with very faint or absent banding in specimens of all sizes. Head ground color chocolate brown dorsally grading to lighter brown ventrally. Ventral surface of head and branchiostegal membranes speckled. Pectoral fin with brown or gray rays, hyaline inter-radial membranes. Analfin membrane uniformly light brown or gray.

Ecology: All specimens were captured in small terra firme rainforest streams with sandy beds and dense emergent aquatic vegetation. Water chemistry parameters were as follows: conductivity 6.8–8.2, pH 5.2–5.5, temperature 22.2–25.0 °C, Dissolved Oxygen 3.4–4.2 mg ̇l -1.

Electric Organ Discharge: The EOD waveforms of G. riberalta were previously described by Crampton et al. (2013) as “ Gymnotus n. sp. ALT”. The head-to-tail EOD waveform (sensu Waddell et al. (2016)) of this species is tetraphasic with a dominant P1–P2 phase flanked by a lower voltage negative P0 phase ( Figure 7 ), following the nomenclature of Crampton et al. (2011). A positive P3 phase after P0 is invariably absent. This EOD waveform conforms to the ht-EOD category “2” of Crampton et al. (2013). The peak power frequency (PPF) of the EOD ranged from 0.946–1.597 kHz, mean=1.158, standard deviation (SD)=0.151 kHz, with no significant disparity between males (0.946–1.597 kHz, mean=1.148 kHz, SD=0.152 kHz, n=13 recorded specimens) and females (0.976–1.292 kHz, mean=1.12 kHz, SD=0.110 kHz, n=7) (two-tailed t-Test p=0.81).

Resting (daytime) pulse rates (1 minute averages) are reported for G. riberalta in the range 33.9–51.9 Hz, mean 41.9 Hz, SD=5.5 (n= 18 specimens) and active (nighttime) pulse rates in the range 44.7–77.1 Hz, mean 3.2 Hz, SD=7.8 (n=19) (two-tailed t-Test night> day, p=7.2 x 10-11). No sex disparity was observed in the mean resting pulse rates (females=40.7 Hz, n=8; males=42.8 Hz, n=10; two-tailed t-Test, p=0.45) and mean active pulse rates (females=62.0 Hz, n=7; males=63.8 Hz, n=12; two-tailed t-Test, p=0.62).

Etymology: Specific epithet derived from the city of Riberalta in the Beni Department of Bolivia, where the type series was collected. Noun in apposition.

Habitats: Gymnotus riberalta inhabits slowly-moving channels (caños) on forested floodplains and aquatic vegetation at the margins of oxbow lakes on seasonally flooded savannahs.

Figures

FIGURE 6. All five valid Gymnotus species from the Upper Madeira Basin, Bolivia and Peru. Scale bars = 10 mm TL. A. Gymnotus carapo madeirensis holotype, UMSS 06964, 178 mm TL. B. Gymnotus chaviro holotype, MUSM 33715, 233 mm TL. C. Gymnotus coropinae, MUSM 20264, 118 mm TL. D. Gymnotus eyra n. sp., INHS 37119, 178 mm TL. E. Gymnotus riberalta n. sp. holotype,CBF 10248, 199 mm TL.

FIGURE 7. Electric organ discharge (EOD) of Gymnotus riberalta males (blue) and females (red). A. Head-to-tail recorded EOD waveforms. B. Accompanying power spectral density (PSD) plot. Note the absence of sexual dimorphism in the EOD waveform.