A New Species of Brachyhypopomus (Gymnotiformes, Hypopomidae) from Northeast Uruguay

A new species of Brachyhypopomus from north and northeast Uruguay is described. The new species is found in tributaries of the Uruguay River and southern Los Patos-Merín system. Brachyhypopomus bombilla can be distinguished from all other species of the genus by the following combination of characters: pigmentation pattern of body consisting of scattered dark brown pigmentation over a brown background, more dense on the dorsal half of the body, epidermal laterosensory canals not depigmented, head depth 61.8–76.4% of HL, caudal filament 6.5–16.6% of TL, anal-fin rays 155–190, fourth supraorbital pore dorsal and halfway between posterior nares and anterior margin of orbit, upper jaw slightly longer than lower jaw, mesocoracoid bone not ossified, maxilla anterodorsal process slender, all branchiostegal rays of equal size, none hook-like. Like many other species of the genus Brachyhypopomus, B. bombilla generates a pulse-type biphasic EOD, with a head-positive phase followed by a head-negative phase. The EOD duration of the new species was significantly lower than the EOD duration of the sympatric B. pinnicaudatus, whereas the EOD rate was significantly higher, thus suggesting that duration and frequency are conspicuous electrophysiological parameters that greatly aid in the description of electric fish species.

Figure 1

Adult male holotype of Brachyhypopomus bombilla (ZVC-P 6287; 136.5 mm TL).

Figure 2

Diagrammatic representation of Brachyhypopomus bombilla skeleton: (A–C) left lateral view of transitional (indicated in parentheses) and first caudal vertebrae in three different individuals (A–B, ZVC-P 5916; C, ZVC-P 5687); (D) lateral view of left suspensorium and jaws; (E) lateral view of left gill arches.

Abbreviations: An–angular; Ant DHS–anterior series of displaced hemal spines; Br1–5–branchiostegal rays 1–5; Ch–ceratohyal; D–dentary; Eh–epihyal; Hhv–ventral hypohyal; Hy–hyomandibular; Iop–interopercule; Mes–mesopterygoid; Met–metapterygoid; Mx–maxilla; Op–opercle; Pop–preopercule; Post DHS–posterior series of displaced hemal spines; Px–premaxilla; Q–quadrate; R–retroarticular; Sop–subopercle; Sy–symplectic. Scale bar = 1 mm.

Figure 3

Distribution of Brachyhypopomus bombilla.

Black squares = type localities; white squares = non-type localities.

Figure 4

EOD waveform of Brachyhypopomus bombilla.

(A) Head-to-tail recordings of the holotype's EOD at 20 and 30 C. The biphasic waveform at 20 C (P1:head positive; P2:head-negative) becomes almost monophasic at 30 C. (B) Temperature sensitivity of EOD waveform during a temperature change. Upper plot represents the temperature step from 20 to 30 C. Lower plot represents the steady decrease of ITS (amplitude of P2/amplitude of P1) during the temperature step.

Figure 5

Temporal pattern of the EOD emission in Brachyhypopomus bombilla.

(A) Head-to-tail recordings of the holotype's EOD at 20 and 30 C (above and below, respectively) show a high regularity. (B) Inter-EOD intervals as function of time recorded at 20 and 30 C (above and below, respectively). Scale bar = 20 ms.

Received: 07 Jul 2005

Accepted: 12 Jun 2006