Bioacoustics of Brachyhypopomus draco

Description. Morphometric and meristic data are presented in Table 1 . Head conical, nearly triangular in lateral view, snout short, eyes small, mouth terminal with upper jaw equal to lower jaw, no teeth in both jaws. Body slender, slightly laterally compressed. Dorsal profile straight from snout to supraoccipital, slightly convex posteriorly to caudal filament. Ventral profile gently convex from lower jaw to anal-fin origin, slightly convex to nearly straight along anal-fin base. Highest body depth located posterior to anal-fin origin. Body depth increasing smoothly from head to body region near the 30th anal-fin ray, clearly gradually decreasing from that point to caudal filament. Caudal filament moderately short, slender and laterally compressed in females, juveniles and males in non-reproductive period; caudal filament paddle-shaped in sexually mature males (see sexual dimorphism, below). Cycloid scales covering uniformly the body, except head and fins. Scales smaller on all anterior quarter portion of body; posterior scales at dorsal and medial body regions two to four times larger in diameter than anterior ones; smallest scales covering region of anal-fin pterygiophores. First anterior perforated scale of lateral line above pectoral fin origin and lateral line extending to caudal filament tip, hardly discernible. Branchial aperture small and slightly anterior to pectoral fin origin. Anus with the presence of urogenital papilla in males and females, although less developed in juveniles. Pectoral fins rounded with pigmented rays and perpendicular insertion; pectoral-fin rays i-ii + 13-15 (15-17 total pectoral-fin rays, n = 47, median = 16). Anal fin relatively long with vii-xii + 148- 186 rays (155-198 total anal-fin rays, n = 41, mean = 181.3) which are pigmented. Anal-fin origin located posterior to posterior edge of pectoral fin. Precaudal vertebrae 21-23 (20-22 anterior, 1-2 transitional; n= 6). Coloration in life. General body color brownish varying from dark brown to yellowish brown, dark brown near dorsal mid-line and clearing ventrally. A highly variable number of dark brown bands, well delineated or not, nearly perpendicular or oblique to longitudinal body axis, running posterodorsally from base of anal-fin rays to nearby lateral line and occurring from head to tip of caudal filament. Anal and pectoral-fin rays speckled brown, with hyaline inter-radial membranes.Adults and juveniles exhibit the same coloration pattern. Character Holotype n Minimum Maximum Mean L1 L2 SD 95% confidence limit Length to the end of anal fin (mm) 137.3 44 50 151.9 96.9 24.55 Percents of length to the end of anal fin Head length 10.1 44 9.4 14.2 11.4 11.08 11.69 1.00 Caudal filament length 29.1 34 17.3 35.2 25.1 23.32 26.86 5.15 Snout to anal fin origin 18.3 44 17.9 23.0 20.3 19.93 20.70 1.27 Depth of caudal filament 2.7 44 1.4 3.1 2.1 2.01 2.24 0.38 Longest anal fin ray 3.7 43 2.9 4.4 3.6 3.52 3.73 0.34 Longest pectoral fin ray 4.1 41 4.1 5.8 5.0 4.85 5.16 0.50 Body depth 9.4 44 8.8 12.2 10.8 10.52 11.08 0.92 Percents of head length Snout length 26.5 50 20.7 30.0 25.6 25.12 26.08 1.68 Gape width 14.5 50 9.5 16.5 13.7 13.32 14.14 1.45 Orbital diameter 9.5 50 9.4 14.4 12.3 11.90 12.64 1.30 Interorbital distance 23.4 50 22.7 35.0 28.4 27.60 29.25 2.91 Posterior nare to eye 3.4 49 2.9 5.4 4.2 4.04 4.34 0.52 Branchial aperture 23.3 50 16.5 26.0 19.9 19.31 20.44 1.99 Head width at operculum 54.1 50 48.4 65.0 57.0 55.96 57.96 3.50 Head width at eyes 37.2 50 31.3 45.6 39.9 39.09 40.63 2.71 Secondary sexual dimorphism. During reproductive period, established through monthly variation analysis of male and female gonads in a B. draco population, males undergo hypertrophy of the distal portion of caudal filament ( Fig. 2 ). This has been observed between August and December in a population studied from the type-locality (A. Schaan, J. Giora and C. Fialho, in preparation). Extremely vertical broadening and lateral compression give the caudal filament a paddle-like shape. After the reproductive period, this structure regresses until the caudal filament resembles those of females and juveniles. In addition, adult males are significantly larger than females. Electric organ discharge . Brachyhypopomus draco generates a continuous train of pulse-type EODs. Adult specimens (with developing or fully developed gonads) ( 72.6-105 mm LEA) exhibited the following EOD parameters. The mean EOD repetition rate (per individual) during the day ranged from 15.7-24.6 Hz (mean among all specimens, 19.6 Hz, SD 2.5, n = 11 individuals fishes), with a standard deviation (SD) of 0.4- 1.2 Hz (mean 0.7, SD 0.3, n = 11), and coefficient of variance of 1.8-7.0 % (mean 3.9 %, SD 1.88, n = 11). The lowest and highest absolute pulse rates recorded from all diurnal recordings were 13.7 and 29.2 Hz respectively. During the hours of peak foraging activity, 1-3 hours after sunset, the mean EOD repetition rate (per individual) ranged from 34.2-45.8 Hz (mean among all specimens 38.6 Hz, SD 5.2, n = 6) with a standard deviation of 2.8-10.8 Hz (mean 2.9, SD 1.1, n = 6) and a coefficient of variation of 8.3-27.3 % (mean 20.7 %, n = 6). The lowest and highest absolute pulse rates recorded from all nocturnal recordings were 8.4 and 80.2 Hz respectively. In sum, adult specimens of B. draco exhibited a distinct increase in pulse rate from the resting day-time state (mean 19.6 Hz) to the nocturnal active state (mean 38.6 Hz). The coefficient of variation of pulse rate during nocturnal activity (mean 20.7 %) was considerably higher than during the day (mean 3.9%) reflecting the greater variability in pulse rate during foraging. Disturbances such as minor vibrations provoked novelty responses, sudden increases in the resting pulse rate from the baseline rate ( Fig. 3 ). The approximately stable resting diurnal EOD punctuated by novelty responses results in a right-skewed distribution of inter-pulse intervals in a histogram ( Fig. 3 ). Some specimens of B. draco displayed spontaneous complete cessations of the EOD for periods of up to 1 minute. Normal EOD activity was resumed after these interruptions. The EODs waveforms of adult specimens of B. draco comprised two components of alternating polarity (P1 and P2) sensu Crampton & Albert (2006), and varied in duration from 1.172 -1.992 ms (mean 1.578, SD 0.147, n = 23) ( Fig. 4 ). The Peak Power Frequency ( PPF ) (see Fig. 4 ) of these EODs varied from 0.6958 -0.8942 kHz (mean 0.7707, SD 0.0534, n = 23). We did not encounter evidence for obvious sexual differences in the EOD waveforms of B. draco , despite evidence for this phenomenon in other species of Brachyhypopomus ( e.g . B. occidentalis , Hagedorn, 1985 , Shumway & Zelick, 1988; B. pinnicaudatus Stoddard et al ., 2003 ; B. brevirostris , Kawasaki & Heiligenberg, 1989 , Crampton, 1996 ). Where a sexual difference exists, this is manifest as an elongation of the second negative waveform phase, P2 and a corresponding reduction in the PPF of the EOD. We observed no consistent EOD waveform shape differences. Likewise, we observed no significant difference between the PPF of five sexually mature males ( 0.7504 -0.8942 kHz, mean 0.8178, SD 0.0675) and 2 sexually mature females ( 0.7385 -0.7949 kHz, mean 0.7667, SD 0.0399) (2-sample T-test, df = 5, n = 7, P = 0.38). The observation of a lower PPF in mature females than males contrasts with cases of sexual EOD differences in Brachyhypopomus , where sexually mature males invariably display lower PPFs. Nonetheless, we did note that the Spectral Power Densities of the two sexually mature females exhibited a more rapid decline from the PPF than in the five sexually mature males ( Fig. 4 ). Males and females with developing gonads also exhibited no obvious difference in waveform shape, and no significant difference in the PPF (males: 0.6958 -0.8362 Hz, mean 0.7518, SD 0.0460, n = 10; females: 0.7111 -0.8194 kHz, mean 0.7642, SD 0.0422, n = 6) (2-sample T-test, df = 14, n = 16, P = 0.59). All recorded specimens had been held in social isolation for several days before they were recorded. This is known to minimize the hormonally-induced modulation of the P2 phase in sexually mature males ( Stoddard et al ., 2003 ). Further investigation is required to explore whether EOD differences emerge in males that are exposed to normal social stimuli ( i.e. the presence of sexually mature conspecifics). All recordings presented here were made during the southern summer (December) when ambient water temperature is typically in the range 24 – 30º C. During the winter, water temperatures in Rio Grande do Sul decline to as low as 10º C. We noted that these seasonal temperature changes have a significant impact on the waveform duration and shape of the EODs of B. draco . The PPF of the EOD declines substantially, and the P2 phase becomes diminished in relative amplitude. We will present detailed observations on this phenomenon elsewhere. Loureiro & Silva (2006) discuss the effect of temperature on the EODs of B. bombilla .