A sciaenid swim bladder with long skinny fingers produces sound with an unusual frequency spectrum

Abstract

Swim bladders in sciaenid fishes function in hearing in some and sound production in almost all species. Sciaenid swim bladders vary from simple carrot-shaped to two-chambered to possessing various diverticula. Diverticula that terminate close to the ears improve hearing. Other unusual diverticula heading in a caudal direction have not been studied. The fresh-water Asian species Boesemania microlepis has an unusual swim bladder with a slightly restricted anterior region and 6 long-slender caudally-directed diverticula bilaterally. We hypothesized that these diverticula modify sound spectra. Evening advertisement calls consist of a series of multicycle tonal pulses, but the fundamental frequency and first several harmonics are missing or attenuated, and peak frequencies are high, varying between < 1-2 kHz. The fundamental frequency is reflected in the pulse repetition rate and in ripples on the frequency spectrum but not in the number of cycles within a pulse. We suggest that diverticula function as Helmholz absorbers turning the swim bladder into a high-pass filter responsible for the absence of low frequencies typically present in sciaenid calls. Further, we hypothesize that the multicycle pulses are driven by the stretched aponeuroses (flat tendons that connect the sonic muscles to the swim bladder) in this and other sciaenids.

Figure 1

Spectrogram (a) and oscillograms (b) for 4 captive Boesemania microlepis calls recorded in a cement tank. The waveform is expanded for the third (c,e) and fourth call (d,f). The pulse-period (blue arrow) and cycle period (short orange arrow) is shown in (c). FFT = 128; overlap = 75%; Hamming window.

Figure 2

Power spectra (left graphs) and cepstra (right graphs) for the third (a,b) and fourth call (c,d) and background noise (e,f) in Fig. 1.

Figure 3

Spectrogram and power spectrum of wild Boesemania microlepis tonal calls recorded in the Bang Parkong River (a) and Krasieo Reservoir (b). Power spectrum FFT = 256; overlap = 75%; Hamming window. The power spectra encompass the tonal sounds and the low-frequency background noise below 0.25 kHz.

Figure 4

Temporal changes in Boesemania microlepis calls. (a) number of calls in three consecutive evenings from 11 to 13 March 2011 and (b) occurrence of calls with varying pulse numbers in the evening of March 12, 2011.

Figure 5

Boesemania microlepis swim bladder, diverticula sonic muscles and aponeurosis. (a) Radiograph showing the swim bladder. (b) Photograph of the swim bladder removed from the fish showing close up of diverticula. (c) Photograph of the swim bladder diverticula and sonic muscles. (d) Photograph of a fixed specimen with the swim bladder removed showing the dorsal aponeurosis and sonic muscle. The fish had a standard length of = 260 mm and a swim bladder length of 116 mm. The black lines in (b) are pins to hold the swim bladder in place. d diverticula, da dorsal aponeurosis, pp parietal peritoneum, sm sonic muscle.