Velocity of swimbladder wall displacement (twitch): A corresponds to the start of the movement, B to the maximal velocity and the half of swimbladder displacement, C to maximal velocity through the return of the swimbladder to its resting position, and D to the end of the movement. The A–B segment represents the velocity increase that corresponds to the main peak sound energy, and the C–D segment represents the recovery phase.
Oscillograms and sonograms of recorded sounds in Pygocentrus nattereri. (A) Sound type 1, or bark (1, initial; 2, main; 3, terminative sequence of sound); (B) sound type 2 (black arrow indicates the first part of the sound); (C) sound type 3. The dotted lines indicate the beginning and the end of each sound.
Maximum displacement (A–B; see Fig. 1) amplitude of cranial (black) and caudal (white) swimbladder sac at different sonic muscle stimulation rates. NS, not significant difference; *, significant differences (P<0.05) between swimbladder sacs (factorial ANOVA, Newman and Keuls test).