Does muscle fatigue limit advertisement calling in the oyster toadfish Opsanus tau?

Many sonic fishes appear to produce advertisement calls at a lower rate than insects, frogs and birds (song). Fish sonic muscles in many species rank among the fastest in vertebrates, suggesting that acoustic signalling is a costly activity. Surprisingly however, sound production in the oyster toadfish Opsanus tau requires negligible oxygen consumption. Male toadfish produce a long-duration tonal advertisement call, the boatwhistle, and both sexes produce short-duration agonistic grunt calls. The question of what limits the calling activity in fishes has not been addressed. We tested the hypothesis that calling in the oyster toadfish is limited by fatigue of the sonic muscles by stimulating them intermittently at the most rapid rate evoked by playbacks of the courtship boatwhistle call (100 ms every 4 s at 200 Hz = 1.5 s stimulation/min) for 5 min and measured swimbladder movement, sound production and glycogen use. Muscles in both sexes showed almost complete fatigue by 5 min (7.5 s of stimulation), although rested control muscles contained over twice as much glycogen in males as in females. Glycogen use was similar in both sexes, but males used 10.8% of their glycogen and females used 23.2%. Glycogen would support muscle contraction at this rate for 15 min in males during mating call production. It appears that sound production in the oyster toadfish is fatigue limited, which dictates a low rate of spontaneous calling that can be elevated for brief bursts of activity.