Sexual Dimorphism of the Growth Rate of the Swimbladder of the Toadfish Opsanus tau

The swimbladders with the attached drumming muscles of 144 toadfish were measured for width and weight. These values, when regressed against the total lengths and weights of the fishes, demonstrated that after an initial period of equal growth rate, the swimbladders of males grew faster than those of females. Bladders of females typically grew to 3 to 4 gm, while those of males were commonly over 8 gm. Bladder-weight bladder-width regressions of males and females were not significantly different, pointing to a morphological similarity of the bladders. Therefore sexual differences in the sound production of this species (males and females make different calls) suggest neural and hormonal differences rather than differences in the structure of the sound-producing organ. Literature on sexually dimorphic swimbladders as sound-producing organs was reviewed, and it was concluded that the toadfish's dimorphism is unlike any yet described.