SYNOPSIS. Respiratory gas bladders are found in the Osteoglossomorpha, Elopomorpha and Euteleostei and are absent in the Clupeomorpha. All teleosts with respiratory gas bladders share a common pattern of air ventilation: during the transfer phase gas is transferred passively from the gas bladder to the buccal cavity. Subsequently, gas is expelled during the active expulsion phase mediated by action of the geniohyoideus muscle causing a positive pressure pulse in the buccal cavity. This is followed by an active intake phase by action of the sternohyoideus muscle creating a negative pressure pulse, which is succeeded by an extensive compressive phase by action of the geniohyoideus muscle forcing fresh air into the gas bladder. Saltatory evolution of gas bladders and their buccal pumps seems to have proceeded by major transformations in structural design without appreciable changes in the pattern of neural control. The hypothesis of symmorphosis in gas bladder design is well corroborated by the independent evolution of accessory esophageal pumps in three unrelated lineages. Evolutionary reversals (Primitive lung evolving into nonrespiratory hydrostatic swim bladder which subsequently reverts back to become a respiratory gas bladder) have occurred repeatedly. Such reversed shifts are facilitated by the conserved neuromuscular pattern during functional transformations. Experimental comparative evidence is offered for the notion that evolutionary innovations may involve the addition of entirely new functions (respiratory) of a structural complex (gas bladder) while the original functions (hydrostatic, hearing and sound production) are rigidly retained. The paucity in Elopomorpha and absence in Clupeomorpha of respiratory gas bladders reflect the lack of functional demands for new habits in the environment rather than the absence of essential preexisting building blocks.

Author notes

1From the Symposium on Vertebrate Functional Morphology: A Tribute to Milton Hildebrand presented at the Annual Meeting of the American Society of Zoologists, 27–30 December 1986, at Nashville, Tennessee.