Meeting Abstract
Approximately 50 families of fishes contain species that are known to breathe air. The ability to breathe air, in addition to ventilate their gills with water, allows these species to survive in oxygen-poor habitats. While the behaviors and anatomical structures used to breathe air are diverse, numerous species possess a respiratory gas bladder and use buccal pumping to ventilate it. It is likely that buccal pumping was used by the first air-breathing fishes and by the tetrapodomorphs that transitioned to living on land, which makes it an evolutionarily interesting behavior. The kinematics of this method of air breathing have been previously described from 2D x-ray videos of a handful of fish species. Here we present the first 3D kinematics of air-breathing in the royal knifefish, Chitala blanci, a facultative air-breather that uses four-stroke buccal pumping. We used X-ray Reconstruction of Moving Morphology (XROMM) to measure cranial bone motions and muscle shortening. As air is released from the gas bladder, the suspensoria abduct and the cleithra retract, depressing the hyoid bars. The buccal cavity then compresses to expel the stale air through the opercular valves. Opercular elevation depresses the lower jaw via the interoperculomandibular ligament, and the cleithra retract by 5 to 10 degrees, causing the hyoid bars to depress by up to 40 degrees to draw in fresh air. The sternohyoideus does not shorten during hyoid depression. Knifefish also rely on large amounts of lateral expansion to widen their laterally-compressed heads—the suspensoria abduct as much as 10 degrees, and the two halves of the lower jaw spread 20 degrees apart from each other, as do the left and right hyoid bars. Comparing the air-breathing kinematics among species may shed light on the evolutionary history of this important behavior.
