DANOS, Nicole: Biomechanics of ventilation in the ratfish, Hydrolagus colliei.

Bony fishes have a bony operculum covering the branchial cavity. Negative pressure can be created in the branchial cavity by opening the operculum to draw water over the gills. Backflow, which has detrimental effects on counter-current efficiency, can be prevented by closing the operculum. Sharks have fleshy flaps over each parabranchial chamber. These do not effectively prevent backflow, nor do they allow them to ‘suck’ water across the gills. In contrast, ratfish have a single fleshy flap over the branchial chamber. I investigated the pressure gradient during respiration to see if they can prevent backflow and also maintain an effective suction pump. I used pressure transducers to measure the pressure in the buccal and branchial cavities and see whether a negative pressure gradient was observed at any time, allowing backflow. Negative pressure gradients were measured but I could not verify whether there was any actual backflow. Initial observations suggested that the opercular pressure might be controlled by movements of the pectoral fins. However, the ventilation rate was higher than the pectoral fin movement frequency. Morphological data from an SEM showed that lamellae are packed closely in the gills (2.97 primary and 22.59 secondary lamellae/ mm). The significance of these densities lies in the small intralammelar space, which in conjunction with the high ventilation rates (and consequently short ventilation cycles) are possible adaptations for a continuous unidirectional flow over the gills. The high viscous forces in the small intralamellar space may be enough to negate the inertial forces created by the short duration of the pressure reversal. Hence the flow may slow down significantly or even stop but not change direction. My data also strongly suggest that further measurements should be made on animals that are accustomed to captivity.