The transition to air breathing in fishes. V. Comparative aspects of cardiorespiratory regulation in Synbranchus marmoratus and Monopterus albus (Synbranchidae)

Extreme heart-rate lability accompanies the air-breathing cycles of Synbranchus marmoratus and Monopterus albus. When air is taken into the buccopharyngeal air-breathing organ of these fishes, heart rate increases sharply above pre-inspiration rates of 3­25 beats min-1 to as high as 40­45 beats min-1. With time, and as O2 is depleted from the air-breathing organ, heart rate gradually declines and drops to its lowest level with, or following, exhalation. Relationships between air breathing and sinus arrhythmia in M. albus were investigated by injecting variable gas volumes and O2 mixtures into the cannulated air-breathing organ. Tests were also carried out on undisturbed fish breathing volitionally in atmospheres containing different O2 levels. Both the volume and O2 content of the inspired gas affect the level and duration of inspiration tachycardia. Additional factors affecting tachycardia are the heart rate prior to inspiration and the time since air was last held. S. marmoratus is a non-obligatory air breather and uses rhythmic branchial aquatic respiration to a greater extent than M. albus, an obligate air breather. While the heart rates of both species are increased during aquatic ventilation, the higher heart rate to ventilation ratio in S. marmoratus (2­3 versus approximately 1 in M. albus) seems attributable to its more proficient aquatic respiratory system. The available cardiorespiratory data for air-breathing fishes indicate that the scope of air-inspiration tachycardia is smaller in lungfishes and other primitive species than in most teleosts. This difference is mainly attributable to the greater chronotropic effect of sympathetic cardiac innervation in teleosts.