Breathing with a mouth full of eggs: respiratory consequences of mouthbrooding in cardinalfish

Mouthbrooding occurs among several groups of fishes. Although a mouth full of eggs can be expected to pose a considerable respiratory problem, to our knowledge no study has examined respiratory consequences of mouthbrooding in fishes, or how hypoxia or strenuous swimming may affect the success of this reproductive strategy. In two species of cardinalfish (Apogon fragilis and Apogon leptacanthus), from the reef at Lizard Island (Great Barrier Reef), we found that mouthbrooding significantly reduced the ability to take up oxygen at low ambient oxygen levels. While the direct energetic cost of mouthbrooding appeared insignificant at rest in well-oxygenated water, mouthbrooding significantly reduced the respiratory scope of the fishes and their capacity for sustained aerobic swimming. The males spat out their eggs in hypoxia. Interestingly, the species with the larger brood, A. fragilis, spat out the brood at a higher water [O2] than did A. leptacanthus, which had a smaller mean brood mass. Moreover, in contrast to mouthbrooding A. leptacanthus, mouthbrooding A. fragilis was unable to increase its ventilatory frequency in response to hypoxia. This suggests a trade-off situation between hypoxia tolerance and brood size. Apparently, A. fragilis has sacrificed hypoxia tolerance in favour of a large brood size to a greater extent than has A. leptacanthus.