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Fig. 6. A female (A, left) and male (A, right) of the cardinalfish Apogon
leptacanthus. The male is mouthbrooding, as revealed by its expanded
lower jaw. The egg mass of this species makes up about 14% of the body mass of
the male, and constitutes a considerable respiratory obstacle. (B) A male of
Apogon fragilis spitting his brood when exposed to hypoxia in a
closed respirometer, thereby significantly increasing his ability to take up
oxygen. Photos by G. E. Nilsson.