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.