Some species of cardinalfish are mouthbrooders, meaning that they incubate their eggs inside their mouths. In cardinalfish, this task is performed only by the males.
After spawning, the male gathers the eggs and places them in his mouth. During this period of several weeks, he cannot eat and will survive on stored energy. The ball of eggs are rotated inside the mouth periodically to evenly expose the eggs to aerated air.
Mouthbrooding offers physical and environmental protection for the eggs, as they are inside the relative safety of the fish’s mouth.
Klaus Stiefel on Flickr