There are several kinds of fish, which carry their impregnated eggs in their mouths. Synodontis multipunctata, or cat-fish-cuckoo, learnt to make use of this mechanism. Sensing odor of its victims spawning eggs, such cat-fish would quickly swim up to them, eat a part of their eggs replacing them with its own, then the “adoptive mother” would take the eggs into its mouth. Moreover, young fish of the cat-fish-cuckoo hatch earlier and eat the remaining native fish eggs.
