In keeping with the remarkable diversity of species of fish extant today, and with the many habitats that they have colonized, reproductive methods are also extremely varied. Some fish, such as salmon and American eels, engage in massive breeding migrations, with millions of adults laying eggs simultaneously and then dying shortly thereafter. While the vast majority of fish lay eggs, up to 35 million in a season in some cases, a great many are live bearers that reproduce through internal fertilization. Many species merely scatter the eggs and leave them to take their chances, while others take great precautions in preparing nests, guarding the eggs, and caring for the young.

In one of the most unusual reproductive twists known in the animal world, female seahorses deposit their eggs in the pouches of the males. The “surrogate mothers” thereafter regulate the salinity content of the water in the pouch, brood the eggs, and bring forth the young. In yet another unusual strategy, certain species of fish all come into the world as females, with some becoming males at a later point in their lives (a phenomenon known as protogyny). Similarly, in a reproductive strategy known as protandry, the fish are all born as males and some later develop into females. Surgeonfish dispense with sexual differences altogether, existing only as females that reproduce by giving birth to clones of themselves. Several common aquarium fish use sex-changing strategies, including wrasses, which all begin their lives as females, and clownfish, which are all born as males.