Most people know about the iconic symbiosis that clown fish share with sea anemones. Another interesting fact about clown fish is one of their reproductive behaviors. Clown fish change gender to increase their chances of breeding and passing on their genes.
A biologist would call clown fish protandrous sequential hermaphrodites. A sequential hermaphrodite is an organisms that starts life as one sex and changes to the other. This type of hermaphrodite can reproduce as a male or female but not both at the same time. The word protandrous refers to creatures who start life as males. Depending on the circumstances, clown fish can turn into females. However, this change is one-way; females can't turn into males under any circumstances.
In the Wild
In the wild, most species of clown fish live in anemones as large harems. The largest fish in the group transforms into a female. The next-largest clown fish in the harem becomes a sexually mature male. The rest of the fish in the harem remain sexually immature, nonbreeding males. If the female clown fish disappears, the next-largest male turns into a female and the new next-largest male fish is promoted to breeding male.
Their unusual sexual reproduction ability offers several benefits to clown fish. Clown fish eggs and the first stages of these fishes' life cycle are pelagic. This means the eggs float on ocean currents. This can bring young clown fish to distant areas. While this spreads the species, it also makes them harder for young clown fish to find mates. By being sequential hermaphrodites, clown fish increase their chances of finding a mate, since any fish can become female. Despite the trouble finding mates, sexual reproduction still offers some benefits over asexual reproduction: Mating allows fish to shuffle their genes around without mutation, producing offspring potentially more robust than the parents.
In the Aquariums
In the confines of an aquarium, clown fish behavior changes. Male/female pairs get along well in marine fish tanks. To ensure a male/female pair, you should purchase two juveniles -- do not buy more than two clown fish for an aquarium. In the wild, other clown fish have enough room to get a break from the breeding pair. In the confines of all but the largest marine aquariums, the breeding pair will bully the nonbreeders excessively. Additionally, a breeding pair will fight with other breeding pairs in most species of clown fish.