Sex Change in Nature - Coral Reef Fish
For many species, it is certain that the majority of their offspring will not survive to adulthood. The solution, then, is to simply have more offspring. But this is where nature finds a limitation. While males of almost every species can produce sperm in very short order, the females, once impregnated, must wait until they give birth (or lay eggs) before they can reproduce again. For some fish common to coral reefs, evolution found an interesting solution.
Not only will the size, color, and markings of fish change as they change sexes, so too will their sexual organs and the gametes (sex cells) that they produce.
Female-to-male (protogyny) fish which once produced eggs are able to instead produce sperm. This is conversely true for male-to-female fish (protandry). While some fish can only undergo a sex change once in their lives, others can go back and forth many times, or even have both sexual organs at once.
The exact biological mechanism that stimulates the sex change process is not fully understood. What is known is that the determining factors are often social in nature. One such is called disinhibitional or suppressional. It suggests that the presence of a male in a group of females prevents or inhibits them from changing sex. Removal of the male causes one or more of the females (usually the larger one) to become male. In the same way, if a female is removed from a male, she will change sex (called stimulation or induction).
One example is the blue-streaked cleaner wrasse. The fish usually separate themselves into small groups of one male and several females (referred to as a harem). In a laboratory setting, when scientists removed the male, the largest female would spontaneously change sex. This is an example of protogyny, and is much more common that protandry (changing from male to female).
In protandy, the male changes to female to improve genetic fitness. One major example of this type of fish is the clown fish, which usually form monogamous relationships.
Should the female die, the male will change sex rather than find another female, and then pair up with a single male clown fish. In this case, the absence of the female is the triggering factor in the male's change.
On an interesting side note, this is the same type of fish seen in the Pixar movie Finding Nemo. In it, a love-struck clown fish's wife dies and he is left to raise a child, Nemo, alone. In reality, Nemo's father would have changed sex so that he might reproduce again with another unpaired male. It has been observed though, that males will sometimes find new female partners rather than change sex, as the process can take up to two months. During this time the fish would be unable to reproduce, and might miss his only opportunity to mate in his lifetime.
While protogyny and protandry increase genetic fitness, it should be noted that in both cases, the original genetic makeup of the fish does not change. Fish born female will contain XX chromosomes, while fish born male will retain their XY chromosomes, no matter what sex they might eventually become.