Imagine a world where all newborns are of the same sex. Later in life some of them change into the other sex. This is not science fiction, but a not-uncommon scenario in sex-changing fish. Why would they do it? Let’s say that it is better to be large female, because they produce more eggs: in this case small individuals should be males as they grow, and then become females when bigger. Or, if small males cannot compete with large ones, it would be more convenient to be small females first and then turn into large, dominant males later on in life. This too-good-to-be-true strategy has a downside. If all newborns are, for example, females and only some become males at a specific age, when larger, the whole population will be composed for the main part by females, with just a few males. This sex ratio imbalance create a problem: the few male fish will be the fathers of the majority of the newborns, fertilizing the eggs of many females. Thus, we will have many mothers and just a few fathers: while every baby fish receives 50% of its genes from the mother and 50% from the father, in his situation, many baby fish will have the same father, being a little more similar to each other, so the overall genetic variability is expected to be lower than in circumstances where the same number of parents are represented by equal numbers of mothers as fathers.

Variation is good, especially in changing environments, where some fish might end up doing better than others. Scientists calculate the “effective population size” estimating the contribution to parenthood made the actual breeders (the available adult fish of reproductive age). The higher the effective population size the better: small populations (with few parents of either sex) are at higher risk of losing genetic variation than larger populations. Overall genetic variability is expected to be lower with sex unbalance than in circumstances where the parents are represented by equal numbers of mothers as fathers. Unexpectedly, sex changing fish did even better than a simulated virtual population with their same characteristics (same number of eggs produced, length of life, etc…) but equal number of males and females (and not the few fathers as in sex changing fish).

The answer lies in the continuous process of sex-changing, a bit like rolling substitutes in sport. Sex-change can give each fish an extra advantage (a bonus) over a lifetime: increased number of offspring is achieved reproducing as a young female, but also, later in life, as a large, older male. But when to change sex? We find indication that if sex-changing fish could change sex a little earlier than they do, their effective population size would be even larger than it is: another intriguing aspect to consider. And finally, think about all of this again but in the opposite direction. Yes, some species change from males to females (many fathers, fewer mothers), but the overall story does not change much. Sex changing fish are very ‘strategic’ and they can produce many offspring. This is why quite a few species do change sex! And, even if fish are the only vertebrate capable of using this reproductive strategy, many invertebrates are sex-changers, including corals, sponges, molluscs, annelids, echinoderms, and crustaceans.