Excluding insects, about 5% of animal species have the ability to change sex, even the most unexpected. In most cases it is hermaphroditism, or the simultaneous presence of both male and female genital organs, but in some species the change occurs over time, upon reaching a certain age, or due to external factors, such as environmental pollution or the temperature.
Simultaneous or sequentialThere are two types of hermaphrodites. Simultaneous are those that produce male and female gametes at the same time: a classic example is the snail, in which the couples inseminate each other, but there are some that are able to self-fertilize, such as the tapeworm, the intestinal parasite. Sequentials, on the other hand, transition from male to female at different stages in their lives. This is the case for many invertebrates such as jellyfish, sea sponges, molluscs and starfish.
The case of the clown fishThe change usually occurs to maximize reproductive success. Clownfish form pairs that live together for years, in symbiosis with a sea anemone. We all know the film “Finding Nemo”, in which the father Marlin loses his wife and children and decides to go in search of the only one left. In real life, the clownfish would probably have found another mate and changed her name to Marlina. This species is very vulnerable to predation in the wild, so it is common for one of the individuals in a pair to die. When this happens, the remaining fish look for a new mate. And if it turns out that both are male, the larger of the two moults into a female.
Reversible polygamyThe timing and modality of the sex change largely depend on the mating system. For some it is permanent, for others it is reversible. Among the latter there is the Gobiodon okinawae, one of the smallest existing vertebrates, which lives in polygamy: when the dominant male disappears, the largest female inverts her sex and becomes the sultan of the group, to then return as a concubine in the presence of another male of larger size. This bidirectional sex change occurs in 66 fish species.
Monosex and neutralityIn the variegated world of animal sexuality, there are other variables. For example, the limpet that lives attached to the rocks of the Mediterranean has neither ovaries nor testicles but only one gonad capable of producing both types of gametes. Then there are the animals that reproduce themselves by parthenigenesis and also those who have no sexual organs, neither male nor female. The first ‘neuter’ cat has been found in the UK, but in this case it is a rare genetic condition due to a lack of fetal development.
Many worlds yet to be discoveredThe variability is so great that science hasn’t yet documented all the ways animals change sex. In 2019, a new species of wrasse was first described off the coast of Zanzibar, Tanzania, which also causes a yellow patch to appear on its head when it transforms its reproductive system from female to male. The Serranus Tortugarum, native to the Caribbean, changes sex even 20 times a day but always remains faithful to the same partner. In reptiles, from crocodiles to sea turtles, it is the temperature around the egg that determines the sex: females are born above 32 degrees and this could become a serious problem due to global warming.
