The animal kingdom presents a great diversity of sexual expressions, and among them, the different types of hermaphroditism stand out. In addition to the best known, simultaneous hermaphroditism, in which the same individual has functional male and female sexual organs simultaneously, there are also numerous species that have sequential hermaphroditism .
In this second type, the organism has the ability to change sex throughout its life . Male creatures during their youth, at some point become female — a process called protoandry —, or vice versa — protogyny , like the dinosaurs in Jurassic Park.
hermaphrodite fish
Among teleost fishes —fishes with bony skeletons— sequential hermaphroditism has repeatedly evolved in different groups, and in each case, the conditions for its occurrence may be different. The best-known example is that of the clownfish , where a clan of young males is dominated by a single female, and when she dies, the eldest changes sex and takes on his new role.
In other species of fish, the change of sex happens for other reasons, they can be environmental, as in cases in which the trigger is demographic —for example, in response to a single-sex environment— or by temperature ; or pollution ; or they can depend on a biological determinism of the individual himself, for example, that the change happens at a specific age.
However, there are still many species of fish in which this type of behavior has yet to be discovered.
The Mexican admiral fish
In 1974, in the states of Oaxaca and Veracruz, a new species of fish was discovered, Millerichthys robustus , originally classified in the genus Rivulus , and colloquially called ‘Mexican admiral fish ‘, the only species of its genus. It is a small fish —no more than 4 or 5 centimeters in length— of the Cynolebiidae family, oviparous, and with a behavior strongly marked by the seasons.
These fish live in seasonal lagoons , that is, in environments that are flooded during the rainy season, and remain dry the rest of the year. This type of environment forces the fish to develop their entire life cycle, from hatching to reproduction, while the lagoon is full.
A peculiar life cycle
In such a hostile environment, animals acquire evolutionary adaptations that sometimes seem surprising to us. When the rainy season begins, the fish grow at an unusually fast rate , sexually mature very early and reproduce continuously, until the lagoon dries up, and they die.
It is in this phase that the embryos, which have protective structures against drought , enter a state of dormancy , called diapause , which allows them to wait until the rains arrive. Something that was a great find in 2017: as far as we know, it is the only species of fish in North America that lives in this peculiar way.
In other words, each new year, in each new rainy season, there is a new generation of fish.
The hermaphroditic annual fish
In fish with life cycles of several years, it seems a good adaptation that the young have a different sex than those that have more time, as in the clownfish. The presence of multiple young males is guaranteed, and when the female is missing, the process facilitates its replacement, maintaining the population and increasing the genetic variability of the offspring.
But in a fish like the annual Mexican admiral, with such a rapid life cycle, and in such extreme conditions, it seems counterintuitive to assume that it could have sequential hermaphroditism . Fish need to reproduce as quickly as possible before the pond dries up, and the time it takes to transition is time wasted not reproducing.
However, a research team led by Dr. Omar Domínguez-Castanedo, from the Mexican Association for the Study and Conservation of Cyprinodontiformes, detected adult specimens with an intersexual coloration and gonadal system , that is, in an intermediate state between the feminine and the masculine. In the subsequent study, they found that Mexican admiral fish do indeed have sequential hermaphroditism, specifically protogyny —that is, changing from female to male.
As appears in the recent publication in the scientific journal Scientific Reports , dependent on the prestigious Nature , this sex change event occurs in a social context of a majority of females, due to competition between them and induced by aggression physical.
The results of this research suggest that when the chances of reproducing are low, due to the competitive environment, the less suitable female to obtain a reproductive partner changes sex, probably in order to have greater biological success as a male.
The first signs of sex change were observed when the fish were just 23 days old, and by 75 days they were fully functional males. Very rapid changes, consistent with the accelerated growth rate of these animals.
It is the first time that a hermaphroditism of this type has been observed in an annual cyprinodontiform. This discovery may open new doors of research on this fascinating evolutionary adaptation.
This article has been reviewed by Drs. Omar Dominguez-Castanedo
References:
Domínguez-Castanedo, O., Uribe, M. C., et al. 2017. Life history strategies of annual killifish Millerichthys robustus (Cyprinodontiformes:Cynolebiidae) in a seasonally ephemeral water body in Veracruz, México. Environmental Biology of Fishes, 100(8), 995-1006. DOI: 10.1007/s10641-017-0617-y
Domínguez-Castanedo, O., Valdesalici, S., et al. 2017. Developmental ecology of annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae): Developmental Ecology of M. robustus. Developmental Dynamics, 246(11), 802-806. DOI: 10.1002/dvdy.24519
Domínguez-Castanedo, O. et al. 2022. Protogynous functional hermaphroditism in the North American annual killifish, Millerichthys robustus. Scientific Reports, 12(1), 9230. DOI: 10.1038/s41598-022-12947-2
