Environment and reproduction under water: (fe)male sex differentiation in fish

Reproduction is essential for the survival of any species. Sexual reproduction is nearly the sole mechanism in that regard in the case of animals, and of course fish as well. Sexual reduction in animals requires two different sexes - male and female - to exist. This is called gonochorism. There are species, including many fish, which are hermaphrodite. Yet in those cases, we find male and female gonads at different points of time (they have one sex when juvenile and the other afterwards) or space (gonad divided in two).

Sexual segregation is genetically determined in the majority of species – as is the case of mammals; although we do not always find sexual chromosomes. However, the sex of fishes is not always ‘written in stone’ or simply genetically determined. Environmental conditions – such as temperature, food availability, the pH, social conditions or the presence of pollutants – can prevail over genetically established rules, causing changes in the development of the fish gonads. Thus, genetically male specimens can be feminised and vice versa.

Several chemical pollutants – known as endocrine disruptors – that started to appear in our environment some time ago have been grounds for concern in recent years. There are many and vary as regards their composition, but some have a structural similarity to estradiol (female hormone). These include synthetic hormones, but also alylphenols that are used as surfactants, some compounds that are used to make plastics, and several pesticides. They are considered to be xenoestrogen due to their effect.

Consequently, fish receive feminising signals from the environment that prevail over the message of their hormonal background. Their impact is greatly noted around discharge points of wastewater treatment plants, as male fish found at those point frequently produce roe in the testicles. That is known as intersex testicles. Of course, this feminisation response has repercussions for the reproductive capacity of the population. In recent years, populations of feminised mullets have been found in all the river estuaries of the Basque Country, particularly in the Urdaibai river estuary near Gernika and in Pasaia.

Thanks to the molecular and histological study of all those fish, we have been able to study the Urdaibai situation since 2007, and we have good news! In 2021, the Gernika wastewater treatment plant and operations were transferred to the Bermeo plant, where the discharge is into the open sea. Consequently, while around 50% of the Gernika male mullet had roe in their testicles (2007-2021), we have not seen this type of fish since 2022.