Strategies Under the Sea: Fish That Change Sex
In the underwater world, sex is not always fixed. For many fish species, changing sex throughout their lives is a natural survival and adaptation strategy.
This phenomenon is part of the complex biological language with which evolution has equipped numerous marine organisms and can be influenced by social, hormonal, or environmental factors.
It is much more common than it may seem, especially in warm or temperate waters, and less frequent in cold waters.
How and why do some fish change sex?
Fish that change sex usually do so in one direction, although some species can change in both directions.
The evolutionary advantage of these changes lies in maximizing reproductive opportunities.
For example, in species with social hierarchies, it can be more advantageous to be born female and become male only when able to dominate a harem. In other cases, when a group has a single dominant female, a male can replace her upon her death.
Common Mediterranean species that change sex
1. Wrasse (Coris julis)
Very common in rocky bottoms and Posidonia meadows, the common wrasse is born female and becomes male when reaching a certain size or age.
Males are larger and more brightly colored. They usually form small harems and defend territories. This change is regulated by social and hormonal signals.
2. Sea bream (Diplodus spp., such as D. annularis)
Some species in this family (Sparidae) start life as males and, upon reaching a certain size or under specific environmental conditions, can transform into females.
This allows them to first contribute simpler gametes (sperm) and later, with more accumulated energy, produce eggs.
3. Gobies (Gobiodon spp., Paragobiodon spp.)
Many goby species can change sex in both directions (bidirectional hermaphroditism).
When two individuals form a pair, one becomes male and the other female based on the group’s needs. If separated, they can change sex again to adapt to a new partner.
4. Grouper (Epinephelus marginatus)
This emblematic Mediterranean species is a protogynous hermaphrodite. Young individuals are female, and only some large ones become male.
This explains why protecting large groupers is crucial: catching a big grouper likely means removing the unique reproductive male in the area.
A global phenomenon: clownfish and flatfish
Sex change is not exclusive to the Mediterranean. Other well-known species also exhibit it:
Clownfish (Amphiprion spp.)
They live in small hierarchical groups within anemones. If the female dies, the dominant male changes sex to take her place, and a subordinate male rises in rank.
Flatfish and other benthic fish
Some benthic species show sex changes related to environmental factors such as water temperature or population density.
Sex change and human impact
These natural strategies can be disrupted by fishing pressure, pollution, and climate change.
Selective removal of the largest individuals (often the reproductive males) can destabilize population balance and hinder species reproduction.
