Ocean acidification is tipping the balance in oyster populations, causing more females to be born. New research shows how changing water conditions are reshaping the future of these marine creatures.
Oyster reef at Hunting Island State Park, South Carolina
Unlike many animals, oysters do not have sex chromosomes. Instead, their gender is dependent on environmental factors—a process known as environmental sex determination.
With rising carbon dioxide levels leading to the increased acidification of the oceans, a team of researchers led by Xin Dang and Vengatesen Thiyagarajan sought to find out how the gender ratio of oysters across generations is affected as a result.
The research process
They placed oysters taken from the wild in two different tanks: one with neutral pH water and the other with slightly acidic water. When the oysters in the acidic tank reproduced, their offspring showed a higher female-to-male ratio compared to those in the other tank.
To further explore these effects, the offspring from the acidic tank were transplanted into natural habitats with either neutral pH and acidic conditions. Regardless of the pH level, the third-generation oysters in both habitats showed higher female-to-male ratios, indicating that the impact of acidification on the oyster’s gender ratios was transgenerational.
This held true even for the offspring of the oysters in the tank with neutral pH levels. Similar to the other group, the offspring were transplanted into natural habitats with either acidic or neutral pH waters. Here, the third-generation offspring in the natural habitat with acidic waters exhibited a higher female-male ratios.
Why it happened
Genetic analysis provided additional insights. Under acidic conditions, genes involved in female development were activated, while genes associated with male development were shut down.
“This study is the first to document a biased sex ratio over multiple generations towards females driven by exposure to low pH,” said Dang. “The results expand our understanding of environmental sex determination and highlight the possible impact of future global changes on reproduction and population dynamics of mollusks and other marine organisms.”
Future research will investigate whether similar mechanisms occur in other marine species and explore the potential application of pH-driven sex determination in oyster aquaculture.
