Award: OCE-1948975

As atmospheric emissions have increased, there has been greater absorption of carbon dioxide gas (CO₂) by the world’s oceans. This process, referred to as ocean acidification, has resulted in increased concentrations of CO₂ in seawater, and lowered pH values. These changes can in turn affect the physiology of marine organisms and can ultimately affect their rates of growth and survival. However, even though elevated CO₂ can cause negative effects on marine species, it is unlikely that all individuals in a population will have the same level of susceptibility. And given that ocean acidification is an ongoing process that will occur across generations of marine species, it is critical that we understand ocean acidification as an evolutionary force.

This project demonstrated how ocean acidification conditions can generate natural selection on fish populations, and how these populations are likely to evolve over the coming decades. There are several components to these processes and several key findings to our research. First, our results suggest that one of the biggest changes that fish populations can experience under ocean acidification conditions is differential mortality during the early larval stage. Because the response is relatively strong and because mortality is a direct component of fitness, this represents a major change in a component of natural selection. Another important finding is that rates of larval mortality are strongly related to egg size. Larger eggs give rise to larger larvae that survive better not just overall, but especially under ocean acidification conditions. This means that the selective effects of ocean acidification need to be viewed through the lens of offspring size evolution. 

Our study found that a major way in which ocean acidification will generate natural selection is by changing the existing pattern of selection on offspring size. In general, offspring size evolves in response to two forces. Larger offspring tend to survive better, but reproductive fitness is the product of both survival and offspring number, and if a mother produces bigger offspring, she must produce fewer of them. We tested for size-number tradeoffs in detail and found that the intensity of the tradeoff was greater than predicted by theory. Follow-up investigations suggest this was because the indirect costs of producing offspring (e.g., the costs of supplying developing offspring with nutrients) get proportionately larger with offspring size. We also found that the tradeoffs between offspring size and number were not affected by CO₂ levels, but eggs and larvae that were average sized or smaller experienced much higher mortality under high CO₂ conditions.

Ocean acidification will thus create an environment that favors offspring that are larger than the current average. However, whether populations will evolve, and the pace at which such changes will occur will depend on the degree to which variation in offspring size is because of genetic variation, and thus transmissible to subsequent generations. Our project used experiments where fish were spawned artificially to create offspring of varying degrees of relatedness (full siblings vs. maternal half siblings vs. paternal half siblings) and we compared variation in egg and larval sizes to measure genetic variation. We found moderate amounts of genetic variation in offspring size, suggesting that our study populations have significant capacity to evolve in response to ocean acidification.

The third major outcome of this project was the development of simulation models to assess the long-term effects of ocean acidification on fish populations by including a contemporary, evolutionary response. Models were built using our measures of natural selection and genetic variation and combining these with rates of CO₂ increase that were projected from available records. We found that contemporary evolution can partially offset the reductions in larval survival and population replenishment that are expected under acidification. As a rough guideline, projections of our study species to the year 2100 suggest that when allowing for an evolutionary response, the decline in population replenishment is only about half as much as projected under ocean acidification without allowing for evolution.

This award supported the training of three graduate students and 12 undergraduate students at California State University Long Beach. Students received training in experimental design, seawater chemistry, data collection, data analysis, research presentations and scientific writing. Six undergraduates conducted independent research projects related to the broader study. This award also supported the development of data-based modules for classroom instruction and outreach to local middle schools and high schools over several years.

 

Last Modified: 07/04/2026
Modified by: Darren Johnson