We tested the individual and combined effects of ocean warming (OW) and ocean acidification (OW) on multiple generations of two foundational copepod (Crustacea) species, Acartia tonsa (warm season) and Acartia hudsonica (cold season). We measured a suite of life history traits (survival, fecundity, egg hatching success, time to adulthood, and sex ratio), which were used to calculate population fitness, the net reproductive rate. Experiments lasted 25 generations for A. tonsa and 11 generations for A. hudsonica. In collaboration with Melissa Pespeni?s lab at the university of Vermont, we measured allele frequencies during the multigenerational experiment of A. tonsa. In addition, separate studies were carried out to look in detail at thermal adaptation of A. tonsa; namely, latitudinal comparisons of thermal tolerance (ability to survive warming) and phenotypic plasticity (change in thermal tolerance with warming) on tolerance to test for local adaptation and the role of gene flow on eroding this adaptation, seasonal studies at one location to test for intergenerational selection for thermal tolerance, and experimental evolution in the laboratory to test for thermal tolerance.

Rapid (a few generations) and complete recovery of fitness (adaptation) was evident for both Acartia tonsa and A. hudsonica in response to OW. There was no evidence of deleterious effects of OA on either copepod species. Importantly, while rapid adaptation to OWA was also observed for both species, it was limited. That is, there was incomplete recovery of fitness. This implies that adaptation to OWA is a costly process for both species. There was clear segregation of allele frequencies by the end of the 25-generation experiment, indicating that the observed changes were driven by selection acting upon extant genetic variation. Non-genetic processes (drift and lab selection) were minor components in the allele frequency segregation. Further analysis revealed, however, a loss of transcriptional plasticity of copepods adapted to OWA.  

While egg hatching success was the main trait accounting for adaptation to OWA in A. tonsa, survival was responsible for adaptation in A. hudsonica.  Complex (nonadditive) interactions between OW and OW were evident in the limited adaptation for both species. A cost of adaptation for A. tonsa was also documented in later studies (65 generations) under OWA conditions. Paradoxically, thermal tolerance was lower in animals adapted to OWA than to ambient conditions.  Both the complex interactions between OW and OA and the hidden costs of adaptation add complexity to predictions of the response of animal populations to climate change.

Comparisons among populations indicate that low latitude populations have higher thermal tolerance, but lower phenotypic plasticity for thermal tolerance. This indicates that low latitude populations are near their thermal limits and are more vulnerable to further warming. While local adaptation to temperature was evident for the low and high latitude populations, gene flow prevents local adaptation at intermediate latitudes. There was also evidence of intergenerational selection for thermal adaptation and plasticity throughout the growth season of both species. Such selection is important for buffering the effects of heat waves (prolonged periods of warming) on animal populations. Finally, lab selection was evident in the experimental evolution studies and should be accounted for in future studies. From all these three approaches, a negative relationship between thermal tolerance and phenotypic plasticity for thermal tolerance was evident.  This means that thermal tolerance may come at the expense of phenotypic plasticity, which is considered a mechanism of short-term adaptation to climate change. Hence, the evolution of thermal tolerance may represent itself a cost of adaptation.  

Our project results imply that while copepods may adapt quickly to ocean climate change, the adaptation is limited and costly. In effect, there is no free lunch to climate change adaptation. Limited copepod adaptation suggests lower fisheries yields in the future, and lessened contribution of these animals (the most abundant in the oceans) to removal of CO₂ from the surface waters to the deep ocean.

This project supported two Ph.D. students (and their dissertations) and several undergraduates, as well as our lab?s participation in the Research Experience for Undergraduates program at Mystic Aquarium and University of Connecticut. Several underrepresented groups in science (people of color, Hispanic) were part of the team. The project also resulted in a fruitful collaboration with the University of Vermont examining the genomics basis of copepod adaptation to climate change. The results of the project have been reported through numerous scientific publications in international journals (Nature Climate Change, Nature Communications, Global Change Biology, Biology Letters, Ecology and Evolution, Evolutionary Applications, and Journal of Plankton Research) and books (The Impacts of Climate Change on Fisheries and Aquaculture), and numerous scientific conferences and lay audience presentations. Several outreach activities for elementary and high school students and underrepresented groups in science, technology, mathematics, and engineering were carried out.     

Last Modified: 12/20/2021
Modified by: Hans G Dam