Simultaneous warming and acidification limit population fitness and reveal phenotype costs for a marine copepod

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: deMayo, James A., Brennan, Reid S., Pespeni, Melissa H., Finiguerra, Michael, Norton, Lydia, Park, Gihong, Baumann, Hannes, Dam, Hans G.
• Other Authors: National Science Foundation, Connecticut Sea Grant, University of Connecticut, University of Connecticut
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2023
• Subjects: Ocean acidification; Copepods
• Online Access: http://dx.doi.org/10.1098/rspb.2023.1033; https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2023.1033; https://royalsocietypublishing.org/doi/full-xml/10.1098/rspb.2023.1033

Phenotypic plasticity and evolutionary adaptation allow populations to cope with global change, but limits and costs to adaptation under multiple stressors are insufficiently understood. We reared a foundational copepod species, Acartia hudsonica , under ambient (AM), ocean warming (OW), ocean acidification (OA), and combined ocean warming and acidification (OWA) conditions for 11 generations (approx. 1 year) and measured population fitness (net reproductive rate) derived from six life-history traits (egg production, hatching success, survival, development time, body size and sex ratio). Copepods under OW and OWA exhibited an initial approximately 40% fitness decline relative to AM, but fully recovered within four generations, consistent with an adaptive response and demonstrating synergy between stressors. At generation 11, however, fitness was approximately 24% lower for OWA compared with the AM lineage, consistent with the cost of producing OWA-adapted phenotypes. Fitness of the OWA lineage was not affected by reversal to AM or low food environments, indicating sustained phenotypic plasticity. These results mimic those of a congener, Acartia tonsa , while additionally suggesting that synergistic effects of simultaneous stressors exert costs that limit fitness recovery but can sustain plasticity. Thus, even when closely related species experience similar stressors, species-specific costs shape their unique adaptive responses.