Copepod study reveals hidden costs of climate change adaptation

Suppose that we could watch twenty generations of whales or sharks adapting to climate change, measuring how they evolve and how their biology changes as temperatures and carbon dioxide levels rise. That could tell us a lot about how resilient life in the oceans might be to a warmer world. But it would also take hundreds of years – not very useful to scientists trying to understand these changes and policymakers looking for measures to limit climate change and adapt to risks today.

In contrast, the copepod Acartia tonsa, a tiny crustacean near the bottom of the food web, serving as an important diet of many economically important fish species, reproduces, matures, and creates a new generation in about twenty days. Twenty generations pass in about one year. To find out more about its ability to adapt to changes in its environment and long-term impacts, a team of scientists, exposed thousands of copepods to the high temperatures and high carbon dioxide levels. The study was conducted at the University of Vermont and led by Reid Brennan. The marine biologist moved to GEOMAR Helmholtz Centre for Ocean Research Kiel in October 2021 to become junior professor and head of the research group “Marine Ecological Genomics” in the research division Marine Ecology.

“It is hugely important to understand if these types organisms can evolve in response global change”, Professor Dr. Brennan says. “They serve as the base of the food web. Without them we would not have any fish in the ocean.” As part of his work at GEOMAR, he will further investigate how copepods react to climate stress, if they are able to adapt over generations, and what kind of evolutionary rescue mechanisms these species develop. Basis for his work are controlled experiments in climate chambers followed by genome sequencing in the laboratory. “This work is highly relevant if we want to understand the future of our ocean. I think it is a major challenge for the field and we still have a lot of work to do.”

At the University of Vermont, Professor Brennan and his supervisor Professor Dr. Melissa Pespeni watched twenty generations of Acartia tonsa developing under rising carbon dioxide concentrations and temperatures conditions as predicted for the future of the oceans. Afterwards, some copepods were returned to the initial conditions resembling today’s ocean for three more generations.

The results, published in the journal Nature Communications, “show that there is hope but also complexity in how life responds to climate change,” Professor Dr. Pespeni says. The copepods persisted and even thrived under experimental climate-change conditions. The scientists recorded many changes in the copepods’ genes related to how they manage heat stress, grow their skeletons in more acidic waters, produce energy, and other cellular processes affected by climate change. This shows that these creatures have the capacity in their genetic make-up – using the variation that exist in natural populations – to adapt over twenty generations, evolving to maintain their fitness in a radically changed environment.

The team’s observations support the idea that copepods could be resilient to the unprecedented rapid warming and acidification of the ocean that has already started to happen in response to human fossil-fuel use. However, after returning to initial conditions, the planktonic animals revealed the hidden cost of the earlier twenty generations of adaptation: The flexibility that helped the copepods to evolve over twenty generations – what the scientists call “phenotypic plasticity” – was eroded when they tried to return to what had previously been benign conditions. The copepods were less healthy and produced smaller populations. They had lost the ability to tolerate limited food supply and showed reduced resilience to other new forms of stress.

The new study supports the assumption that copepods are among a broad group of species predicted to be resilient to rapid climate. “But we need to be careful of overly simple models about how well species will do and which ones will persist into the future that look at just one variable,” Professor Brennan emphasises. And the scientists’ new study of copepods points to a larger truth about the intricate economy of evolution: There may be unforeseen costs to quickly evolving in a suddenly-hot world.

Original publication:

Brennan, R.S., de Mayo, J.A., Dam, H.G. et al. (2022): Loss of transcriptional plasticity but sustained adaptive capacity after adaptation to global change conditions in a marine copepod. Nature Communications, doi: https://doi.org/10.1038/s41467-022-28742-6