Ocean acidification impacts many marine biota. Although evolutionary responses should occur during persisting environmental change, little is known about the adaptability of copepods. Therefore, we set up a 3½ yr long selection experiment, maintaining Acartia tonsa populations in seawater treated with 200 and 800 μatm CO2, and feeding them with algae cultured under corresponding CO2 conditions. In 3 reciprocal transplant experiments, roughly 1 yr apart, we measured developmental rates, C:N and C:P ratios, egg production and hatching rates of the different lines. In the transplant experiments, we observed significantly lower developmental rates in the high CO2 treatment independent of the selective history. Egg production and hatching success were unaffected by the experimental conditions, but we observed an earlier hatching of eggs from females with a high CO2 selective history. Over the experimental period, beneficial adaptations of the copepods cultured under high CO2 conditions of elevated seawater pCO2 and associated food quality were not detected. However, towards the end of the experiment, copepods cultured under elevated pCO2 and fed with high CO2 algae showed increased body mass and decreased prosome length.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2022) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2023-06-08. The seawater carbonate chemistry data are the data measured at the start of the long-term selection experiment.