How vulnerable are marine organisms to unfolding ocean acidification? Apart from being species- and habitat-specific, the answer may even differ between times of the year. Other than open ocean species, most coastal organisms naturally experience large seasonal pH fluctuations, to which they have adapted. Murray and co-workers monitored pH conditions in the spawning habitat of a common coastal marine fish, while sampling wild spawning adults repeatedly over the season and conducting standardized CO₂ exposure experiments on their offspring. This demonstrated that offspring CO₂ sensitivity is not constant, but decreases seasonally with the increasing acidification in their habitat. These findings imply that realistic assessments of species CO₂ sensitivities should account for the pH/CO₂ variability in the parental environment.