Mollusk shells are valuable recorders of climate and environmental changes of the past down to a daily resolution. To explore this potential, we measured changes in the composition of shells of two types of bivalves recorded at the hourly scale: the king scallop Pecten maximus and giant clams (Tridacna) that engaged in photosymbiosis. We find that photosymbiosis produces more day–night fluctuation in shell chemistry but that most of the variation is not periodic, perhaps recording weather.