Maternal effects could influence the persistence of species under environmental change, but the adaptive significance of many empirically estimated maternal effects remains unclear. Inferences about the adaptive significance of maternal effects depend on the correlation between maternal and offspring environments, the relative importance of frequency- or density-dependent selection and whether absolute or relative fitness measures are used. Here, we combine the monitoring of the environment over time with a factorial experiment where we manipulated both the maternal and offspring environment in a marine bryozoan (Bugula neritina). We focused on temperature as our environmental variable as temperature commonly varies over short time scales in nature. We found that offspring from mothers kept in warmer water were smaller and more variable in size, but had increased dispersal potential and higher metamorphic success than offspring from mothers kept in cooler water. Our results suggest that, under frequency- or density-independent selection, mothers that experienced higher temperatures compared with lower temperatures were favoured. Under frequency- or density-dependent selection, there were indications that mothers that experienced higher temperatures would be favoured only if their offspring encountered similar (warmer) temperatures, though these results were not statistically significant. Analysis of time series data on temperature in the field shows that the maternal thermal environment is a good predictor of the temperatures offspring are likely to experience early in life. We suggest that future studies on maternal effects estimate environmental predictability and present both absolute and relative estimates of maternal fitness within each offspring environment.