It is well understood that offspring size in ectotherms is highly responsive to changes in temperature, but the processes that underlie this response are not as well studied. We investigated when temperature changes induce a shift in the offspring size of the common tropical slipper limpet, Crepidula cf. marginalis as well as the duration of time it takes for offspring to reflect the temperature experienced by their mother. After collecting the juveniles at Playa Venado, Panama, the animals were raised in the laboratory at either 24°C or 28°C, temperatures normally experienced in their natural habitat, and moved reciprocally between the temperatures. In the first experiment, we moved animals to the other temperature after oviposition to establish whether differences in offspring size is attributed to either the temperature experienced during oogenesis or embryogenesis. In the second experiment, we moved animals after the hatching of their first brood and measured the following three broods to establish the length of time that the legacy of the first temperature remains. We determined the thermal experience of the mother during oogenesis significantly impacts offspring size and that offspring size is affected for at least two broods after a temperature change. These results indicate offspring size is determined by the temperature experienced prior to oviposition and that this thermal legacy lingers for no more than two brooding cycles. It is unknown whether this rapid thermal acclimation is an adaptive feature or a response to an oogenetic physiological limitation. Further research into the ways in which temperature affects offspring size will provide a deeper understanding into the potential for organisms and ecosystems to respond to temperature changes induced by climate change.