AbstractIn most animal taxa, large mothers (or those with high nutritional status) produce large offspring, leading to a maternal size-offspring size correlation, that is, a positive correlation between maternal size and offspring size. Here, we used the natural variation in maternal size between three natural populations of Buccinanops deformis (a marine snail with direct development, nurse egg feeding, and a single embryo per egg capsule) to study maternal investment and offspring size. The main objectives were to compare offspring size and maternal investment traits within and between populations and to evaluate the relationship between maternal size and offspring size. Although not supported in every population, our results show that maternal size was positively correlated with offspring size, thus representing an example of the maternal size-offspring size correlation in a species in which there is no competition for food between capsule mates because only one embryo develops per capsule. These findings also suggest that in B. deformis larger mothers produce more offspring and provide their offspring with more resources, and that this between-population variation in offspring size is related to differences in the number of nurse eggs allocated per egg capsule and in egg capsule size. The ubiquity of the maternal size-offspring size correlation in B. deformis needs to be tested further across populations, because factors other than maternal size could influence offspring size variation in this marine gastropod.