Middle Stone Age (MSA) shellfish remains were studied for indications of changes in subsistence adaptations. Optimal Foraging Theory was applied to the MSA shellfish species from Blombos Cave and Klasies River, South Africa, to model which shellfish would be most profitable to collect in terms of meat yield and handling costs. This ranking is compared to the composition of the archaeological shellfish assemblages. Several developments are considered to be the result of changes in prey choice. It appears that at Klasies River, MSA people mainly collected high ranked mid-intertidal shellfish, and that their foraging strategy was strongly influenced by changes in the coastal environment. At Blombos Cave, when the sea was further from the cave, a revaluation of the prey ranking caused an increase in . Perna perna densities in the M2 and M1 phases, compared to the older M3 phase. . P. perna was not only selected because, as a bivalve it preserves better during transport, but also because it can easily be mass collected. At Blombos Cave there are changes in species representation and a decrease in average . Turbo sarmaticus opercula weight. This tentatively suggests that shellfish collection may have intensified during later phases. In general, MSA collecting trips were optimized according to the available shellfish, taking into account the distance to the resource and their yield.