Salinity adaptations of the gastropodsLittorina mandshurica andL. squalida were studied at different stages of their life cycles.L. mandshurica exhibited a higher degree of adaptation to variations in salinity thanL. squalida at all developmental stages. Egg masses did not protect embryos from desalination; the larvae inside the egg masses were capable of adapting to diluted seawater. As larvae developed in egg masses, their adaptive capabilities increased. At the time of hatching, the salinity tolerance of larvae differed little, if at all, from that of adults. It is assumed that pelagic egg capsules provide an advantage in the distribution of a species.