Within life-history trait variations for two herbivorous intertidal lacunids, Lacuna pallidula, a direct developer and Lacuna vincta a planktotroph, were compared and related to their ecology and to marine invertebrate life-history theory. Aspects of life-history theory covered included; reproductive investment, the relationship between egg size and egg fecundity, the Egg-Juvenile-Period (EJP), the implications of egg size for offspring status and maternal effects. Similar patterns of growth and reproductive investment were observed for adult females of both species, although absolute rates of growth and reproduction were differently affected by macroalgal diet. Further, differences in response to the favourability of macroalgal diet were observed for the two species, notably in the manner in which eggs were packaged. Variations in both egg size and egg numbers in spawn masses were observed for the two species, among populations within both species and within Lacuna pallidula populations. For Lacuna pallidula, these variations were shown to be mediated by both maternal macroalgal diet and population source. Maternal diet directly affected the size of hatching offspring but not the size of eggs produced. Consequently, egg size was not a good indicator of hatching size for either species. The EJP was determined for both species for a range of temperatures. Greater variations in the EJP and juvenile size were observed in Lacuna vincta. This was attributed to the ability of the larvae of this species to delay metamorphosis and to display positive growth during the delay phase. Both temperature and microalgal diet were shown to affect patterns of growth and development in Lacuna vincta larvae. Sources of naturally occurring cues for inducing settlement and metamorphosis in Lacuna vincta larvae were investigated to understand further the distribution patterns of this species on macroalgal types. Extension of the work investigated the suitability of various artificial cues for inducing metamorphosis and the effects of larval age and nutritional status of larvae upon latency of response to established inducing cues. Overall, L. pallidula displayed greater variation in traits and was more sensitive to environmental change than L. vincta. This finding is discussed in light of the two species larval strategy.