The gastropod genus Alderia (Allman, 1845) (Opisthobranchia: Sacoglossa) contains a rare case of poecilogony, or variable larval development within a species. This paper reviews the alternative larval morphs and dispersal strategies expressed by Alderia spp., and presents new data on larval ecology, environmentally induced changes in development, and rates of metamorphosis for larvae differing in age and life history. Recent morphological and molecular analyses revealed a cryptic poecilogonous species in the previously monotypic genus Alderia. The newly described Alderia willowi Krug et al., 2007 occurs south of Bodega Harbor, California, and was the subject of all prior studies by Krug and co-workers. Unlike its strictly planktotrophic congener Alderia modesta (Lovén, 1844), A. willowi produces either small feeding larvae that have a 30-day pre-competent period or large larvae that need not feed to metamorphose. Individuals can vary the developmental mode of their offspring, with a variable proportion switching from lecithotrophy (prevalent in summer) to planktotrophy in winter or spring; a similar shift is induced in some adults upon transfer to the laboratory. In a second dispersal polymorphism, a variable percentage of lecithotrophic larvae undergo spontaneous metamorphosis within 2 days of hatching, while their siblings disperse until induced to settle by carbohydrate cues from the host algae Vaucheria spp. The percentage of spontaneous metamorphosis is uncorrelated with fecundity and is generally between 15–30%, a possible product of stabilizing selection on this bet-hedging dispersal strategy. Despite their different ages, competent larval morphs produced by alternative developmental pathways are similar in size, swimming behavior, and responses to dissolved settlement cues. However, competent planktotrophic larvae and older lecithotrophic larvae initiated metamorphosis faster after settlement than newly hatched lecithotrophic larvae, suggesting a link between planktonic period and habitat choice. Although rare, poecilogonous species like A. willowi offer special insights into the evolutionary causes and ecological consequences of alternative life histories.