Within-species phenotypic variation has long attracted the attention of ecologists, taxonomists, geneticists, and evolutionary biologists. This article focuses on marine gastropods and reviews how scientists have addressed the problem of intraspecific variation in shell morphology since the 1930s. The molluscan classification at the species level relied mainly on conchological traits, even after important biological ideas were advocated such as the biological species concept, New Systematics,

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... pulation genetics, and modern evolutionary synthesis. The first half of this article briefly explains the ontogeny and ecology of Monetaria species to examine the conceptual background of their inter-and intraspecific classification developed in Germany in the 1930s. This is to emphasize how phenotypically plastic traits are not desirable as taxonomic traits under the biological species concept. The last half summarizes studies primarily by North American and Scandinavian scientists of the relationship between morphological variation and inducible defense against predators, i.e., crabs. Their systems with Littorina and Nucella 宮地賞受賞総説 M. annulus M. moneta M. caputserpentis M. caputdraconis 4 1 3 are an excellent example of how rearing experiments using marine gastropods successfully contributed to progress in ecology. A lesson from history is that to draw robust, appropriate conclusions from the observed phenotypic variation in both taxonomy and evolutionary ecology, it is critical to quantify the genetic and environmental components separately.