Flattening of coiled shells has occurred in several gastropod lineages, while the evolutionary process is little known. The subfamily Fossarininae (Gastropod; Trochidae) is unique, because the four genera of this family vary considerably in extent of shell coiling/flattening. Broderipia and Roya have zygomorphic shells that have lost coiling, while the sister genera, Fossarina and Synaptocochlea have respectively turbiniform and auriform shells. We compared biology and morphology among these four genera to detect the selection pressure which has driven loss of coiling. We discovered that Broderipia iridescens live symbiotically in rock pits of sea urchins, and Roya eximia live on intertidal rock surfaces exposed to strong waves. Comparison of the morphology of soft bodies in Fossarininae revealed that the columellar muscle of flattened species has been drastically elongated as true limpet family, Nacellidae. The flattering and loss of coiling of the shell in Broderipia and Roya caused acquisition of a zygomorphic flat body, retraction of coiled visceral mass, and expansion of the foot sole. These morphological changes would have improved tolerance against strong waves and the ability to cling to rock surfaces in Roya, and would have enabled the commensal life in narrow space in sea urchin pits in Broderipia.