Aso-kai is a lagoon separated by sand spit from the Miyazu Bay, a marginal water of the Sea of Japan. We collected individuals of Phenacolepas pulchella, belonging to the order Neritimorpha, from inside of oyster reefs, as the first record of such inhabitation. Shell surface of P. pulchella accumulated blackish deposits. Electron probe micro-analysis revealed that the deposits consist mainly of manganese oxides (MnOn). Geochemical setting of the habitat may facilitate accumulation of the redox-sensitive element, Mn.