The larval shell morphology of 10 bivalve species of the family Mytilidae (Adula falcatoides, Crenella decussata, Crenomytilus grayanus, Modiolus kurilensis, Musculista senhousia, Mytilus coruscus, Mytilus galloprovincialis, Mytilus trossulus and Septifer keenae, and an unidentified species Mytilidae indeterminate) from the Sea of Japan is described. The following morphological features were comparatively examined: larval shell outlines including shape and size of umbones, and anterior, posterior and ventral margins, hinge morphology, ligament location, sculpture, colour, and eye-spot outlines. Some dimensional parameters of larval shells are given. The time interval of occurrence of larvae in the plankton, water temperature in this period, and shell length of competent larvae are presented. It is demonstrated that, in spite of sometimes similar external morphological characters, identification of mytilid larvae creates no difficulties if all distinguishing characters are used.

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