Contrasting population makeup of two intertidal gastropod species that differ in dispersal opportunities

► We study the population structure of two topshell species that differ in geographic range. ► The New Zealand endemic Diloma subrostrata displayed little population structure. ► The geographically widespread Diloma nigerrima displayed more population structure. ► D. nigerrima population structure did not consistently increase with geographic distance. ► Dispersal by rafting plays a role in establishing structure among D. nigerrima populations.