A survey of biogenic coralline sands in the littoral fringe of a tropical island in Japan brought a new amphipod species to light. This species represents the first record of the subterranean genus Metaniphargus from the West Pacific. The majority of the species in this genus occur in the Caribbean, but a report from Hawaii and now from Japan defies the endemic Caribbean status it kept for so long. Metaniphargus shiroi sp. nov. is described, and morphological comparisons are made with closely resembling species from Hawaii and the Cayman Islands (genus Metaniphargus), and the Great Barrier Reef and California (genus Dulzura). Involvement of non-congeners in the comparisons is necessary as character overlap is abundant. These comparisons suggest that the presence of form-related body types in the shallow marine interstitial realm is circumtropical and follows habitat suitability rather than sudden dispersal or vicariance events.