The aim of this study was to determine the paleoecological and paleoenvironmental significance of the trace fossil Macaronichnus segregatis by examining its extant counterpart, feeding burrows of the opheliid polychaete Euzonus, which was periodically sampled on a wave-dominated sandy beach on the Pacific coast of Japan from June to December 2006. The distributions of Euzonus and its feeding burrows normal to the shoreline ranged from 5 m to 40 m. The vertical thickness distributions ranged from 32 cm to 123 cm (Euzonus) and from 34 cm to 126 cm (burrows). These parameters increase with decreasing beach-face gradient. The midpoints (median levels) of the vertical distributions invariably occurred near the midforeshore (corresponding to high-tide level). Foreshore width and vertical thickness also increase with decreasing beach-face gradient. These findings suggest that changes in the distributions of Euzonus and its burrows in the midforeshore environment vary with beach morphodynamics. The vertical thickness of M. segregatis–bearing beds may be a behavioral response to—and, hence, be indicative of—ancient beach morphodynamics. In addition, the consistent occurrence of the midpoint of trace-bearing beds suggests that beds containing M. segregatis might prove useful as an index of midforeshore level.