Sea pens are ecologically important habitats for associated marine organisms, serving as ecosystem engineers in sandy or muddy seafloor environments. In such areas, sea pens can form habitats with high population densities known as "sea pen fields". However, the presence and importance of sea pen fields have not been well studied in shallow waters in East Asia. Here, we report a sea pen field of Virgularia sp. aff. gustaviana in the shallow waters of Ushibuka Marine Park, in the Amakusa Islands of southern Japan. The average colony numbers of the field across all depths (7-20 m) was 10.3 colonies/m(2) (live colonies) to 13.6 colonies/m(2) (all: live+dead colonies+holes), and the area of the sea pen field was at least similar to 50,000 m(2). At a depth of 15 m, the substratum consisted of sand and fallen leaves of terrestrial origin, and the highest sea pen density was observed (averages=17.2 live colonies/m(2), =25.8 total (live+dead+holes) colonies/m(2)). At a depth of 20 m, the substratum consisted of broken shells and rocks and had the lowest density (live colonies: average=0.8 colonies/m(2), all: average=1.0 colonies/m(2)). There were significant differences in colony number of Virgularia sp. aff. gustaviana between the "sand", "sand+leaves", and "broken shells/rocks" substrates. We hypothesize that the strength of the water currents caused by local geographic features and tidal movements produce suitable sedimentation and habitat for this species of sea pen. Therefore, we suggest that preserving the natural coastline is crucial to protect this and other sea pen fields in shallow waters and their benthic marine communities.