A corallimorpharian barren in southwestern Kochi, Japan

Abstract In shallow water zooxanthellate coral communities, large aggregations of benthic groups other than scleractinians have been occasionally noted, and sometimes linked with various disturbances. Here we report on a large aggregation of a corallimorpharian, Rhodactis sp., at a site off Himeshima Island, Kochi, Japan from surveys in July 2023 and August 2024. The overall area reached approximately 2100 m2 in 2024, with corallimorpharians accounting for approximately 70% of all benthic coverage. No such aggregation was noted in similar surveys in July 2016. While the cause of this aggregation formation is not known, it is possible that unusually cold winter seawater temperatures in 2018-19 may have caused favorable conditions for Rhodactis, given that benthic space was available. Regardless of the cause, this is among the first reported corallimorpharian outbreaks in Japan, and we recommend continued monitoring of this site.