Reconfirmation of Ecklonia radicosa (Laminariales) from Mageshima Island, Kagoshima, the Southernmost Distributional Limit in Japan

Abstract

A brown alga, Ecklonia radicosa (Kjellman) Okamura (Laminariales), is one of the canopy-forming algae in the kelp forest, and it is distributed at the lowest latitude among Japanese kelp species. Although the southern distributional limit of this alga is known as Mageshima Island, Kagoshima Prefecture, its presence was believed to have disappeared from this site due to climate change, as there were no records and specimens from the site for at least the last two decades. In this study, the existence of this alga was reconfirmed in October 2023 at a site from a depth of 35 m offshore of Mageshima Island. This alga was first confirmed on Mageshima Island in 1948. However, the latest collection’s specimen from the site, deposited at major algal herbaria, including Kagoshima University, dates back to April 1971. Even though a long-term dredge survey was conducted more than 250 times between 2006 and 2022 using T/S Nansei-Maru, Kagoshima University, E. radicosa was not confirmed. In the reconfirmation of this alga, the blade was greatly grazed, suggesting a browsing effect by herbivorous parrotfish; however, as the sorus was confirmed at the lower portion of the blade, maturation and alternation of generations seemed to continuously occur at the study site.