Ecotypic differentiation in maturation seasonality among Sargassum horneri (Fucales, Phaeophyta) populations in Hiroshima Bay, Seto Inland Sea, Japan

Seasonal changes in plant length and maturation rate were surveyed for eight populations of Sargassum horneri in and around the Hiroshima Bay area, Seto Inland Sea, Japan. Six Populations were chosen from the innermost to the exterior part of Hiroshima Bay, and two populations from the inner and outer parts of Yanai Bay, which is adjacent to Hiroshima Bay. A seasonal gradient in the peak of plant length and maturation was observed among populations from the inner to the outer parts of the two bays. Populations inhabiting the inner area of the bays reached the maximum plant length and peak in maturation in autumn, and decayed in winter. Only germlings were found at the habitats in the inner area of the bays in spring. On the contrary, populations inhabiting the mouth and outer area of the bays contained no mature plants in autumn, but plants gradually began producing receptacles during winter and reached the peak in maturation in spring. Seedlings from two inner-bay and two outer bay populations were transplanted to the inner part of Hiroshima Bay in order to compare their seasonal growth and maturation. Although summer growth of seedlings was inhibited due to the effect of sedimentation, each seedling exhibited the original maturation seasonality of the population of its original habitat. We concluded that differences in seasonality among the S. horneri Populations are attributable to a genotypic difference, rather than to phenotypic plasticity.