Growth and maturation of the ‘autumn‐fruiting type’ of <i>Sargassum horneri</i> (Fucales, Phaeophyta) and comparisons with the ‘spring‐fruiting type’

Abstract

SUMMARY The growth and maturation period of the autumn‐fruiting population ot Sargassum horneri C. Agardh (Phaeophyta) was investigated in Hiroshima Bay, Seto Inland Sea. Surveyed traits were compared to those of the spring‐fruiting type and ecological features of this species were discussed. The annual lifetime of the autumn‐fruiting population could be divided into four phases according to the daily increase in thalius length:phase I from December to May (increase in length &lt; 0.1 mm/ day), phase II from May to September (= 0.3–1.0 mm/ day), phase Ml (from September to December &gt; 10 mm/day) and phase IV which was the senescence phase from December to March. Receptacle formation‘as observed in November and gamete release from November to February. Conversely, the spring‐fruiting type germinated in April and exhibited swifter growth in its early stage of development than the autumn‐fruiting type. Rapid increase in thalius iength in autumn was common in both fruiting types, although the spring‐fruiting type continued to grow during winter. Receptacle formation of the spring‐fruiting type started in February but gamete release was not observed until April and May. The difference in life‐history patterns of both types of S. horneri was in the overwintering period. The autumn‐fruiting type spent that season as germi‐ings or as young plants exhibiting slow‐paced growth, while the spring‐fruiting type overvwintered as adult thal‐li preparing for gamete release in spring.