Life cycle of Dilophus okamurae (Phaeophyceae) and its associated invertebrate fauna in Onagawa Bay, Japan

The life cycle of Dilophus okamurae and the associated benthic invertebrate fauna were studied monthly or bimonthly from June 1998 to September 1999 at a depth of 4-7 m in Onagawa Bay along the Pacific coast of north-eastern Honshu, Japan (38 degrees 28'N, 149 degrees 29'E). Dry biomass ranged from 230 g/m(2) in June to 10 g/m(2) in October. Sporophytes with tetrasporangia appeared during July to September 1998. Gametophytes with oogonia or antheridia were observed for the first time in May 1999 during observation. The absence of mature sporophytes in 1999 suggests that the life cycle is completed over two years. The sympatric gastropods Lirularia iridescens, Homalopoma sangrarense, Conotalopia minima and Cantharidus callichroa synchronized with seasonal changes in biomass of this algae. In contrast, Barleeia angustata and B. trifasciata fluctuated independently. The juvenile sea urchin Strongylocentrotus nudus appeared in the algal population starting in October at 0.7 mm mean test diameter at high density, and disappeared after March.