Variations in phlorotannin concentrations among the developmental stages of brown algae have been reported; however, the phlorotannin concentration plasticity associated with fluctuations in environmental factors make it difficult to determine the essential ontogenetic variation. The phlorotannin concentrations in five perennial sargassacean species where newly sprouted branches appear in summer and become fertile the following spring were examined every month during a year; and correlation with the developmental or seasonal environmental factors was determined. Although the phlorotannin fluctuated greatly throughout the year, the fluctuation patterns were relatively similar among the five species: phlorotannin showed a peak during July and August; gradually decreased in the winter; and increased in April. Performing a multiple regression analysis, the phlorotannin concentration did not correlate with thallus size in all species; and phlorotannin amounts were significantly affected by ambient abiotic factors in some species. The phlorotannin contents in newly sprouted branches were always higher than those in the long main branches during all seasons. When the phlorotannin contents were determined monthly for S. fulvellum (Turner) C. Agardh where the thalli were cultured from embryos in outdoor tanks, the phlorotannin concentrations were 3-4% of the dry matter (DM) in the juveniles and decreased to less than 1% of the DM in thalli >7.5 cm in length. However, the phlorotannin in these cultured thalli suddenly increased to 5.3% DM after being transplanted to the inshore coast; and then the concentration gradually decreased. The data show higher phlorotannin concentrations in younger sargassacean algae thalli and fluctuation of the phlorotannin amounts with extrinsic environmental factors.

The density (fresh weight/volume) of the detached thalli of four sargassacean species (Myagropsis myagroides, Sargassum horneri, S. patens, S. siliquastrum) was measured weekly to determine their floating period. Thalli of the four species were collected with respect to the difference in their reproductive stage, and their floating abilities at different stages were determined. Floating thallus was ballasted with pre-weighed leads until the final addition of a weight caused sinking. The density and floating period of sargassacean thalli were dependent on the species and the specimens' reproductive stage. The floating period of thalli detached before maturation was estimated to be between 4 and 14 weeks. However, thalli detached during maturation had a floating period of between 1 and 8 weeks, and those detached after maturation sank within 2 weeks. Sargassum horneri had the lowest density and the longest floating period among the four species investigated and this may explain the high abundance of S. horneri in floating seaweeds observed in the Sea of Japan.

Annual net production of the five Sargassaceae species was investigated in Yoro, western Wakasa Bay, Sea of Japan. The Sargassaceae species were collected at a depth of 2.0-2.5 m and subjected to stratified clipping for 13 months. Annual net production was estimated at 2407 g dry wt/m 2 for Sargassum patens, 2132 g dry wt/m 2 for Sargassum macrocarpum, 1458 g dry wt/m 2 for Sargassum siliquastrum, 1197 g dry wt/m 2 for Myagropsis myagroides, and 1471 g dry wt/m 2 for Sargassum piluliferum. The ratio of annual net production to maximum biomass (P/B max ) for these species ranged between 1.3 and 1.7, being higher than in previous studies which used the same method. Winter storms with strong wave exposure and relatively calm condition in Yoro were considered to augment P/B max . The total losses estimated from the different units of stratified clipping were compared. If larger and less segregated units were applied, the total loss would decrease. It was suggested that a 50 cm stratum would be applicable to the larger species (3 m or longer) and a stratum of 20 cm or less would be applicable to mid-sized or smaller species (less than 3 m).

No abstract

The sea urchin Anthocidaris crassispina (A. Agassiz) is a dominant herbivore on rocky shores in the warm temperate region of Japan. To clarify the relationship between macroalgal community and A. crassispina on rocky shores, A. crassispina collected in the Sargassum area and neighboring Corallina area were compared with respect to their density, growth and reproduction. Density of A. crassispina was higher in the Corallina area than in the Sargassum area. A. crassispina in the Sargassum area reached a larger size and had higher gonad indices than those in the Corallina area throughout the year. The annual reproductive cycles were almost the same in the two different habitats. These results indicate that Sargassum spp. support better growth and reproduction of A. crassispina .