For last three decades, the distribution of temperate Sargassum species has declined, and that of tropical Sargassum species has increased along the coast of Kochi Prefecture, southwestern Japan. However, little is known about the ecological and physiological characteristics of tropical Sargassum species in Kochi Prefecture. In the current study we performed culture experiments to understand how the growth of a tropical Sargassum species, S. ilicifolium, is affected by seawater temperature. We also conducted a 31-month field study to understand the seasonality of this species. The culture experiments showed that the relative growth rates of S. ilicifolium thalli sharply increased from 11 ℃ to 12 ℃ and gradually increased from 12 ℃ to 15 ℃. From 14℃ to 30 ℃, the relative growth rates of the tropical S. ilicifolium were higher than those of temperate species. The relative growth rates of the germlings were optimal between 25℃ and 30℃, while the germlings at 10℃ had almost no growth. The results of the field study indicated that S. ilicifolium formed macroalgal beds steadily every year. We proved that this species has the ability to grow, even during winter along the coast of Kochi Prefecture.
The zooxanthellate scleractinian coral Alveopora japonica is endemic to the northwestern Pacific, ranging from Taiwan to Korea and Japan. This coral has recently expanded into seaweed beds in the temperate waters of these regions, as the seawater temperature increased. The largest known bed of A. japonica (1940 m2) was found in 2012 at Yashiro Island, in the Seto Inland Sea in the temperate region of Japan, where the minimum seawater temperature is the lowest among the regions inhabited by this coral. We surveyed the coverage of seaweeds and A. japonica in this coral bed during the seaweed-abundant and seaweed-sparse seasons in 2014. Throughout the year, the coverage of Ecklonia kurome and sargassaceous plants was low in places where the coverage of A. japonica was high. The maximum seawater temperature was within the optimal range for the growth of these seaweeds, indicating that the increasing temperature did not adversely affect seaweed growth. Evidently, a local environmental stress rather than the increase in seawater temperature caused the decline of the canopy-forming seaweeds that, in turn, allowed the coral bed to expand. Our research provides baseline data to compare with and analyze future changes at the edge of A. japonica distribution.
In the asexual life cycle of the marine red seaweed Bangia fuscopurpurea gametophytic thalli produce multiple monospores that develop into thalli as clones. We investigated the effects of heat stress on the production and release of monospores in B. fuscopurpurea from Esashi, in northern Hokkaido Island of Japan. Non-lethal high temperatures of 25℃ and 28℃ strongly promoted monospore discharge, whereas no spore release was observed at 30℃, the limiting growth temperature of Esashi B. fuscopurpurea. These findings differed from previous reports using B. fuscopurpurea collected at Fukaura, the northern Japan, and at the Fujan province of southern China, for which growth and monospore release were observed at 30℃. Thus, the temperature range promoting asexual propagation with monospore discharge in B. fuscopurpurea varies and appears to be unrelated to the thermal conditions of harvesting areas. Since each B. fuscopurpurea strain had a unique upper-limit temperature for survival and release of monospores was accelerated under non-lethal high temperature conditions, the temperature range enabling the asexual life cycle program seems to be restricted by the degree of heat stress tolerance of the B. fuscopurpurea strains themselves.
The present paper presents the findings of field surveys conducted at Southern Primorye and Sakhalin, far-eastern Russia, and northern Hokkaido, Japan, to supply information of the present situation of saccharinian kelp species in southern Primorye, far-east Russia, and describes geographical size variation of the first- and second-year kelp S. japonica in the Sea of Japan. The study examined the relationship between environment and kelp forest using satellite imagery, and the effects of coastal topography. Appearance and density of major saccharinian kelps in three sampling stations of Vladivostok were influenced by topographical environment (inner, mid, and mouth part of bay), and growth of sea urchins was dependent on the standing crop of these major saccharinian kelps. The mouth part of bays in Vladivostok and Nakhodka commonly faced the open Sea of Japan, so topographical condition was similar, but occurrence of both kelp and sea urchin in these two areas were remarkably different. Growth of first- and second-year kelp individuals showed geographical difference, suggesting that the growth of first year S. japonica was controlled by lower temperature, and inhibited growth of first year kelps may be related to higher growth of second year individuals.