| Budget Amount *help |
¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 1997: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1996: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
Raphidophyceanflagellates belonging to the genera Heterosigma and Chattonella cause red tides and kill cultured fish such as yellowtail in the coastal sea of Japan in summer. These flagellates have overwintering cyst stages during their life cycles. The cysts play an important role to initiate red tides and to expand their distribution to new areas. Therefore, to clarify the process and conditions for encystments is essential to understand bloom dynamics of these red tide flagellates. However, there is a paucity of information on the process and conditions for encystments. In the present study encystment processes of H.akashiwo were investigated in northern Hiroshima Bay, the Seto Inland Sea. And the conditions for encystment were examined on C.ovata, C.verruculosa, and H.akashiwo in culture. The following results were obtained.
(1)When a natural population of H.akashiwo in the final stage of a red tide was collected and incubated in the dark with autoclaved marine sediment, a portion of the cells in the population changed into preencystment small cells and finally into cycts, which were identical to H.akashiwo cysts found in the field. The newly formed cycts required at least2 weeks of mandatory dormancy period before germination.
(2)According to the weekly measurements of a red tide of H.akashiwo carried out in northern Hiroshima Bay, newly formed cycts were substantially supplied to the bottom after the peak of bloom, and they could not germinate for at least a week, but could do so after2 weeks.
(3)In C.ovata and H.akashiwo, nitrogen limitation was effective in inducing morphological changes into small cells like preencystment small cells of C.antiqua and C.marina under the laboratory culture conditions. These induced small cells of C.ovata and H.akashiwo, however, failed to change into cysts. After the treatment of N-limitation, cells of C.ovata could survive in the dark for more than 4 weeks.
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