| Budget Amount *help |
¥3,100,000 (Direct Cost: ¥3,100,000)
Fiscal Year 2006: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 2005: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 2004: ¥1,700,000 (Direct Cost: ¥1,700,000)
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| Research Abstract |
Symbioses between didemnid ascidian and prokaryotic algae are the only obligate symbiosis systems in chordates, but there were only a few reports on their taxonomy and distribution in Japan. The present study deals with the distribution of the photosymbiotic ascidians in Ryukyu Archipelago (Yaeyama Islands, Amami-ohshima Island, Tokara Islands, Satsunan Islands) and Chichijima Island (Ogasawara Islands). We also described two new species of photosymbiotic Diplosoma (i.e., D. ooru and D. simileguwa) from Okinawajima Island. Molecular phylogeny of photosymbiotic didemnids based on 18S rDNA sequences supported the monophyly of each photosymbiotic genus. Indicating that the establishment of the symbiosis occurred at least once in each of the photosymbiotic genera, i.e., Didemnum Lissoclinum Trididemnum, and Diplosoma. Morphological survey on the processes of vertical (trans-generational) transmission of photosymbionts showed that the mode of symbiont transmission from parent colonies to their progenies (embryos or larvae) is much various among the species.
Temperature and light intensity are supposed to be the principal factors that confine the distribution along the latitude line of pnotosymbiotic organisms such as ascidian-Prochloron symbiosis system. In subtropical area like the Ryukyus, the variation of water temperature is larger than that in tropical waters. We demonstrated that the occurrence of seasonal fluctuation of sexual reproduction in some photosymbiotic didemnids in Okinawajima island (the Ryukyus). As for the photo-adaptation, we focus on MAAs (mycosporine-like amino acids) as ultraviolet (UV) absorbing substances, and demonstrated the bathymetrical decrease of the MAA contents in the photosymbiotic ascidian Didemnum molle. We also found the significant difference of the MAA contents and chlorophyll contents among the color morphs of D. molle.
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