University of the Ryukyus · Tropical Biosphere Research Center · 教授

Research keywords環境ストレス・dispersal・Infectivity・Monosaccharide・Coral・白化・Fishery・environmental

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• Environmental pH signals the release of monosaccharides from cell wall in coral symbiotic alga2023 · Yuu Ishii, Hironori Ishii, Takeshi Kuroha, Ryusuke Yokoyama, Ryusaku Deguchi, Kazuhiko Nishitani, Jun Minagawa,Masakado Kawata, Shunichi Takahashi, Shinichiro Maruyama
• Photo-movement of coral larvae influences vertical positioning in the ocean2021 · Behaviour can have profound consequences for the dispersal potential of an organism. In the marine environment, larvae rely heavily on oceanic currents to migrate from one area to another. As oceanic currents are faster in the shallows, the vertical positioning of larvae during dispersal is a key factor regulating the distance individuals can travel. Up until now, the vertical positioning of coral larvae has been largely explained by buoyancy, as well as changes in physical and chemical cues. However, here we show that in larvae of coral Pocillopora verrucosa, vertical positioning is influenced by photo-movement. We examined the reaction to light of five coral species in the laboratory and found that only larvae of P. verrucosa, but not other species, displayed a positive photo-response (i.e. an accumulation of larvae close to the light source). This reaction was observed irrespective to the orientation of light from the top, bottom or side. In the field, P. verrucosa larvae accumulated in the top halves of transparent chambers at all depths (1, 7, 15 m), whereas such behaviour failed to occur in dark chambers. Our results demonstrate that light can play an important role for coral larvae to regulate vertical positioning during dispersal and provides a hypothesis that positive photo-movement might allow larvae to disperse further and contribute to the wide geographical distribution of P. verrucosa in the Indo-Pacific.
• Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus2021 · Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus. There have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, this study reported the genome of a kleptoplastic sea slug, Plakobranchus ocellatus, and found no evidence of photosynthetic genes encoded on the nucleus. Nevertheless, it was confirmed that light illumination prolongs the life of mollusk under starvation. These data presented a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic analysis showed that genes for proteolysis and immunity undergo gene expansion and are up-regulated in chloroplast-enriched tissue, suggesting that these molluskan genes are involved in the phenotype acquisition without horizontal gene transfer.
• Moonrise timing is key for synchronized spawning in coral Dipsastraea speciosa2021 · Synchronized mass coral spawning typically occurs several days after a full moon once a year. It is expected that spawning day is determined by corals sensing environmental change regulated by the lunar cycle (i.e., tide or moonlight); however, the exact regulatory mechanism remains unknown. Here, we demonstrate how moonlight influences the spawning process of coral, Dipsastraea speciosa. When corals in the field were shaded 1 and 3 d before the full moon or 1 d after the full moon, spawning always occurred 5 d after shading commenced. These results suggest moonlight suppresses spawning: a hypothesis supported by laboratory experiments in which we monitored the effects of experimental moonlight (night-light) on spawning day. Different night-light treatments in the laboratory showed that the presence of a dark period between day-light and night-light conditions eliminates the suppressive effect of night-light on spawning. In nature, moonrise gets progressively later during the course of the lunar cycle, shifting to after sunset following the day of the full moon. Our results indicate that this period of darkness between sunset and moonrise triggers synchronized mass spawning of D. speciosa in nature.

• サンゴ礁及び周辺生態系由来の環境酵素によるフェーズシフト抑制効果の検証2024 · サンゴ礁生態系衰退に伴い、サンゴ優占から藻類優占へのフェーズシフトが急速に進んで いる。本研究では、サンゴ及び周辺生態系由来の環境酵素が、サンゴの加入・着底に悪影響 を及ぼす藻類や病原菌を除去して、サンゴ優占群集の維持に寄与している可能性を検証する。
• サンゴ骨格を用いた蓄積型栄養塩評価の新展開2024 · 陸域負荷がサンゴの生育に及ぼす影響の実態解明に向けて、これまで蓄積してきたサンゴ飼育実験技術及びサンゴ骨格分析技術を活用して、サンゴ群体の骨格試料を用いたリン酸塩負荷の評価手法、すなわち骨格に蓄積しているリン酸塩(蓄積型栄養塩)と骨格密度の測定方法を新たに開発する。
• 環境適合型サンゴの養殖・移植技術の確立2024 · 地球温暖化により、サンゴ礁生態系の貧弱化が進んでおり、過去30年でサンゴ被度は世界で20％も減少している。過去20年以上、サンゴ礁保全対策の一つとして、サンゴの養殖・移植事業が期待され、国内外で積極的に推進されてきた。しかし、移植されたサンゴの死亡率が高く、保全対策としての有効性が疑問視されている。死亡率が高い理由は、移植後の環境にサンゴが適応できなかったためで、今後は移植サンゴの生存率の向上が求められている。そこで本研究課題では、「サンゴの環境適応機構」に関する研究の知見を活用し、「環境適合型サンゴの養殖・移植技術の確立」を目的とする。
• サンゴの高温適応を考慮した新たな将来予測の実現2023 · 基盤研究(A)

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