Onset of symbiosis and distribution patterns of symbiotic dinoflagellates in the larvae of scleractinian corals
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The establishment of symbiosis in early developmental stages is important for reef-building corals because of the need for photosynthetically derived nutrition. Corals spawn eggs and sperm, or brood planula larvae and shed them into the water. Some coral eggs or planulae directly inherit symbiotic dinoflagellates (Symbiodinium spp.) from their parents, while others acquire them at each generation. In most species examined to date, the larvae without dinoflagellates (aposymbiotic larvae) can acquire symbionts during the larval stage, but little is known regarding the timing and detailed process of the onset of symbiosis. We examined larval uptake of symbiotic dinoflagellates in nine species of scleractinian corals, the onset of symbiosis through the early larval stages, and the distribution pattern of symbionts within the larval host, while living and with histology, of two acroporid corals under laboratory conditions. The larvae acquired symbiotic dinoflagellates during the planktonic phase in all corals examined which included Acropora digitifera, A. florida, A. intermedia, A. tenuis, Isopora palifera, Favia pallida, F. lizardensis, Pseudosiderastrea tayamai, and Ctenactis echinata. The larvae of A. digitifera and A. tenuis first acquired symbionts 6 and 5 days after fertilization, respectively. In A. digitifera larvae, this coincided with the formation of an oral pore and coelenteron. The number of symbiotic dinoflagellates increased over the experimental periods in both species. To test the hypothesis that nutrients promotes symbiotic uptake, the number of incorporated dinoflagellates was compared in the presence and absence of homogenized Artemia sp. A likelihood ratio test assuming a log-linear model indicated that Artemia sp. had a significantly positive effect on symbiont acquisition. These results suggest that the acquisition of symbiotic dinoflagellates during larval stages is in common with many coral species, and that the development of both a mouth and coelenteron play important roles in symbiont acquisition.
KeywordsCoral Species Scleractinian Coral Symbiotic Alga Coral Larva Symbiotic Dinoflagellate
We thank the staff of the Sesoko Station of the Tropical Biosphere Research Center, University of the Ryukyus, for providing research facilities. We also thank Kenji Iwao, Masaya Morita, Akira Iguchi, Yossi Loya, Takeshi Hayashibara, Naoko Isomura, Hironobu Fukami, Nami Okubo, Makoto Kitamura, Kenji Iwai and staff of Okinawa Churaumi Aquarium for providing samples. We are also grateful to Mamiko Hirose for valuable suggestions regarding histological observations and the reviewers who helped to improve this manuscript. This research was funded by the 21st Century Center of Excellence (COE) program of the University of the Ryukyus, a Ministry of Education, Culture, Sports, Science and Technology Grant-in-Aid for Young Scientists (B) No. 20770017 (SH), and the Australian Research Council Centre of Excellence for Coral Reef Studies at the University of Queensland.
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