Vertical zonation of endosymbiotic zooxanthellae within a population of the intertidal sea anemone, Anthopleura uchidai
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Intertidal organisms commonly form zonation bands along the shore. Environmental stressors often determine the vertical position of each zonation band. These stressors may similarly affect the distribution pattern of endogenous species in their intertidal hosts. To evaluate this possibility, we investigated the distribution pattern of endosymbiotic zooxanthellae in the genus Symbiodinium in a population of the intertidal sea anemone Anthopleura uchidai. We used molecular genetics to identify the Symbiodinium clades and found that A. uchidai has two clades of Symbiodinium, clades A and F. These Symbiodinium clades were disproportionally distributed along the vertical gradient of the intertidal shore. Anemones on the upper shore exclusively possessed clade F Symbiodinium while clade A Symbiodinium became dominant in the sea anemones on the lower shore. Photosynthesis activity assays showed that these Symbiodinium clades had similar net productivities at 23.3 and 31.8 °C at all irradiance levels. At 35 °C, however, clade A Symbiodinium exhibited substantially lower net productivities than clade F Symbiodinium, demonstrating that these Symbiodinium clades have distinct tolerances to thermal stress. These results suggest that the thermal gradient across tidal height is a major factor shaping the zonation pattern of Symbiodinium clades in A. uchidai.
KeywordsPhotosynthetically Active Radiation Zonation Pattern Depth Gradient Infected Snail RFLP Pattern
We thank K. Fukami for his valuable comments on this study. We also thank Y. Kumekawa, K. Matsuyama, K. Ohga, N. Yokoyama for their field assistance, and C. Keogh for English editing. Two anonymous reviewers provided useful comments. This study was performed through the Program to Disseminate Tenure Tracking System of the Ministry of Education, Culture, Sports, Science and Technology, the Japanese Government.
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