Coral colony density and colony size distribution have been used as an indicator of the coral population structure to study coral population dynamics accompanied by recruitment, growth and mortality. In this study, we conducted traditional Photo-quadrat method (PHOTS) in the daylight and UV-excited fluorescence observation method (Fluorescence-PHOTS) in the night and investigated the Alveopora japonica colony density and colony size distribution at Tateyama, Chiba, Japan. We conducted two preliminary experiments about 1) UV-excited fluorescence properties and 2) diurnal polyp behavior of A. japonica. 1) The tip of tentacles and oral disc of A. japonica polyps emitted blue-green fluorescence and the spectra showed 492 nm of the peak wavelength by UV excitation. 2) The relative size of the contracted colonies in the night measured by Fluorescence-PHOTS was about 0.9% of the maximum expanded colonies in the daylight measured by PHOTS, because of the retraction of the fluorescent polyp areas to their skeletons. Following these results, we conducted PHOTS in the daylight and Fluorescence-PHOTS shortly after sunset. Due to high contrast between A. japonica and others, it is easier to detect A. japonica colonies by Fluorescence-PHOTS. The measurement of the number of colonies was more accurate in the night than in the daylight because small colonies beside large colonies were distinguishable without adjoining problem. The colony size of Fluorescence-PHOTS measured within an hour after sunset was estimated at about 34% of that of PHOTS. Difference of colony size distributions of both methods was significant. The distribution of Fluorescence-PHOTS was shifted to a smaller size class compared to that of PHOTS because of polyp contraction. This study revealed diurnal polyp behavior, colony density and difference of colony size distributions of A. japonica.

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