While giant sea anemones, which are called “host sea anemones”, harbor anemonefish providing them with valuable refuge, it was unclear what the fish provided in return. However, recent studies of this symbiotic relationship focusing on zooxanthellae residing inside the anemone have clarified the benefit from the fish to their host: their territorial defense against anemone feeders such as Chaetodon and their excrement play important roles in anemone growth and survival by facilitating the photosynthesis activities of the zooxanthellae. Accordingly, the survival rate of an early settler anemone is expected to be high or low in the presence of a large or small number of potential partners (anemonefish species), respectively. Anemonefish regularly use 6 out of 10 species of host anemones as reproductive sites, and the other 4 species are only used as temporary refuges or as nurseries by some individuals at the juvenile life stage. Accordingly, benefit from the former and the latter for anemonefish must be large and small, respectively. Based on large and small benefits from the symbionts, combinations of host anemones and anemonefish can be classified into four types (type A-D, Table 4). Out of 270 possible combinations between 10 host anemones and 27 anemonefishes (geographical distributions of 221 combinations are overlapped), only 82 have been found, of which only 67 could be classified into the four types; the other combinations were unclassifiable due to lack of information on spawning of the fish. We found large differences in the numbers of observed combinations among the four types (Table 4). A review of the literature revealed that combinations are largely influenced by interspecific competition between anemonefishes for host anemones, host specificity of early settler anemonefish, overlap of animal environment requirements and geographical distributions. Available published information indicates that major tropical sea currents largely influence the geographical distributions of host anemones. It should be noted that these analyses could not have been conducted without regard to the worldwide classification of host sea anemones (Dunn 1981).

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In order to identify the obstacles to the self-rehabilitation of the branching Acropora coral communities in the Sekisei Lagoon, one of the largest coral reefs in Japan, we investigated the basic parameters for establishing coral communities, such as larval recruitment, adult survival and growth, and nutrient concentrations in water column and sediment. We compared these parameters between 2 areas: “damaged” area, where branching corals have not been recovered for >20 years, and a “healthy” area, where corals continue to dwell at present or did until the recent past (3 years ago). Larval recruitment, which is important for natural rehabilitation of coral communities, was very low in all study sites (less than 3 settlers/176cm²), particularly in the sites of the “damaged” area (less than 0.5 settlers/176cm² in average). Although adult survival and growth did not differ significantly between the 2 areas, they did differ among sites and coral transplant methods (touching the sea floor or raised up by a stake). Nutrient (NO₂ and PO₄) and chlorophyll-a concentrations in the water column were significantly high in the “damaged” area. It remains to be determined whether these slightly higher concentrations in water influence the competition between corals and algae. The results suggest that low larval recruitment rate and slight eutrophication in the water column are one of the main causes for the delayed rehabilitation of branching Acropora corals in the Sekisei Lagoon. Further, the optimal method of transplantation must be investigated at each site, even within a narrow area, when restoration is conducted by transplanting cultured corals.

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