Distribution of marine organisms and its geological significance in the modern reef complex of the Ryukyu Islands

Extensive investigations of biota in the reef complex around the Ryukyu Islands have revealed ecologic specificity of many benthic organisms and have shown that characteristic assemblages are found in each of the topographic zones and sub-areas. The moat is divisible into a nearshore seagrass bed and an offshore sand bottom. Both inner reef flat and outer reef flat are characterized by abundant occurrences of hermatypic corals and nonarticulated coralline algae. However, the former is dominated by branching and foliaceous forms of corals and various, large, fleshy, erect forms of algae, whereas the latter is dominated by encrusting and tabular forms of corals, lacking these algae. Corals and coralline algae are not present on the reef crest, which is covered by rubble and gravel, where algal turf and Sargassum are spreading. Encrusting and tabular forms of corals flourish on the shallower part of the reef slope, with high coverage, while, with increasing depth, the coverage decreases and the dominating coral forms change, with hemispherical and encrusting forms on the middle part of reef slope, and foliaceous and encrusting forms on the deeper part of reef slope. Nonarticulated coralline algae are distributed throughout the reef slope. The composition of coral and coralline algal assemblages changes dramatically with increasing depth. Foraminiferal-algal nodules, rhodoliths, are the most abundant constituent on the island shelf, commonly with Cycloclypeus carpenteri. There are likely to be two types of shelves in tropical to subtropical regions: nutrient-rich Halimeda-dominant and nutrient-poor rhodolith-dominant. Sediments abundant in bryozoan skeletons occur occasionally on the shelf.