Variations in Pleistocene coral assemblages in space and time in southern and northern Central Ryukyu Islands, Japan

We studied the taxonomic and morphological compositions of Pleistocene coral assemblages in 24 quadrats in Okinawa-jima, southern Central Ryukyus, and in 14 quadrats in Kikai-jima, northern Central Ryukyus. A total of 72 species belonging to 35 genera were identified in Okinawa-jima, whereas 65 species coming under 33 genera were recorded in Kikai-jima. Cluster analysis of the Bray-Curtis (BC) similarity coefficient produced 8 coral associations and 3 ungrouped taxa; they correspond to a wide range of reef environments, from shallow lagoon to lower reef-slope settings. The analysis of taxonomic similarities among quadrats leads to the identification of 7 clusters and 5 isolated quadrats. The segregation of quadrats indicating a same reef zone into distinctive groups or single branches in the cluster diagram reflects the within-reef-slope variability in community structure. Specifically, encrusting Acropora palifera, indicative of the upper reef-slope zone, shows a great variability of associated coral species and morphologies. We also report differences in the geographic distribution of Acropora palifera and Platygyra contorta between present-day and Pleistocene reefs, which may be linked to changes in sea surface temperature. The decrease in the total species and generic richness recorded in modern reefs is not clearly detected in the fossil record. In addition, MDS ordination of the BC similarities highlights patterns of reef zonation and geographic differences in taxonomic composition. One-way tests of similarity (ANOSIM) show that differences in the composition of upper reef-slope coral assemblages between Okinawa-jima and Kikai-jima are not larger than those observed between southern and central Okinawa-jima. Finally, we report various types of coral successions. Sea-level changes resulted in transitions from one reef zone to another, whereas local disturbances are likely to have been responsible for the successions of distinctive coral assemblages with overlapping depth ranges, which reflect the within-reef-zone variability.