Rhodoliths from deep fore-reef to shelf areas around Okinawa-jima, Ryukyu Islands, Japan

The distribution, abundance, composition, and growth history of rhodoliths were investigated based on 222 grab samples and 202 submarine photographs taken from 223 sites arranged at regular intervals and on 13 additional samples (5 dredge and 8 grab samples). These samples were collected at water depths ranging from 15 to 970 m around Okinawa-jima, Ryukyu Islands, Japan. The rhodoliths grow in deep fore-reef to shelf areas at water depths of 50 to 135 m. Where rhodoliths occur, they cover 45% of the sea bottom. The rhodoliths are primarily spheroidal to ellipsoidal in shape (with mean diameters usually less than 8 cm); internally they are primarily composed of nongeniculate coralline algae and an encrusting foraminifer Acervulina inhaerens. The rhodoliths have envelopes of well-preserved, concentric to irregular laminations or, much more commonly, are bored and display various degrees of bioerosion. Constructional voids (primary spaces between encrusters) and borings range from empty to completely filled with unlithified and lithified mixtures of micrites and bioclasts. The bioerosion is more extensive with increasing water depth and is progressively much more pervasive at water depths greater than 90 m. The rhodoliths are covered with nongeniculate coralline algae and A. inhaerens associated with other epilithic skeletal and nonskeletal organisms. The living biotic cover on rhodoliths is relatively great down to water depths ~ 100 m; below this, the cover decreases rapidly with increasing water depth. Rhodoliths with similar size, shape, and composing organisms to those in the Ryukyu Islands are commonly found on deep fore-reef to shelf areas or on the banks and seamounts of tropical reef regions, likely as the combined result of ecological degradation (=decreased number and coverage) of hermatypic corals and the relative predominance of nongeniculate coralline algae and encrusting foraminifers in such areas. The slow accretion rates of rhodoliths (< 0.1 mm/year) indicate that their formation is commonly to frequently intermittent, probably because of their burial in the surrounding sediment.

Research Highlights
► Rhodoliths cover ~30% of the sea bottom of deep fore-reef to shelf areas.
► The living biotic cover on rhodoliths is great (>40%) down to water depths ~100m.
► The bioerosion is more extensive with increasing water depth.
► Rhodolith formation is frequently intermittent because of their burial.
► Similar rhodoliths extends fore-reef to shelf areas of tropical reef regions.