Deep-water macroid beds of the Ryukyu Islands, Japan: Encrusting acervulinids as ecosystem engineers

Encrusting acervulinid foraminifera can produce centimeter-sized, free-living coated grains named macroids. Macroid beds are an interesting feature of subtidal environments and have been recognized as important carbonate producers, habitat-forming structures, and paleoenvironmental indicators. Macroids provide a hard, three-dimensional substrate serving as microhabitat for a wide range of diverse organisms. As yet, the most extensive known occurrence of macroid beds in the western Pacific Ocean is found on the insular shelf of Kikai-jima, a coral reef-related island in the central Ryukyu Islands. The surveyed beds occur at water depth of 61-105 m arrayed on a ca. 6-km2 area. Despite their importance for the benthic communities of the Ryukyu Islands, only in the last decade have sampling efforts led to a more comprehensive understanding of the beds' distribution, their structure and associated communities, and the data concerning the influence of environmental factors on macroid beds and their ecological dynamics. In this study, the available information on the biodiversity associated with the recently described Ryukyu macroid beds are reviewed. This review is intended to inform and influence future research and policy planning on this largely unexplored, highly diverse marine ecosystem.