A field experiment on the microhabitat preference of algal-symbiont-bearing larger foraminifera in a reef-crest algal turf zone, Ishigaki Island, Okinawa, Japan

Abstract

Larger benthic foraminifera with algal endosymbionts are prolific carbon producers in coral reef environments. Algal turf zones on reef crests host extremely high densities of larger foraminifera, which are abundant not only in algal turfs but also on reef rubble, both of which provide complex three-dimensional habitats. However, the microhabitat preference of larger foraminifera on these substrata has not been studied experimentally. To examine the effect of vertical position and surface configuration on foraminiferan distribution, I conducted a field colonization experiment in a reef-crest algal turf zone off Ishigaki Island, Okinawa, Japan, using three surface types (smooth surfaces, surfaces with small holes, and surfaces covered with artificial turf) of artificial substrata (bricks), on which the upper section was separable from the lower. After 10 weeks, colonization results indicated that vertical position, rather than surface type, influenced the small-scale distribution of Peneroplis, which was more abundant on the upper than on the lower sections of the experimental substrata. In contrast, surface type rather than vertical position influenced the distribution of Amphistegina lobifera and some reef-flat species of the Calcarinidae (Baculogypsina, Calcarina, and Neorotalia), which were more abundant on artificial turf substrata. The abundance of A. lobifera and reef-flat calcarinid species on artificial turf substrata was comparable to that in natural habitats (algal turfs and reef rubble). The preference of Peneroplis, which are omnivorous and free-living, for upper surfaces is likely due to positive phototaxic behavior and the availability of food and nutrients associated with sediment accumulation. On the other hand, the abundance of suspension-feeding, crawling Amphistegina and some calcarinid species in the dense, entangled three-dimensional substratum probably stems from the presence of numerous attachment sites and refuges from strong water motion and high light intensity.