In recent years, the diversity of symbiotic dinoflagellates of the genus Symbiodinium has become intensively investigated. Found in a variety of marine invertebrate hosts, Symbiodinium are a key element in the observed high diversity within many sub-tropical and tropical shallow marine ecosystems. One group of marine invertebrates for which Symbiodinium diversity has been examined is zooxanthellate zoanthids (Cnidaria: Anthozoa: Zoantharia) of three genera; Zoanthus, Isaurus and Palythoa. Focusing on specimens in southern Japan, zoanthid species from Zoanthus and Palythoa show varying patterns of both levels of Symbiodinium variation within individual colonies, and levels of Symbiodinium variation (=“flexibility”) over geographic ranges (i.e., latitudinal/longitudinal variation, etc.). Here I examine and compare the results of previous studies based primarily on sequences of internal transcribed spacer of ribosomal DNA (ITS-rDNA), and focus on two zoanthid species with apparently very different Symbiodinium symbiotic system characters; Zoanthus sansibaricus and Palythoa tuberculosa. Symbiodinium (subclade C1z) within Z. sansibaricus are characterized by extremely high levels of ITS-rDNA sequence microvariation, while Symbiodinium within P. tuberculosa (subclade C1/C3) do not have such microvariation. I suggest that three factors may potentially contribute to apparently higher flexibility and diversity of Symbiodinium in Z. sansibaricus; 1) multiple copies of ITS-rDNA sequences, 2) strong reliance on sexual reproduction in Z. sansibaricus vs. asexual reproduction in P. tuberculosa, and 3) niche specialization by Z. sansibaricus vs. “generalist” P. tuberculosa,. Through an examination of past literature and results, it appears that both multiple intragenomic copies of ITS-rDNA and multiple types of Symbiodinium within Z. sansibaricus due to niche specialization are the most likely reasons behind such unusually high microvariation. It is hoped that in the near future more intensive investigations of Symbiodinium in a variety of host species will help researchers more fully understand the mechanisms driving varying symbiont flexibility and diversity.

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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.

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The plasticity of allocation between life history traits and of sex allocation in the sessile colonial coral Goniastrea aspera was examined by field survey and transplant experiment at two zones (lower intertidal and reef edge) of a fringing reef at Sesoko Island, Okinawa. A field survey showed that colony density, colony size, and polyp size were greater at the lower intertidal zone than at the reef edge zone. Mortality of transplanted fragments was higher at the reef edge zone than at the lower intertidal zone. These results suggest that the environment at the lower intertidal zone was more favorable for G. aspera. There was, however, no significant difference in the growth rate and reproductive effort of the transplanted fragments between the two zones. In contrast, sex allocation to testis was higher at the lower intertidal zone than at the reef edge zone in the transplanted fragments. Both spawning of gametes and brooding of planulae are known in G. aspera, and natural colonies at the lower intertidal zone brooded more planulae than those at the reef edge zone. Therefore, it is suggested that G. aspera produced more sperm at the habitat where number of brooded planulae was larger. It is considered that, in order to increase the possibility of cross-fertilization, G. aspera colonies exhibited the plasticity to allocate more resources to sperm where number of brooded planulae was larger.

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Cadmium (Cd) of coastal surficial sediments from four islands (25 sites) in the Yaeyama region, Okinawa, was examined for an initial assessment of Cd pollution. Sediment samples sieved through a 63-μm mesh were leached by 0.5 N hydrochloric acid, and extracted Cd was determined using a flameless atomic absorption spectrophotometer. The concentrations of Cd analyzed from the study sites were in the range of 0.017 to 0.316 μg/g. Generally, Cd contamination in the study region has not been particularly remarkable; however, high concentrations were observed near the mouths of rivers that run through agricultural fields and in harbors, suggesting Cd pollution sources, such as phosphate fertilizer and sewage sludge, attributed to anthropogenic activity.

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