We examined whether increased concentration of ammonium or phosphate prevents bleaching in the coral Galaxea fascicularis exposed to a high temperature to test the hypothesis that bleaching occurs when cnidarian hosts fail to supply their symbiotic algae with sufficient inorganic nutrients. Isolated polyps of G. fascicularis were incubated at 32°C for 9 days either in filtered sea water (FSW), 100μM NH₄Cl in FSW, or 10μM Na₂PO₄ in FSW. The density of zooxanthellae and areal concentration of chlorophyll decreased to about a half regardless of when inorganic nutrients were present and absent in the treatments. The present results suggest that the loss of zooxanthellae at a high temperature is not due to a reduction in the supply of inorganic nutrients to zooxanthellae by their host.

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Seasonal changes in species composition and size frequency distributions of six sympatric coexisting species of Trapezia associated with the hermatypic coral Pociliopora damicornis were investigated to analyze their population structure, mechanisms promoting coexistence, and the organizing processes of the symbiont community. All Trapezia species showed a long recruitment season, but they could be divided into two groups according to variations in their size frequency distribution pattern throughout the year: Young T. ferruginea and T. guttata occurred all year round, while young T. cymodoce, T. areolata, T. digitalis, and T. sp. were absent part of the time (a few months). Recruitment of T. cymodoce and T. areolata occurred from April to August and from February to March, respectively. Young T. digitalis were abundant only in July. Even among larger colonies, those harboring only one or two Trapezia species were rather common. This suggests that agonistic interactions occur among these crab species. Although it is also true that host colonies harboring several species are sometimes found. Since no conspicuous hierarchy in inter-specific agonistic interactions has been demonstrated (Tsuchiya and Yonaha, 1992), new crab recruits of any species can invade colonies lacking strong competitors.
Although these crab species require very similar resources, i. e. pocilloporid corals as their habitat and mucus produced by host corals as food, their coexistence on larger single colonies is promoted by several mechanisms: 1) different recruitment seasons, 2) weak hierarchy of agonistic interactions among the six species, and 3) microhabitat segregation between adults and young on a colony.

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Time course of enhancement of calcification of the coral Galaxea fascicularis in light and decay of light-enhanced calcification in darkness were studied. The hourly rates of calcification and of photosynthesis/respiration of isolated polyps of G. fascicularis were measured successively in light or in darkness using the alkalinity anomaly technique. Corals were kept in darkness for 15-19h before experiments. Calcification rate during 50min incubations in light was about four times higher than that during previous incubation in darkness. Light-enhanced calcification did not decrease abruptly when the coral was returned to darkness but decayed gradually to about 46%, 3h after illumination was terminated. The rate of light-enhanced calcification increased gradually when corals were incubated in light. The photosynthetic uptake of CO₂ by corals held in light was 3.7 times faster than CO₂ uptake due to calcification. Dark respiration increased after 50min incubation in light. The slow decay of light-enhanced calcification in darkness suggests that there is an internal pool of substances which enhance calcification, and that accumulation of the substances might sustain high rates of calcification in darkness.

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Coral bleaching occurred at various places around the Ryukyu Islands in summer 1998. Several researchers have suggested links to high sea surface temperature. However, the observation of coral bleaching around Ishigaki-jima of the Yaeyama Island Group where the bleaching has been extensive since July shows that there was already a sign of the event during the Baiu rainy season.
The study of corals on Ishigaki-jima presents the following:
1) Coral bleaching was probably caused by the continued high sea surface temperature from July through September 1998.
2) The corals had already been severely damaged by the sedimentation of red soil from the disturbed land surface during the Baiu season of the year which had nearly twice as much rainfall as normal year. The subsequent high sea surface temperature in summer contributed to accelerated bleaching.
The observation of bleaching in moats around Ishigaki-jima in summer 1998 suggests that the progress of bleaching be divided into the following three time stages:
1) Early stage: from around June 20 through early August
2) Mature stage: from middle August through early September
3) Final stage: from middle September through middle October
The bleaching process divided by these stages is synchronous, with a time lag, with the seasonal change in sea surface temperature and the prevailing weather conditions in Okinawa.

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The bleaching of hermatypic corals that occurred in the summer of 1998 throughout the Nansei Islands, southwestern Japan, was the most extensive one in recent years. Field investigations using a large quadrat (10×10m) method distinguished this bleaching from prior episodes as (1) the largest in scale, (2) the most severe, and (3) causing the highest mortality. At two shallow reef flat sites, a total of 85.9 and 92.2% in coverage of hermatypic corals were bleached and mortality reached 60.7 and 78.7% in the most thermally susceptible acroporids. Bleaching damage varied among species and habitats. Dominant coral species were classified into four categories of bleaching; I. both rates of bleaching and mortality were high (e. g. Acropora hyacinthus, A. pulchra), II. rate of bleaching was high, but mortality was low (e. g. A. digitifera, Montipora digitata), III. both rates of bleaching and mortality were low (e. g. Platygyra ryuhyuensis, Porites cylindrica), IV. mortality (due to bleaching) was higher than rate of bleaching (Leptoseris gardineri). Acroporids in Category I were thermally more susceptible than those in Category II as the surface area per volume of skeleton was larger. Species categorized into Category II and III were considered to be best adapted to severe thermal environments such as the shallow reef flat and moat. The mass bleaching during the summer to autumn of 1998 was considered to be caused mainly by extraordinary high thermal stress. Meteorological data showed that average sea water temperatures have risen by 0.7°C in summer and 2.0°C in winter over the past 85 years, resulting in more frequent and serious disturbance by bleaching on coral reefs in recent years.

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Short-term changes in a fish and coral community on an outer reef at Ishigaki Island, Ryukyus Islands of Japan, were quantified and related to severe coral bleaching that occurred in the summer of 1998. Prior to July 1998, the benthic substrata along the transect consisted mostly of living Acropora spp. By late September 1998, however, most of the living coral had died and the dead coral skeletons were coated with filamentous algae by late October 1998. In the fish community, both the number of individuals and species per transect did not change one month after the coral bleaching, however, species diversity decreased. Although the number of individual omnivores and carnivores did not differ before and after the coral bleaching, the number of coral-polyp feeders decreased. In contrast, the number of herbivores, particularly two species of acanthurids, increased one month after the coral bleaching. Differences in abundances of hervivores and coral-polyp feeders after the coral bleaching appear to be related to changes in the benthic habitat. These results suggest that the two acanthurid fish species responded to the increase in algal biomass by migrating from adjacent habitats to the outer reef.

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Extensive bleaching of corals occurred around Akajima, Okinawa, in the summer of 1998. Bleaching was observed in a few corals in late July 1998, but by the end of August many pocilloporiid and acroporiid corals were bleached. The bleaching frequency, relative to total coverage of corals, at 15 sites (1.3-7.2m in depth) around Akajima ranged from 55.9 to 97.4% in late September 1998. We measured 2m x 2m quadrats at each site. Three out of 4 sites that showed over 90% bleaching were located in either a moat or inside of a bay, where water exchange was slow. One site was characterized by a dominant occurrence of branched corals of the genus Acropora. This group was more easily bleached than other morphologic groups around Akajima. Sea surface temperature over 30°C (normally about 28°C) was recorded for a month in August. This high temperature may have been a primary factor of the coral bleaching. Coral diseases caused by viruses or bacterial infection after bleaching were not observed. Geographical features, rate of water exchange, and composition of coral species seem to be responsible for the differences in extent of bleaching.

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In the summer of 1998, mass coral bleaching occurred in many reefs around the Ryukyu Islands. Various sized colonies of completely bleached Pocillopora damicornis were collected in order to study the effect of the coral bleaching on the structure of its associated community. Although six species of Trapezia were collected in this survey, and it was comparable to its species richness on healthy Pocillopora, bleached colonies harbored fewer individuals and species of obligate symbionts than unbleached colonies, before the bleaching event. Even on the larger colonies, of which the volume of interbranch space was > 222cm³, no obligate symbionts were found. Heterosexual pairs of the symbionts such as Trapezia spp., Alpheus lottini and Paragobiodon spp. were rather uncommon and the abundance of ovigerous females of Trapezia was lower and their clutch size was smaller than usual. It was quite unusual that large numbers of the symbiont crab Cymo melanodactylus were found on completely bleached colonies of P. verrucosa and Acropora aspera.

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Changes in living coral coverage along five transects on Shiraho reef and one transect on Kabira reef, Ishigaki Island, Ryukyu Islands were investigated before and after the 1998 bleaching event. The coverage of living corals after the bleaching decreased by 50% of their original values. Branching types of Montipora, Acropora and Porites were most severely bleached and their mortality was also high. Bleaching was intermediate to high for massive Porites whose recovery was better than branching corals. Heliopora coerulea was less susceptible to bleaching and mortality was low. Higher than 30°C sea surface temperature prevailed over the south sea of Japan, which induced higher than 30°C reef water temperature on the coral reef flat and resulted in coral bleaching.

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The author has witnessed two coral bleaching events taken place at Sesoko and Okinawa islands in 1980 and 1998. Although the final reports of the 1998 event are not accessible, the author tries to make some comparisons among the events according to his own report on the 1980 event (Yamazato 1981) and his personal experiences of the 1998 event. Coral bleaching in 1980 around Sesoko Island, Okinawa, Japan appears to have been a local event, while that of 1998 was much more extensive affecting all islands of the Ryukyu Islands and Southern Kyushu. Abnormally high surface temperatures were observed on the reef flat during daytime low tide in the summer of 1980, while it was observed continuously throughout the summer of 1998, affecting not only coral reef waters but also coastal waters both day and night. In 1980, over 40% of coral colonies bleached once and about 10% died, the rest of the bleached corals recovered by the following January. In the 1998 event, according to visual observations of the author made on the same reef section, almost 90% of corals and soft corals bleached and died, with only a few colonies of a few species that recovered. In addition to these personal observations, some results of new surveys are presented and comparisons between the two events are made.

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Coral bleaching was described in Kikai-jima (Kikai Island) in October, 1998. Mortality of coral species belonging to the families Acroporidae and Pocilloporidae was the highest of all scleractinian corals in this island. In contrast, some species of the genera Psammocora and Pavona were never or seldom damaged. These results were similar to those observed in the other Indo-Pacific regions. Significant differences in mortality among families could be attributed to the their geological ranges. The species that were free from appreciable damages by bleaching belong to the families which first appeared in the geological periods when the climate was globally warm (greenhouse periods).

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This study investigated zooxanthellae densities, chlorophyll a and c₂ concentrations, and zooxanthellae morphologies in replicate samples of Stylophora pistillata colonies before and during the 1998 coral-bleaching event in Okinawa, Japan. During the bleaching event sea surface temperatures reached 31 °C and Stylophora pistillata colonies supported significantly reduced mean chlorophyll a and c₂ concentrations per cm², and significantly more pale zooxanthellae cells in hospite than before the bleaching. Although the number of healthy zooxanthellae decreased during the coral-bleaching event, the total density of zooxanthellae in S. pistillata colonies (i. e., including the pale zooxanthellae) did not change significantly. Thus, the presence of pale zooxanthellae within colonies numerically compensated for the loss of healthy zooxanthellae. We suggest that pigment degradation in zooxanthellae and subsequent cell shrinkage, rather than zooxanthellae expulsion, was the mechanism involved in S. pistillata bleaching.

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[in Japanese]

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