The genotypes of zooxanthellae associated with eight scleractinian corals and one hydrocoral from a fringing reef of Sesoko Island (Okinawa, Japan) were determined using the single stranded conformational polymorphism (SSCP) of large subunit ribosomal DNA (28S rDNA). Preliminary RFLP analysis of small subunit (18S) rDNA showed that four pocilloporid corals harbored zooxanthellae of clade C, except one branch of Stylophora hystrix kept at 8% light which contained zooxanthellae of clade A and C. The hydrocoral Millepora intricata harbored zooxanthellae of clade A and C. RFLP analysis failed to distinguish three morphophysiological types described by Titlyanov et al. (2001a, b). SSCP analyses of 28S rDNA showed that zooxanthellae from the nine coral species gave at least fourteen distinct SSCP patterns. While zooxanthellae from S. hystrix and S. caliendrum had identical SSCP patterns, SSCP patterns of zooxanthellae from other coral species studied were distinct from each other. In six out of nine coral species, branches from the same colonies kept under different light conditions contained zooxanthellae that showed different SSCP patterns. However, no clear relationship was found between zooxanthellae genotypes and light intensity. The present results suggest that three morphophysiological types are not unique but that each type consists of zooxanthellae of several genotypes.

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Species composition of soft corals from the Yaeyama and Miyako island groups, of Sakishima Is., located at the southern part of the Ryukyu Archipelago were studied during 1992 and 1993. SCUBA collections were carried out at 18 sites around the islands down to 30m. The collection, comprising approximately 200 specimens, yielded 45 species. The survey established two new species: Sinularia higai and S. tanakai, and in addition revealed 14 new zoogeographical records. Among the latter are Protodendron and Rhytisma species, which are the first records of these genera in the Ryukyu Archipelago. The obtained results along with a previous survey on Sesoko Is. show the genus Sinularia have the highest species diversity on the reefs, comprising 33-45% of total species number. Synthesizing the existing data for the south Ryukyu Archipelago yielded a total of 56 soft coral species (11 genera) of the families Clavulariidae, Tubiporidae, Alcyoniidae and Briareidae.
Since the survey was conducted prior to the recent mass coral bleaching event (1998) in the region, it is of prime importance to monitor changes in soft coral species diversity and abundance there. Such data will indicate possible shifts in benthic community structure on the reefs and the rate of recovery.

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The purpose of this study was to examine the physiological responses of the coral host and their algal symbionts to elevated temperatures. Isolated polyps of the coral Galaxea fascicularis were exposed to three temperatures (24, 28 and 30°C) for 7 days and were then allowed to recover for 15 days at 24°C. Both zooxanthellae density and the maximum photosynthetic quantum yield of in hospite zooxanthellae, measured by pulse-amplitude-modulation (PAM) fluorometry, decreased after 7-day exposure to 28 and 30°C. In corals exposed to 28°C, both parameters returned to the initial level after 15-day recovery, but those exposed to 30°C died during the recovery period. A high release of healthy-looking zooxanthellae was evident in the coral exposed to 30°C, while those exposed to 24 or 28°C mainly released degraded zooxanthellae particles. The mitotic index of both retained and expelled zooxanthellae was highest at 30°C. The coral host appeared to lose the capacity of controlling algal cell division and of preferential expulsion of degraded algal cells after 7-day exposure to 30°C. The growth rate of corals was lowest at 30°C. The reduced growth rate, the low release of degraded zooxanthellae particles and high release of healthy-looking zooxanthellae, and increased mitotic index of zooxanthellae, in corals exposed to 30°C for 7 days, suggest that high temperature caused a physiological dysfunctioning at the level of host and that this change may be at least partly responsible for bleaching and the disruption of symbiosis.

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The most severe and large scale-bleaching event on record occurred in 1998 world-wide. In Mauritius, three surveys covering 2 habitats at 4-5 sites were conducted to describe and quantify the event. Bleaching was recorded at all sites and habitats. However, relatively few corals were bleached (<10%) except at one localised site (Le Bouchon: 38.6%) indicating that Mauritius was one of the few islands in the Indian Ocean to have sustained a comparatively mild bleaching episode. On the reef flat Acropora species were generally more susceptible to bleaching than non-Acropora species. In contrast on the reef slopes, non-Acropora species were more affected than the Acropora species. Furthermore, susceptibility to bleaching differed within colonies and species. The main cause of this bleaching event was most likely due to an increase in seawater temperature and solar radiation, exacerbated by lowered salinity from higher seasonal rainfall.

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Destruction and recovery of hermatypic coral communities during and after the 1998 bleaching event were compared quantitatively among three representative field sites (inner reef flat, outer reef flat, and reef slope) at Urasoko Bay, Ishigaki Island, southwestern Japan. As a result of bleaching, corals drastically decreased in number and coverage at all study sites and final coral coverage, at the heavily destroyed outer reef flat, was only 8.2% of that prior to bleaching. Differential susceptibilities and survival were noted among coral species and among study sites. Bleaching damage most severely affected the outer reef flat, followed by the reef slope, whereas it less severely affected the inner reef flat, where sea water temperatures were highest. The degree of bleaching response among sites was not simply thermal-dependent but species-related. Several species of Acropora (A. digitifera, A. nobilis, A. formosa, A.pulchra, A. microphthalma, A. hyacinthus, A. echinata, A. subglabra, and so on) suffered the most damage from bleaching, and consequently the rich coral communities disappeared from the outer reef flat and the reef slope. At the inner reef flat, some species of Montipora (M. digitata, M. cactus, M. stellata, M. aequituberculata) recovered considerably after bleaching, despite conspicuous bleaching during the early bleaching phase. The present results indicate the possibility that some species inhabiting under thermally severe conditions have potentially acclimatized enough to endure such stress, and consequently they could live for a long period of more than six months in a bleached condition. Although the species richness (number of species) decreased at all sites, the species diversity indices often increased as a few dominant species of Acropora were reduced, and as a consequence, species evenness increased.

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