Sea surface temperatures were warmer throughout 1998 at Sesoko Island, Japan, than in the 10 preceding years. Temperatures peaked at 2.8 °C above average, resulting in extensive coral bleaching and subsequent coral mortality. Using random quadrat surveys, we quantitatively documented the coral community structure one year before and one year after the bleaching event. The 1998 bleaching event reduced coral species richness by 61% and reduced coral cover by 85%. Colony morphology affected bleaching vulnerability and subsequent coral mortality. Finely branched corals were most susceptible, while massive and encrusting colonies survived. Most heavily impacted were the branched Acropora and pocilloporid corals, some of which showed local extinction. We suggest two hypotheses whose synergistic effect may partially explain observed mortality patterns (i.e. preferential survival of thick‐tissued species, and shape‐dependent differences in colony mass‐transfer efficiency). A community‐structural shift occurred on Okinawan reefs, resulting in an increase in the relative abundance of massive and encrusting coral species.

ABSTRACT. This study investigated the process of zooxanthellae degradation in hermatypic corals.The number of degraded zooxanthellae in corals taken fmm rllfferent light conditions amounted to 1 to 6 % a day, which was similar to the number of dividing amxanthellae. Zooxanthellae degradation takes place only at night in the connecting sheet and tentacle but both at night and during the day in the gastroderm of the mesenteries. Zooxanthellae degradation continues for about 6 h. DNA staining with DAPI (4'6-diamidino-2-phenylindole) and light, UV and electron microscopic examinations showed that zooxanthellae under degradation lost DNA, protein of pyrenoids and lipid drops. The degraded zooxanthellae particles contained 'accumulat~on bodies', unpacked thylakoids, starch grains and a pyrenoid starch envelope. Under starvation experirnenls the number of degraded zooxanthellae in Stylophora pistillata increased in the tissue, as did thefi ~e l e a s e .It is concluded that hermatypic corals are capable of regulating their zooxanthellae p o p u l a t i a by digestion and extrusion of zooxanthellae remnants.

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