The thermal environment of reef-building corals is critical to their distribution and survival. Recent evidence has revealed that the mean global temperature has increased by 1 degree Celsius over the past century. Examination of tropical sea temperatures reveal that they have also increased by almost 1°C over the past 100 years and are currently increasing at the rate of approximately 1-2°C per century. Japanese waters are no exception. Waters off Okinawa have increased by 1.4°C since 1930 and are currently increasing at the rate of 2.3°C per century (95% confidence interval, 1.4-3.1°C). Coral bleaching occurs when the thermal tolerance of corals and their zooxanthellae are exceeded, probably due to an increased sensitivity of the zooxanthellae to chronic photoinhibition. The consequences of bleaching can be devastating, with corals dying in vast numbers and such aspects as coral reproduction being severely curtailed. In 1998, the world's coral reefs experienced the worst bleaching on record. Japanese reefs were severely effected, with bleaching coinciding with a massive temperature anomaly in July-August 1998. Vast numbers of corals died. The intention of this paper is collect what we know about coral bleaching from biochemical, physiological and ecological perspectives and to discuss to how reefs (especially around Okinawa) might change in the next century. A single important issue surfaces in these deliberations. If temperatures continue to increase, then corals will experience greater incidences of bleaching and mortality unless they can acclimate physiologically or adapt genetically. The conclusions that stem from this issue are of great concern. Firstly, available evidence suggests that corals are not acclimating physiologically to any really extent to the sporadic and seasonal changes in sea temperature. Secondly, the rate of change in sea temperature arguably exceeds the capacity of coral populations to genetically adapt fast enough. The third conclusion is the most worrying. If corals are not changing fast enough, then coral reefs will experience more frequent and more intense bleaching. Given the outcome for many coral reefs across the planet during the 1998 episodes of bleaching, this has to be of great concern to coral reef users and managers everywhere.

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Coral reefs have been degrading faster than they have been recovering (Done 1992; Ginsburg 1994; Hughes 1994; Grigg and Birkeland 1997; McManus and Vergara 1998; Wilkinson 1993, 1998) and they have been accruing in their degraded states because they achieve alternate stable states of algae instead of corals (Knowlton et al. 1981; Knowlton 1992; Done 1992; Hughes 1994), and because rates of damage are much greater than rates of recovery (Muzik 1985; Birkeland 1997a). With the accumulation of reefs in alternative stable states, the total area of some reefs previously dominated by coral is decreasing over time (Muzik 1985; Knowlton et al. 1981, 1990; Wilkinson 1993), and the balance has been continuously in the negative for corals over the past few decades (LaPointe 1989; Hallock et al. 1993; Birkeland 1997a). Furthermore, human activities are now bringing about environmental changes on a global scale (Smith and Buddemeier 1992; Hoegh-Guldberg 1999; Tsuchiya 1999; Wilkinson et al. 1999) which alter the basic conditions and nature of environmental processes in the domain.

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[in Japanese]

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The developmental changes in cnida composition of the coral Pocillopora damicornis was studied. At least two types of microbasic p-mastigophores (type I and type II MpM), large and small holotrichous isorhizas (HI), microbasic b-mastigophores (MbM) and spirocysts were observed in P. damicornis. The nematocyst composition of planulae was similar to that of one or two day old primary polyps, but markedly different from that of adult colonies. The large HI were observed only in planulae and primary polyps. MbM were most abundant in planulae and primary polyps while type I MpM were most abundant in adult colonies. The size of MbM and type II MpM was different between planulae or primary polyps and adult colonies. While large HI were retained at least for one week after settlement, spirocysts appeared to decrease temporarily after settlement. The present observations suggest that large HI are used for defense against predators at early stages of development and spirocysts are used for attachment to the substrate during settlement. The present study showed that the cnida composition and the dimensions of certain types of nematocysts change during the course of development of P. damicornis.

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Six species of compound ascidians found in a Fijian seagrass bed dominated by Syringodium isoetifolium were divided into two groups, each occupying somewhat different microhabitats provided by the seagrass. Didemnum molle and Lissoclinum bistratum, both with algal symbiont Prochloron sp., were abundant in high light microhabitats. D. molle was mostly attach to seagrass blades (maximum colony density: 980m^-2), while L. bistratum occurred both on seagrass and sediment surfaces in places with sparse seagrass cover (maximum colony density: 11, 500m^-2). Trididemnum clinides also had the symbiont Prochloron sp., but it mostly occupied dark microhabitats such as the sheaths of the seagrass. The other 3 species, Didemnum cuculiferum, D. sp. cf. albopunctatum and Trididemnum discrepans, lacked algal symbionts and were rare, all occupying dark places such as seagrass sheaths in areas with dense seagrass cover. Sympatric ascidians, thus, co-exist in seagrass beds and show a different microhabitat use. Ascidians were not distributed evenly over the area of the seagrass bed, but were concentrated in an area between 30 and 84m from the shore, independent of the distribution of seagrass biomass. In dense seagrass patches, light intensities varied greatly between the top and the basal part of the seagrass, and persistence and stability of seagrass as an attachment substrate were also different between leaf blades and sheaths. Populations of D. molle on the seagrasses included many smaller colonies. There were no colonies as large as those in the population on the more stable nearby rock substrates. The small size of the seagrass blades (1.5mm in diameter), their short lifetime (1.5mo) and their lower persistence and stability as an attachment substrate may explain the small size of the colonies on the seagrass.

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Changes of cnida composition during larval development are described for a scleractinian coral Acropora nasuta. Two types of cnidae, a microbasic b-mastigophore nematocyst and a spirocyst, were observed in planulae; whereas, four types were identified in polyps: microbasic p-mastigophore, holotrichous isorhiza, and the two types of cnidae found in planulae. The cnidae of planulae appeared 3-4 days after fertilization and gradually increased in number until the 8th day. The appearance of cnidae 3-4 days after fertilization and the maximum number of cnidae on the 8th day after fertilization coincided with the period of first settlement and maximum settlement, respectively. On the 15th day of in vitro culture, the number of nematocysts remained, but the spirocysts decreased until they were virtually absent in the larvae. The spirocysts might be used up during the repeated trial of settlement. We supposed that the nematocysts act as a defense against predators, whereas the spirocysts were used for initial attachment to the substrata. The number of cnidae will be a useful indicator to find larval maturity.

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The spatial distribution and morphologies of massive Porites colonies were investigated over a large area (1100m×200m) in the sandy moat of Miyara fringing reef, Ishigaki Island, Japan. Massive Porites (diameter>50cm) showed a preferential distribution for deeper depths (i. e., 2 to 3m). This distribution pattern suggested that massive Porites colonies were transported to deeper more stable habitats, as “mobile colonies”. Twenty-three percent of the colonies were mushroom shaped, with narrow stems attached to the substratum. Significant portions of mushroom shaped colonies (69%) were dislodged and 66% of them were tilted toward the dominant direction of water flow. The mushroom morphologies appear related to the high mobility of sandy sediments around the colonies, which interferes with ordinary growth of the colony base. Mobile colonies seems important for maintaining local coral populations in sandy habitats, which are often unsuitable habitats for larval settlement. In this case, physical environmental factors, such as water movement and micro-geomorphology, influence the distribution and population structure of coral communities in the sandy shallow habitats.

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