The topographic complexity of coral reef structure naturally provides temporal and spatial variations in environmental factors. Among these physical environmental factors, water-flow is an important environmental factor in coral reef habitat that had been overlooked as a stress ameliorator for reef-building corals. This review will summarize the recent studies as well as previous findings concerning the effects of water-flow on corals. The interaction among major stress factors and water-flow will be discussed with special attention to the aspects of coral bleaching phenomenon.

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In order to investigate long-term changes in a zooxanthellate cnidarian community in the temperate-subtropical area, we marked off a 41m² area at Taisho Lava Field, Sakurajima, Kagoshima, Japan. Colonies of three zooxanthellate cnidarian species (encrusting anemone Zoanthus sansibaricus Carlgren, soft coral Stereonephthya japonica Utinomi, and bubble anemone Entacmaea quadricolor sensu Fautin and Allen) in the survey area were monitored yearly in August from 1995 to 2004. Seawater temperature data were also obtained to investigate possible environmental influences on observed cnidarian changes. Sequences of internal transcribed spacer region of ribosomomal DNA (ITS-rDNA) were also obtained to identify the dinoflagellate Symbiodinium spp. endosymbionts found within each cnidarian species (subclade C1/C3 in Z. sansibaricus and E. quadricolor, clade G in S. japonica). Our results show that abnormally high ocean temperatures in 1998 (›2.0°C than average summer sea water temperatures) were followed by significant decreases in both cover and colony number in S. japonica, which showed no sign of recovery to the previous level. The difference in changes between the three zooxanthellate cnidarian populations may be primarily due to physiological differences at high ocean temperatures in Symbiodinium spp. hosted by the three cnidarian spp..

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Okinawa zoanthids have only recently begun to be investigated intensively. Recent research has confirmed the presence of several previously unknown species in the shallow coral reef waters surrounding Okinawa. Here, to aid zoanthid researchers in field identification, a dichotomous key of brachycnemic zoanthids (families Sphenopidae, Zoanthidae, Neozoanthidae) is provided, with characters almost exclusively based on ecology and morphology, although all taxa listed have also been examined using molecular phylogeny. With this key, five species of Palythoa, four species of Zoanthus, and one taxon each of Sphenopus, Isaurus, Neozoanthus, as well as an undescribed zoanthid genus are identifiable. Although not common, other as of yet undescribed brachycnemic zoanthids are likely to exist in Okinawa waters, and it is hoped this key will help spur further research into their biodiversity.

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Previous studies have demonstrated the importance of refuge structure from grazers on settlement substrata in survivorship of scleractinian coral spats. The present study reports some evidence showing that the importance of refuges varies depending on growth speed of coral spats. Survival of spats of the fast-growing scleractinian coral, Acropora solitaryensis that had settled on plain surfaces of settlement plates or in artificially-made micro-crevices (MC) serving as refuges on the plain plate surfaces was monitored in situ over the first year of life. Survival rate of A. solitaryensis spats dropped rapidly to less than 50% during the first two months but stabilized afterward and maintained high values both in MC (33%) and on the plain surfaces (17%) by the end of the one year experimental period. The higher spat survival rate observed in MC confirmed refuge effects on spat survivorship while many spats surviving on plain surfaces, which were not seen in slow-growing coral species in a previous comparable study, suggest that spats of fast-growing coral species that can attain a large enough size to survive damage from grazers early may depend less on refuge structure on settlement substrata. This may allow fast-growing coral species to extend potential areas for successful recruitment, more regardless of surface structure of settlement substrata, promoting its population development.

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The use of line-intercept transect (LIT) method combined with image analysis, i.e. photo line intercept transect (PLIT) method, where digital photos of corals along a transect line are taken and the length of corals intercepting the transect line are measured, was evaluated for coral reef monitoring. Coral coverage (%) and generic diversity were examined with the PLIT at eleven coral reefs and the results were compared with that from a photo-quadrat method (PHOTS) as a standard. Although PLIT significantly underestimated the coral coverage compared to PHOTS, the difference between the two methods was approximately 8% and was not large. PLIT can be used as a quick, easy and inexpensive method to estimate coral coverage for management level monitoring. However, PLIT considerably underestimated the number of coral genera and would not be appropriate if diversity is included in the monitoring measurements.

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