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The coral surface mucus layer provides a vital interface between the coral epithelium and the seawater environment and mucus acts in defence against a wide range of environmental stresses, in ciliary-mucus feeding and in sediment cleansing, amongst other roles. However, we know surprisingly little about the in situ physical and chemical properties of the layer, or its dynamics of formation. We review the nature of coral mucus and its derivation and outline the wide array of roles that are proposed for mucus secretion in corals. Finally, we review models of the surface mucus layer formation. We argue that at any one time, different types of mucus secretions may be produced at different sites within the coral colony and that mucus layers secreted by the coral may not be single homogeneous layers but consist of separate layers with different properties. This requires a much more dynamic view of mucus than has been considered before and has important implications, not least for bacterial colonisation. Understanding the formation and dynamics of the surface mucus layer under different environmental conditions is critical to understanding a wide range of associated ecological processes. KEY WORDS:Mucin · Mucopolysaccharide · Coral surface microlayer · CSM · Surface mucopolysaccharide layer · SML Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 296: [291][292][293][294][295][296][297][298][299][300][301][302][303][304][305][306][307][308][309] 2005 We feel that it is time that the study of mucus was revisited for at least 3 reasons. Firstly, mucus plays an important part in coral disease which has been responsible for significant coral mortality, particularly in the Caribbean (Porter et al. 2001) and more recently in the Indo-Pacific provinces (Sutherland et al. 2004). In this context mucus may function both as a protective physicochemical barrier (Peters 1997, Santavy & Peters 1997, Hayes & Goreau 1998, Sutherland et al. 2004) and as a growth medium for bacteria, including potential pathogens (Ducklow & Mitchell 1979b, Rublee et al. 1980, Toren et al. 1998, Banin et al. 2001, Lipp et al. 2002. But what do we actually know about the dynamics of the in situ mucus layer or its physico-chemical properties that might influence bacterial entrainment, growth or inhibition? Apparently we know very little. Even the most fundamental measure, the rate of mucus production, is extremely difficult to assess and is poorly defined in the literature. When, for example, authors report increased mucus production rates due to environmental stress, is this actually an increase in synthesis or merely an increase in the release of stored mucus?Secondly, coral algal symbionts play an important role in governing the composition of mucus, with 20 to 45% of daily net photosynthate being released as mucus and dissolved organic carbon (Davies 1984, Crossland 1987, Bythell 1988, Edmunds & Davies 1989. It follows then that during bleaching, when densities of algal symbionts are significant...

The shallow-water reef coral Goniastrea aspera Verill 1865 has previously been reported to demonstrate differences in within-colony susceptibility to bleaching at elevated sea temperatures; parts of the colony which are exposed to the highest solar radiation are more thermotolerant than areas which are less exposed. In this paper, we show that at elevated experimental sea temperature the 'high light' surfaces lose fewer symbiotic algae, have lower levels of oxidative stress, higher levels of host antioxidant-enzyme copper zinc superoxidase dismutase (CuZnSOD), and host heat-shock proteins 60 and 70, compared to the less exposed surfaces. In addition, 'high light' surfaces show less chronic photoinhibition and greater Photosystem II (PS II) recovery potential when exposed to high irradiance at ambient sea temperature. In contrast, no differences were noted in algal defences (e.g. antioxidant enzymes and stress protein production, and xanthophyll cycling) either at elevated or ambient temperatures. These results are noteworthy because they suggest that corals which acclimatise to high irradiance can, as a result, develop increased thermotolerance which may prevent bleaching at high sea temperatures. Importantly, they also demonstrate the significance of the host tissues in maintaining the intact symbiosis of G. aspera under thermal stress.

Marked seasonality is observed in sea-surface temperature (SST) and photosynthetically active radiation (PAR) received by shallow water corals at Phuket, Thailand (7" N latitude). This seasonality is reflected in both algal densities and algal chlorophylls (a and C?) in 4 massive species of scleractinian corals, namely Coeloseris rnayeri, Goniastrea retiformis, Porites lutea, and G. aspera. Algal numbers and algal chlorophylls are generally maximal at the end of the wet season (November) and minimal at the end of the dry season (March to May). Algal densities are reduced by up to 53 % in May compared to November in some species, whde algal chlorophylls may be up to 4-fold greater in November. Concomitant with these changes are differences in algal cell volume in 3 out of the 4 species studied and alterations in host tissue depth, with smaller algal volume and greater tissue depth at the end of the wet compared with the dry season. Over the period 1993 to 1996 algal numbers showed a significant negative correlation with monthly (30 d) and short-term (1 to 2 d) values of both SST and PAR dose, while algal chlorophylls displayed a significant negative correlation with PAR dose in the 5 d preceding collection. Rising SST and increasing PAR in the dry season are paralleled by decreasing algal numbers and algal chlorophylls on an annual basis. Recovery of algal densities and a build-up in algal chlorophylls follow reductions in SST and PAR at the end of the wet season. In one of the years, when coral 'bleaching' was widespread at the study site (May 1995), the interaction between SST and PAR resulted in an 86 % reduction in algal densities in bleached corals compared to their annual maximum density. This study highlights the importance of seasonal fluctuations in physical environmental parameters and concomitant changes in the coravalgal symbiosis. Furthermore, it emphasises the need for greater understanding of the potential effects of seasonal and inter-annual variability on the status of algal symbionts, as well as identification of critical times of the year when visible bleaching may not be obvious but when algal numbers and algal chlorophylls are low and effects on coral physiology may be far-reaching.

Solar irradiation has been cited as a possible cause of bleaching in corals, either acting alone or in conjunction with other environmental factors. However, evidence of a solar involvement in naturally occurring bleaching is s t d largely conjectural. We have recorded a particular type of naturally occurring bleaching damage at intertidal sites at Phuket, Thailand for a number of years which has a strong directional component. Use of tidal data, sun track analysis, and solar irradiance measurements have enabled us to show that this bleaching directly corresponds to sun altitude and azimuth. Our work has shown that for the massive coral Goniastrea aspera, bleaching is induced during periods of subaerial exposure with high sun altitude and irradiance. Furthermore, on-site measurements of solar irradiance mitigate against the biologically damaging effect of shorter wavelength ultraviolet radiation (UVR) as a major causative factor. Desiccation, heating, or photochemical reactions by photosynthetically active radiation (PAR) (400 to 700 nm) remain possible candidates for further investigation