The hermatypic coral Stylophora pistillata has a wide bathymetric distribution (0 to 70 m). Within this range, light intensity decreases exponentially. Deep-water colonies are generally planar in morphology, with the upper part being dark and the bottom-facing part pale. Shallow-water colonies are generally subspherical and ivory in coloration. We studied the effects of photoacclimation on photosynthesis, respiration, and calcification in S. pistillata colonies along its bathymetric range over a reef profile (5 to 65 m) in Eilat, Gulf of Aqaba, Red Sea, during winter and summer, using a submersible respirometer. Respiration rate, light-saturated rate of photosynthesis (P max ), compensation light intensity (E c ), and light intensity of incipient saturation (E k ), all decreased with depth. In contrast, the efficiency of photosynthesis (α) increased with depth. All colonies displayed 'lightenhanced calcification' during daytime and decreasing calcification rates with depth. These results indicate an adjustment in harvesting and utilization of light by the algal symbionts to the light environment. At all light intensities except the lowest ones, there was a consistent ratio of calcification to photosynthesis, in agreement with the concept of light-enhanced calcification. In the deepest, lowlight corals, there was no evidence for support of calcification by photosynthesis, and we assume that these colonies subsist mainly by preying on zooplankton.

In contrast to the abundance of literature on the relationship between fish assemblages and habitat structure in the upper 30 m of coral reefs, the deeper (> 40 m) parts of coral reefs are rarely studied. We examined changes in reef fish diversity and habitat structure along an increasing depth gradient, including the unknown deep reef. We ran visual and video transects along a substantial depth gradient (0 to 65 m) in the northern Red Sea and extended the known depth distribution for 48 reef species. We found a change in assemblage composition highly correlated to both the depth gradient and a reduction in the abundance of branching corals with depth. The number of reef fish species declined with depth and we also measured a high species turnover as measured by beta diversity (β t , β w ) in the deep reef. This pattern is mainly due to the replacement of the abundant damselfishes in the shallow reef, which prey on zooplankton, by zooplanktivorous sea basses and wrasses in the deep reef. The steep reduction in branching corals, which most damselfishes use for cover, may be the main factor contributing to this change. We found a peak in species richness (alpha diversity) at 30 m, a peak in β w at 50 to 65 m, and peaks in β t at 30 to 50 m and 50 to 65 m. The 30 m depth stratum shows species of both shallow and deep assemblages generating a transition zone with characters of both deep and shallow habitats. The fish assemblage continues to change with depth, and future research will determine if there exists a depth threshold at which the assemblage will stabilize. KEY WORDS: Depth gradient · Deep reef fish · Gulf of Aqaba · Red Sea · Twilight zone Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 371: [253][254][255][256][257][258][259][260][261][262] 2008 Nishihira 2002). Habitat structure and related abiotic parameters have been shown to be some of the main factors structuring fish communities (McGehee 1994, Lara & Gonzalez 1998, Wantiez & Chauvet 2003, Brokovich et al. 2006). In the context of depth gradients, decreased light levels may hinder the ability of mobile organisms to forage (Rickel & Genin 2005), can decrease algae growth rates (Russ 2003) and can modify coral assemblages and resulting habitat structure. Srinivasan (2003) demonstrated that the distribution of some coral reef fishes over different depths is established at and/or soon after settlement, and suggests that factors associated with depth may explain differences in settlement, growth and survival, and warrant further investigation.While scuba diving has enabled researchers to study these patterns in shallow coral reefs with relative ease and safety, deep habitats are still rarely studied in detail because of decreasing bottom times with depth, and the inherent risks of breathing elevated partial pressures of oxygen and nitrogen. The literature now includes a number of studies that use remote video, submersibles and mixed gas diving to study reef communities of fishes, invertebrates a...

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The light-limited environment of tropical coral reefs has not been intensively studied due to technical limitations. Studying this vast part of the coral reef is paramount to understanding the ecological and physiological significance of coral-algae symbiosis and defining the boundaries imposed on its bathymetric distribution by the underwater light field. In the present study we describe morphological changes in colonies of the coral Stylophora pistillata and track changes in its carbon sources (autotrophic/heterotrophic behavior) along its full bathymetric distribution. The growth form of hermatypic corals must compromise between an optimal light-trapping surface facilitating photosynthesis and other structures and/or mechanisms that enhance exploitation of nutrientrich sources such as zooplankton. That architectural modulation is constrained within the speciesspecific structural and biological characteristics. We found that the profusely branched S. pistillata colonies shift between subspherical morphology at high-light environments to a planar structure at depth. The stable carbon isotopic composition (δ 13 C) of the host coral tissue changed from a value of -15 ‰ in shallow water to -23 ‰ at the deep reef. The latter value indicates either a carbon source with a stable isotope composition equal or below -23 ‰ or, alternatively, internal carbon cycling between host and algae that involves isotopic fractionation (ε). The δ 13 C values showed significant correlation to morphological traits, but contradicting trends were found within the traits. A clear shift to heterotrophy was not apparent, which, therefore, suggests that the internal cycling and Rubisco activity are the dominant processes determining isotopic composition.KEY WORDS: Corals · Zooxanthellae · δ 13 C · Deep coral reef · Light · Calcification · Skeleton Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 381: [167][168][169][170][171][172][173][174] 2009 tinct δ 13 C signatures (Deniro & Epstein 1978). In the special case of corals, the 2 partners that make up the holobiont interact at the basic metabolic level that includes reciprocal fluxes of energy and nutrient-rich compounds (Muscatine et al. 2005). Evidence supporting the autotrophic nature of zooxanthellate corals is found in shallow-water corals, where the δ 13 C of both the coral host and the symbiont algae are similar. Muscatine et al. (1989) reported that as depth increased, so did the difference between zooxanthellae and coral tissue δ 13 C, and the latter approached that of oceanic particulate organic carbon. This suggests that carbon is translocated at all depths and deep-water corals draw significantly on allochthonous sources of carbon. In addition, the δ 13 C of the coral tissue is significantly more positive (contains more 13 C, -10 to -14 ‰) than that of the zooplankton (ca. -20 ‰), a primary food source (Muscatine et al. 2005, Swart et al. 2005. This indicates that, for those corals studied, the supply of carbon to the c...

Photosynthetic coral reef structures extend from the shallow sundrenched waters to the dimly lit, "twilight" mesophotic depths. For their resident endosymbiotic dinoflagellates, primarily from the genus Symbiodinium spp., this represents a photic environment that varies ∼15-fold in intensity and also differs in spectral composition. We examined photosynthesis in the scleractinian coral Stylophora pistillata in shallow (3 m) and mesophotic settings (65 m) in the northern Red Sea. Symbiodinium spp. in corals originating from the mesophotic environment consistently performed below their photosynthetic compensation point and also exhibited distinct light harvesting antenna organization. In addition, the non-photochemical quenching activity of Symbiodinium spp. from mesophotic corals was shown to be considerably lower than those found in shallow corals, showing they have fewer defenses to high-light settings. Over a period of almost 4 years, we extensively utilized closed circuit Trimix rebreather diving to perform the study. Phylogenetic analysis showed that shallow corals (3 m) transplanted to a deep reef environment (65 m) maintained their initial Symbiodinium spp. community (clade A), rather than taking on deep low-light clades (clade C), demonstrating that shallow S. pistillata acclimate to low-light mesophotic environments while maintaining their shallow photosynthetic traits. Mesophotic corals exhibited static depth-related chlorophyll content per cell, a decrease in PSI activity and enhanced sigmoidal fluorescence rise kinetics. The sigmoidal fluorescence rise kinetics we observed in mesophotic corals is an indication of energy transfer between photosynthetic units. We postulate that at mesophotic depths, a community of adapted Symbiodinium spp. utilize a unique adaptation to lower light conditions by shifting their light harvesting to a PSII based system, where PSII is structured near PSI, with additional PCP soluble Einbinder et al.Novel Photosynthetic Characteristics of Mesophotic Photosymbionts antenna also trapping light that is funneled to the PSI reaction center. In this study, we provide evidence that mesophotic Symbiodinium spp. have developed novel adaptive low-light characteristics consisting of a cooperative system for excitation energy transfer between photosynthetic units that maximizes light utilization.