Contribution of fluorescence to the spectral signature and perceived color of corals

Solar-induced fluorescence from pigments in the host tissues of reef corals can make a significant contribution to their spectral signature and can affect their apparent color as perceived by a human observer. The relative magnitudes of the reflectance and fluorescence contributions to the spectrum can vary as a function of illumination conditions. We have combined measured coral reflectance and fluorescence spectra with modeled downwelling spectral irradiances to investigate the contribution of fluorescence to coral spectra and color. Variations in depth, fluorescence efficiency, and solar zenith angle were modeled. Fluorescence enhancement is greatest when the coral absorbs light at wavelengths that are transmitted well by seawater and emits light efficiently at wavelengths that are moderately attenuated. The methodology provides a means of predicting the combined reflectance and fluorescence spectral signatures and perceived colors of fluorescent corals under arbitrary illumination conditions. The striking patterns and hues of coloration in many corals and anemones have attracted attention for many years (Fox and Pantin 1944; Kawaguti 1944; Stephenson 1947), but an understanding of the possible function of such coloration remains elusive (Wicksten 1989). Observers long ago noted that in some cases the intense coloration is enhanced by or largely attributable to the fluorescence of pigments contained in the animal tissues (Phillips 1927; Kawaguti 1944; Shibata 1969). The effect is most striking when specimens appear orange or red (Limbaugh and North 1956; Marden 1956) at depths at which those colors are absent from the downwelling light because of the spectral filtering of seawater (Smith and Baker 1981). Investigations of coral fluorescence have focused on qualitative observations of the color of the fluorescence and quantitative measurements of its spectral characteristics (Catala 1959; Shibata 1969; Logan et al. 1990; Mazel 1995), and not on its contribution to spectral signature or perceived color under natural lighting conditions. Lythgoe (1979) noted that fluorescence played very little role in coloration in the terrestrial environment but that in the spectrally limited underwater environment it had a greater potential to produce vivid color effects. Some of the fluorescent pigments in corals have recently been identified (Matz et al. 1999) as forms of the green fluorescent protein (GFP) originally found in the hydromedusa Aequorea victoria (Chalfie et al. 1994). The identification of the molecular nature of a pigment does not in itself reveal 1 Present address: Marine Physical Laboratory, Scripps Institution for Oceanography, 8820 ShellBack Way, La Jolla, California 92093. Acknowledgments This work was supported by the Coastal Benthic Optical Properties (CoBOP) program of the Environmental Optics Program at the Office of Naval Research. Curt Mobley and Robert Maffione provided valuable input into the concept and methodology of the work. We thank the Caribbean Marine Research Center (CMRC) at Lee Stocking Island, Bahamas, for field support. its function. In Aequorea, GFP performs an energy transfer function in bioluminescence (Morin and Hastings 1971), converting the blue emission of the bioluminescence reaction to green. Corals, however, are not bioluminescent, and the function of the fluorescence has variously been ascribed to (1) providing photoprotection in high-light conditions (Kawaguti 1969; Salih et al. 2000), (2) enhancing photosynthesis in low-light conditions (Schlichter and Fricke 1990), or (3) both, depending on the positioning of the fluorescent pigment relative to the zooxanthellae (Salih et al. 1998; Dove et al. 2001). These functions have not been conclusively demonstrated for corals in general. Gleason (1993) found that the green fluorescent pigment in Porites astreoides, while more prevalent at shallower depths, did not provide protection against ultraviolet radiation. Mazel et al. (2003) concluded that the green fluorescent pigment in Caribbean corals is not performing either of the two functions mentioned above. Here, we explore the potential for fluorescence to influence the color of corals. Corals can be colorful even without fluorescent pigments in the host tissues. The predominant color of corals is a beige or brown shade due to the absorption properties of lightcapturing pigments in the symbiotic dinoflagellates (zooxanthellae) that are present in the gastrodermal tissues of most reef species. Other colors can be associated with nonfluorescent pigments in host tissues (Kawaguti 1944; Shibata 1969; Dove et al. 1995, 2001; Lukyanov et al. 2000). Even when they are present, fluorescent pigments do not necessarily result in a perceived color effect under daylight illumination, at least in human observers. Many corals that exhibit striking fluorescence when illuminated with ultraviolet or blue light in darkness appear as ordinary brown specimens under ambient daylight illumination, indistinguishable from neighboring nonfluorescent specimens (pers. obs.). This can be explained by several factors, including the spectral distribution of the ambient illumination, the excitation and emission characteristics of the fluorescent pigments, and 391 Fluorescence and coral color the fluorescence efficiencies of the pigments. We used in situ and laboratory measurements of coral reflectance and fluorescence and the modeling of downwelling spectral irradiance to explore the contribution of fluorescence to the spectral signatures of corals as a function of variation in depth, solar zenith angle, and fluorescence efficiency. Because human perception of coral color has been the basis of several studies of the ecological distribution of coral varieties (Gleason 1993, 1998; Takabayashi and Hoegh-Guldberg 1995), we compute a quantitative measure of the color enhancement effect as it would affect a human observer. The results could also be used by researchers interested in the interaction of coral color with the visual systems of reef fish or invertebrates.