ABSTRACT
Background At depths below 10 m, reefs are dominated by blue-green light because seawater selectively absorbs the longer, ‘red’ wavelengths (600 nm and more) from downwelling sunlight (Fig. 1)[1,2]. Consequently, many reef sh have visual pigments matched to shorter wavelengths, which are transmitted better by water [3-5]. In addition, ecological studies of sh vision must correct for the spectrum available at the depth where they live [1,6,7] and therefore routinely correct spectral sensitivity measurements from the laboratory for the available (mostly downwelling) light on the reef. is reduces the relevance of red light to reef sh even more. However, this procedure ignores the fact that several marine organisms, including deep sea sh, produce their own red bioluminescence and are capable of seeing it [8,9]. e purpose of this study was (1) to see “with our own eyes” whether there
is indeed a lack of red light at depth in the euphotic zone during daytime and (2) to identify the observed sources of natural red uorescence in sh in particular. is work combines results from several studies carried out on coral reefs in the Red Sea and the Great Barrier Reef and has been supplemented by observations and measurements on sh in the laboratory.
