Does red eye fluorescence in marine fish stand out? In situ and in vivo measurements at two depths

Abstract

Since the discovery of red fluorescence in fish, much effort has been made to elucidate its potential contribution to vision. However, whatever that function might be, it always implies that the combination of red fluorescence and reflectance of the red iris is sufficient to generate a visual contrast. Here, we present in vivo iris radiance measurements of T. delaisi under natural light fields at 5 and 20 m depth. We also took substrate radiance measurements of shaded and exposed foraging sites at those depths. To assess the visual contrast that can be generated by the red iris, we then calculated iris brightness in the 600-650 nm \"red\" waveband relative to substrate radiance. At 20 m depth, T. delaisi iris radiance substantially exceeded substrate radiance in the red waveband, regardless of exposure, and despite substrate fluorescence. Given that downwelling light in the 600-650 nm range is negligible at this depth, we can attribute this effect to iris fluorescence. As expected, contrasts were much weaker in 5 m - despite the added contribution of iris reflectance, but we identified specific substrates and conditions under which the pooled radiance caused by red reflectance and fluorescence still exceeded substrate radiance in the same waveband. Due to the negative effect of anesthesia on iris fluorescence these estimates are conservative. We conclude that the requirements to create visual brightness contrasts are fulfilled for a wide range of conditions in the natural environment of T. delaisi.