Red fluorescence: a novel light emitting mechanism to enhance prey detection in Tripterygion delaisi?

Abstract

Since the discovery of red fluorescent fish, scientists argue whether it is of ecologically relevance or merely a side effect of pigment evolution. Red fluorescence could theoretically be involved in many different functions ranging from foraging over species recognition to camouflage. Despite growing evidence supporting the functionality of red fluorescence, we still lack knowledge concerning the type of function and the evolutionary processes influencing it. Within this thesis, I therefore first focused on identifying the ecological drivers of fluorescence and then assessed whether fluorescence might be used in a context of prey detection facilitation. Using the black-faced triplefin Tripterygion delaisi as model species, I conducted empirical and experimental studies to address these points. In the first chapter, I investigated why fish fluoresce more efficiently when originating from deep water compared with shallow water individuals by identifying the environmental triggers causing this effect. After conducting physiological experiments under controlled ambient light conditions, I confirm that fluorescence increases its efficiency with decreasing brightness and is regulated through phenotypic flexibility (Chapter 1). In the following chapters, I focused on the question whether red fluorescence is used to enhance prey detection. By illuminating the environment with longer wavelengths, which are nearly absent below 10 m depths, fish capable of emitting red fluorescence could theoretically increase their foraging success by enhancing the visual contrast between prey and natural background. This, however, requires red fluorescence to exceed the ambient light and the emitted substrate radiance in the longer wavelength range (> 600nm). I tested this by taking spectral measurements of substrate radiance and in vivo iris fluorescence in the field. After calculating the brightness contrast between these components, I can confirm that iris fluorescence always exceeds substrate radiance in deeper water (Chapter 2). Contrary to my predictions, however, I also identified several conditions in shallow water within which red fluorescence is likely to generate a visual contrast. Since a visual contrast at least in deeper water is highly likely, I continued my research by testing if fish are indeed more successful in catching prey under “fluorescence friendly” narrow-spectral, blue-green light conditions compared with broad-spectral, “white” light conditions. I predicted that under the blue-green light typical for deeper water, fish emitting red fluorescence are able to enhance the visual contrast between prey and the blue-green background. This contrast could facilitate prey detection, increasing foraging success. Shallow water environments are characterized by broader, more “white” spectra. Here, I predicted that such contrast cannot be generated and hence, foraging should be less efficient. I tested this under dim light (two levels of shading) to encourage the expression of fluorescence (Chapter 1). The results show that fish were more successful in foraging under heavily shaded, blue-green light conditions, compared with the broad-spectral or brighter treatments (chapter 3). I conclude that iris fluorescence is likely to be of ecological relevance to T. delaisi and might act as a contrast-enhancing mechanism to facilitate visual tasks under dim light.