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

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

Fish with red fluorescent eyes forage more efficiently under dim, blue-green light conditions

Abstract Background Natural red fluorescence is particularly conspicuous in the eyes of some small, benthic, predatory fishes. Fluorescence also increases in relative efficiency with increasing depth, which has generated speculation about its possible function as a “light organ” to detect cryptic organisms under bluish light. Here we investigate whether foraging success is improved under ambient conditions that make red fluorescence stand out more, using the triplefin Tripterygion delaisi as a model system. We repeatedly presented 10 copepods to individual fish (n = 40) kept under a narrow blue-green spectrum and compared their performance with that under a broad spectrum with the same overall brightness. The experiment was repeated for two levels of brightness, a shaded one representing 0.4% of the light present at the surface and a heavily shaded one with about 0.01% of the surface brightness. Results Fish were 7% more successful at catching copepods under the narrow, fluorescence-friendly spectrum than under the broad spectrum. However, this effect was significant under the heavily shaded light treatment only. Conclusions This outcome corroborates previous predictions that fluorescence may be an adaptation to blue-green, heavily shaded environments, which coincides with the opportunistic biology of this species that lives in the transition zone between exposed and heavily shaded microhabitats

The consistent difference in red fluorescence in fishes across a 15 m depth gradient is triggered by ambient brightness, not by ambient spectrum

BACKGROUND: Organisms adapt to fluctuations or gradients in their environment by means of genetic change or phenotypic plasticity. Consistent adaptation across small spatial scales measured in meters, however, has rarely been reported. We recently found significant variation in fluorescence brightness in six benthic marine fish species across a 15 m depth gradient. Here, we investigate whether this can be explained by phenotypic plasticity alone, using the triplefin Tripterygion delaisi as a model species. In two separate experiments, we measure change in red fluorescent brightness to spectral composition and ambient brightness, two central parameters of the visual environment that change rapidly with depth. RESULTS: Changing the ambient spectra simulating light at −5 or −20 m depth generated no detectable changes in mean fluorescence brightness after 4–6 weeks. In contrast, a reduction in ambient brightness generated a significant and reversible increase in mean fluorescence, most of this within the first week. Although individuals can quickly up- and down-regulate their fluorescence around this mean value using melanosome aggregation and dispersal, we demonstrate that this range around the mean remained unaffected by either treatment. CONCLUSION: We show that the positive association between fluorescence and depth observed in the field can be fully explained by ambient light brightness, with no detectable additional effect of spectral composition. We propose that this change is achieved by adjusting the ratio of melanophores and fluorescent iridophores in the iris. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13104-016-1911-z) contains supplementary material, which is available to authorized users

Visual modelling supports the potential for prey detection by means of diurnal active photolocation in a small cryptobenthic fish

Active sensing has been well documented in animals that use echolocation and electrolocation. Active photolocation, or active sensing using light, has received much less attention, and only in bioluminescent nocturnal species. However, evidence has suggested the diurnal triplefin Tripterygion delaisi uses controlled iris radiance, termed ocular sparks, for prey detection. While this form of diurnal active photolocation was behaviourally described, a study exploring the physical process would provide compelling support for this mechanism. In this paper, we investigate the conditions under which diurnal active photolocation could assist T. delaisi in detecting potential prey. In the field, we sampled gammarids (genus Cheirocratus) and characterized the spectral properties of their eyes, which possess strong directional reflectors. In the laboratory, we quantified ocular sparks size and their angle-dependent radiance. Combined with environmental light measurements and known properties of the visual system of T. delaisi, we modeled diurnal active photolocation under various scenarios. Our results corroborate that diurnal active photolocation should help T. delaisi detect gammarids at distances relevant to foraging, 4.5 cm under favourable conditions and up to 2.5 cm under average conditions. To determine the prevalence of diurnal active photolocation for micro-prey, we encourage further theoretical and empirical work