Red fluorescence of the triplefinTripterygion delaisiis increasingly visible against background light with increasing depth

Abstract

The light environment in water bodies changes with depth due to the absorption of short and long wavelengths. Below 10 m depth, red wavelengths are almost completely absent rendering any red-reflecting animal dark and achromatic. However, fluorescence may produce red coloration even when red light is not available for reflection. A large number of marine taxa including over 270 fish species are known to produce red fluorescence, yet it is unclear under which natural light environment fluorescence contributes perceptively to their colours. To address this question we: (i) characterized the visual system of Tripterygion delaisi, which possesses fluorescent irides, (ii) separated the colour of the irides into its reflectance and fluorescence components and (iii) combined these data with field measurements of the ambient light environment to calculate depth-dependent perceptual chromatic and achromatic contrasts using visual modelling. We found that triplefins have cones with at least three different spectral sensitivities, including differences between the two members of the double cones, giving them the potential for trichromatic colour vision. We also show that fluorescence contributes increasingly to the radiance of the irides with increasing depth. Our results support the potential functionality of red fluorescence, including communicative roles such as species and sex identity, and non-communicative roles such as camouflage.

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

“…T. delaisi shows highest sensitivity in the green wavelength range [26]. The stenospectral treatment might therefore have been perceived brighter by the fish, regardless of the real overall brightness.…”

Method for Determining the Contribution of Fluorescence to an Optical Signature, with Implications for Postulating a Visual Function

“…There are alternative ways to approach the measurement and modeling of fluorescence contribution (e.g., Bitton et al, 2017;Taboada et al, 2017). While these differ in detail from the approach described here they lead to the same end: an estimate of the contribution of fluorescence to the optical signal.…”

“…Arnold et al (2002) demonstrated all criteria for a fluorescence function in a bird (budgerigar, Melopsittacus undulatus), including a behavioral response. The quantitative approaches of Mazel et al (2004), Taboada et al (2017), and Bitton et al (2017) indicated the potential for fluorescence to play a visual role in a stomatopod (Lysiosquillina glabriuscula), frog (Hypsiboas punctatus) and fish (Trypterygion delaisi), respectively. Another quantitative treatment (Iriel and Lagorio, 2010) indicated that the fluorescence of the flowers investigated was too weak relative to reflected light to have a functional role.…”

“…This is not the only possible way to do this, but it has been successfully applied to the analysis of fluorescing marine organisms (Mazel and Fuchs, 2003;Mazel et al, 2004). Alternative approaches are described, for example, in the analyses of the contribution of fluorescence to the appearance of flowers (Iriel and Lagorio, 2010), a frog (Taboada et al, 2017), and a red-fluorescent fish (Bitton et al, 2017).…”

Do the fluorescent red eyes of the marine fish Tripterygion delaisi stand out? In situ and in vivo measurements at two depths

“…Except for the breeding season, when males develop prominent coloration, individuals are highly cryptic against their natural background, with no obvious sexual dimorphism. Tripterygion delaisi displays highly fluorescent irides with an average peak emission (λ max ) of 609 nm with a full width at half maximum range of 572 nm to 686 nm (Bitton et al, 2017). Furthermore, it can perceive its own red fluorescence (Bitton et al, 2017;Kalb, Schneider, Sprenger, & Michiels, 2015), which is regulated from nearly absent to maximum brightness through melanosome dispersal or aggregation in melanophores in less than 30 s (Wucherer & Michiels, 2014).…”