The Red‐Fluorescing Marine Fish Tripterygion delaisi can Perceive its Own Red Fluorescent Colour

Abstract

Many marine fishes show conspicuous red fluorescent body patterns. Recent work suggests that red fluorescence may be used as a visual colour cue in these species. Very few studies, however, have as yet been able to demonstrate that red fluorescent fish can actually perceive their own fluorescence. This is the first study to our knowledge in which a red fluorescent fish is trained to assess whether it can recognize red fluorescence. We used the triplefin Tripterygion delaisi, a species with conspicuous red fluorescent eye rings. Training and testing involved repeated binary choices between grey and red fluorescence cues. The training and testing were carried out under broad spectral illumination. The final testing phase involved cyan light illumination, mimicking natural ambient light at depth. When testing all nine combinations of three grey brightness levels against three red fluorescence brightness levels, individuals made significantly more correct choices than the random expectation under broad as well as cyan illumination. Under cyan illumination, fish trained on red chose the correct cue more often compared to fish trained on grey. An analysis of the effect of the brightness levels suggests that fish did indeed make their choices based on chromatic more than achromatic cues: The three grey levels did not affect the proportion of correct choices. We conclude that T. delaisi can perceive and respond to levels of fluorescence that are similar to its own. We also discuss the difficulties that can arise from using a binary choice design on a fish with a cryptobenthic lifestyle. We argue in favour of using sequential choice designs in future studies of T. delaisi.

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

“…To better understand the importance of red fluorescence for the visual ecology of the triplefin T. delaisi , we characterized its visual system, separated the iris colour into its reflectance and fluorescence components, quantified the light environment of triplefin habitat and used these data to parametrize visual models. We found that triplefins have (i) double cones with photoreceptor sensitivities differing by an average of 14 nm, which, it may be reasonable to assume, contribute to a trichromatic colour vision system as has been reported in other marine fish species [50], (ii) fluorescent irides that convert wavelengths of light available in a wide range of depths to longer wavelengths lacking in the stenospectral zone and (iii) irides with fluorescence that contributes to the perceived chromatic contrast of the iris against an achromatic substrate [27]. Our results strongly suggest that T. delaisi should be capable of perceiving their own fluorescence in various natural light environments, the signalling importance increasing with depth.…”

“…However, the difference in the peak sensitivity of the two members (average of 14 nm) is much smaller than found in the reef fish Rhinecanthus aculeatus (50 nm difference), the only species for which experimental evidence exists for colour discrimination through double cones [50,51]. Only behavioural tests on trained T. delaisi would confirm that their ability to detect their own fluorescence [27] is facilitated through trichromacy. Nonetheless, our results demonstrate that fluorescence produced by the irides of T. delaisi could contribute to the signal as perceived by conspecifics.…”

What makes fish vulnerable to capture by hooks? A conceptual framework and a review of key determinants

“…Various mechanisms of chromatic sensitivity exist across fish taxa; genes coding for vertebrate opsin proteins evolved in jawless fishes and proliferate throughout the jawed fishes (Collin et al, 2003;Van-Eyk, Siebeck, Champ, Marshall, & Hart, 2011;Kalb, Schneider, Sprenger, & Michiels, 2015). Clear aquatic environments primarily reflect blue-green light (Kalb et al, 2015) but wavelengths of light attenuate with depth depending on the optical properties of water (including phytoplankton, organic and inorganic particulates, and products of vegetative decay; Lythgoe, 1975). Contrast is somewhat important for fish feeding, and predatory fish may increase the contrast of lit areas via concealment in shade (Helfman, 1981) or modify their behaviour to maximize prey detection and self-concealment (Huveneers et al, 2015).…”

Controlled iris radiance in a diurnal fish looking at prey

“…While doing so, it frequently alternates between both ocular sparks and the "off" state by eye rotation and tilting (ESM 3, ESM 4). Both ocular sparks have been observed in the field down to 30 m. Previous work already indicated that T. delaisi can perceive its own fluorescence [19,30]. The blue ocular spark also falls well within its spectral sensitivity (LWS cone lambdamax = 468 nm [19]).…”