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Red fluorescence of the triplefin Tripterygion delaisi is increasingly visible against background light with increasing depth

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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.

No MeSH data available.


Radiance of T. delaisi iris with and without the contribution of fluorescence at 6 m (a) and 20 m (b), along with the respective ambient irradiance.
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RSOS161009F5: Radiance of T. delaisi iris with and without the contribution of fluorescence at 6 m (a) and 20 m (b), along with the respective ambient irradiance.

Mentions: Based on its spectral shape, the reflectance of the iris of T. delaisi (without the contribution of fluorescence) is reddish-brown (figure 4). The wavelength of peak fluorescence excitation averaged 525 nm across all eight eyes characterized with a full width at half maximum range of 452–575 nm (figure 4). The wavelength of peak fluorescence emission averaged 609 nm with a full width at half maximum range of 572–686 nm (figure 4). The practical fluorescent efficiency of the peak excitation wavelength (proportion of non-reflected photons converted to longer-wavelength photons) averaged 3.34% (range: 2.00–6.24). The contribution of red fluorescence to the visual signal of the iris varied with depth (figure 5). At 6 m (figure 5a), for example, the contribution of the fluorescence had little influence on the relative radiance of the irides, which never surpassed the long-wavelength irradiance available in the ambient light field. By contrast, the contribution of fluorescence to the relative radiance at 20 m was great (figure 5b), such that the relative radiance surpassed the long wavelength irradiance available in the ambient light field. Quantification of perceptual contrasts determined that between 6 and 12 m fluorescence did not contribute perceptively to the chromatic component of the signal (figure 6a). However, below 14 m the contribution of fluorescence to the chromatic component of the signal was greater than 1 just-noticeable-difference, the minimum threshold for colour discrimination (1.25 JND at 14 m), and subsequently increased with depth at an average rate of 0.53 JND m−1. The contrast between the iris and an achromatic background depended more on the brightness of the background than on the contribution of the fluorescence (figure 6b). In simulations in which the background reflectance averaged 15% or less, the iris was perceptively brighter near the surface (6 m), slightly increasing with increasing depth. When the background reflectance averaged more than 15%, the iris was perceptively darker, but less so with increasing depth.Figure 4.


Red fluorescence of the triplefin Tripterygion delaisi is increasingly visible against background light with increasing depth
Radiance of T. delaisi iris with and without the contribution of fluorescence at 6 m (a) and 20 m (b), along with the respective ambient irradiance.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5383848&req=5

RSOS161009F5: Radiance of T. delaisi iris with and without the contribution of fluorescence at 6 m (a) and 20 m (b), along with the respective ambient irradiance.
Mentions: Based on its spectral shape, the reflectance of the iris of T. delaisi (without the contribution of fluorescence) is reddish-brown (figure 4). The wavelength of peak fluorescence excitation averaged 525 nm across all eight eyes characterized with a full width at half maximum range of 452–575 nm (figure 4). The wavelength of peak fluorescence emission averaged 609 nm with a full width at half maximum range of 572–686 nm (figure 4). The practical fluorescent efficiency of the peak excitation wavelength (proportion of non-reflected photons converted to longer-wavelength photons) averaged 3.34% (range: 2.00–6.24). The contribution of red fluorescence to the visual signal of the iris varied with depth (figure 5). At 6 m (figure 5a), for example, the contribution of the fluorescence had little influence on the relative radiance of the irides, which never surpassed the long-wavelength irradiance available in the ambient light field. By contrast, the contribution of fluorescence to the relative radiance at 20 m was great (figure 5b), such that the relative radiance surpassed the long wavelength irradiance available in the ambient light field. Quantification of perceptual contrasts determined that between 6 and 12 m fluorescence did not contribute perceptively to the chromatic component of the signal (figure 6a). However, below 14 m the contribution of fluorescence to the chromatic component of the signal was greater than 1 just-noticeable-difference, the minimum threshold for colour discrimination (1.25 JND at 14 m), and subsequently increased with depth at an average rate of 0.53 JND m−1. The contrast between the iris and an achromatic background depended more on the brightness of the background than on the contribution of the fluorescence (figure 6b). In simulations in which the background reflectance averaged 15% or less, the iris was perceptively brighter near the surface (6 m), slightly increasing with increasing depth. When the background reflectance averaged more than 15%, the iris was perceptively darker, but less so with increasing depth.Figure 4.

View Article: PubMed Central - PubMed

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.

No MeSH data available.