Modelling fish colour constancy, and the implications for vision and signalling in water

Abstract

Colour vision and colour signals are important to aquatic animals, but light scattering and absorption by water distorts spectral stimuli. To investigate the performance of colour vision in water, and to suggest how photoreceptor spectral sensitivities and body colours might evolve for visual communication, we model the effects of changes in viewing distance and depth on the appearance of fish colours for three teleosts: a barracuda, Sphyraena helleri, which is dichromatic and two damselfishes, Chromis verater and Chromis hanui, which are trichromatic. We assume that photoreceptors light-adapt to the background, thereby implementing the von Kries transformation, which can largely account for observed colour constancy in humans and other animals, including fish. This transformation does not, however, compensate for light scattering over variable viewing distances, which in less than a metre seriously impairs dichromatic colour vision, and makes judgement of colour saturation unreliable for trichromats. The von Kries transformation does substantially offset colour shifts caused by changing depth, so that from depths of 0 to 30 m modelled colour changes (i.e. failures of colour constancy) are sometimes negligible. However, the magnitudes and directions of remaining changes are complex, depending upon the specific spectral sensitivities of the receptors and the reflectance spectra. This predicts that when judgement of colour is important, the spectra of signalling colours and photoreceptor spectral sensitivities should be evolutionarily linked, with the colours dependent on photoreceptor spectral sensitivities, and vice versa.

Assessing Sexual Dicromatism: The Importance of Proper Parameterization in Tetrachromatic Visual Models

“…Also of great importance is the consideration that color constancy mechanisms, approximated by the von Kries correction in the receptor noise model, limit the influence of the light environment on the perceived contrast between color patches. Behavioural tests [83] and modelling experiments [84] in vertebrates have shown that chromatic adaptation can work over large changes in ambient light environments.…”

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

“…To determine how the fluorescent component of the red iris signal contributes to the conspicuousness of triplefins to conspecifics at different depths, we modelled the chromatic and achromatic contrasts between 6 and 36 m of (i) the iris with fluorescence and without (reflectance only), (ii) the iris reflectance (no fluorescence) against achromatic backgrounds and (iii) the fluorescing iris (relative radiance) against achromatic backgrounds. We produced backgrounds of different brightness using achromatic spectra (equal intensity across all wavelengths from 400 to 700 nm) with varying mean brightness because many marine substrates are achromatic [44,45] (mean brightness = 5% to 50% in 2.5% intervals). We calculated the chromatic and achromatic contrasts in just-noticeable differences (JNDs), where scores above 1.0 indicate that the colours are distinguishable from one another, using Vorobyev and Osorio's receptor-noise model [33].…”

“…We assumed that most interactions between individuals occur at distances of less than 10 cm for which light scattering would be negligible [44]; therefore we did not include an attenuation coefficient and distance parameter in any model. We included the measured irradiance, which combines the downwelling and side-welling light fields, at the appropriate depth as the adapted background (von Kries correction) [44,46]. We compared the signals with and without the contribution of fluorescence at depths ranging from 6 to 36 m, in 2 m intervals.…”

Disruptive colouration in reef fish: does matching the background reduce predation risk?

“…With increased viewing distances (similar to those used by coral trout when attacking prey in the wild) it is likely that the black and white bars on the humbug merge into one grey object, as is predicted with many of the bright and intricate reef fish colour patterns (Marshall, 2000;Vorobyev et al, 1999;Wilkins et al, 2016). Indeed, as the reef contains many dappled shadows and grey areas, merged grey objects are likely to be well camouflaged through a direct match to background luminous intensity.…”