Conspicuous visual signals do not coevolve with increased body size in marine sea slugs

Abstract

Many taxa use conspicuous colouration to attract mates, signal chemical defences (aposematism) or for thermoregulation. Conspicuousness is a key feature of aposematic signals, and experimental evidence suggests that predators avoid conspicuous prey more readily when they exhibit larger body size and/or pattern elements. Aposematic prey species may therefore evolve a larger body size due to predatory selection pressures, or alternatively, larger prey species may be more likely to evolve aposematic colouration. Therefore, a positive correlation between conspicuousness and body size should exist. Here, we investigated whether there was a phylogenetic correlation between the conspicuousness of animal patterns and body size using an intriguing, understudied model system to examine questions on the evolution of animal signals, namely nudibranchs (opisthobranch molluscs). We also used new ways to compare animal patterns quantitatively with their background habitat in terms of intensity variance and spatial frequency power spectra. In studies of aposematism, conspicuousness is usually quantified using the spectral contrast of animal colour patches against its background; however, other components of visual signals, such as pattern, luminance and spectral sensitivities of potential observers, are largely ignored. Contrary to our prediction, we found that the conspicuousness of body patterns in over 70 nudibranch species decreased as body size increased, indicating that crypsis was not limited to a smaller body size. Therefore, alternative selective pressures on body size and development of colour patterns, other than those inflicted by visual hunting predators, may act more strongly on the evolution of aposematism in nudibranch molluscs.

The Role of Life-History and Ecology in the Evolution of Color Patterns in Australian Chrysomeline Beetles

“…This difference in color pattern between the beetle and its background was obtained separately for red, blue and Frontiers in Ecology and Evolution | www.frontiersin.orggreen (hereafter referred to as red, blue and green pattern differences, respectively). As the values for pattern differences were of a greater range than that described by Cheney et al (2014), we did not classify them as "different" or "not different." Rather, we used the values of pattern differences as a continuous measure to compare against the ecological data.…”

“…This was performed separately on the red, green and blue layers of each sample in MATLAB, using the Image Processing Toolbox. A log-scaled power spectrum curve was obtained from the FFT, followed by a rotational average of the amplitudes produced (Cheney et al, 2014). The absolute difference in area between the power spectrum curves of the beetle and each background sample provided a quantification of the difference in color pattern between the beetle and its background.…”

“…All the images were then scaled in Adobe Photoshop CS6 based on the graduations present in each image, such that the number of pixels per cm was consistent for all the images. The beetle in each image was manually defined following Cheney et al (2014) using a tablet and stylus. Four samples of the background were also obtained by shifting the beetle outline to separate, arbitrarily chosen positions within the background area of the same image.…”

From crypsis to mimicry: changes in colour and the configuration of the visual system during ontogenetic habitat transitions in a coral reef fish

“…Cheney et al, 2014;Boileau et al, 2015). In addition, the coral trout might also use differences in luminance contrast (ΔL) to detect dottybacks against their habitat background.…”

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

“…Matching the background luminance is important in disruptive colouration; however, whether disruptive markings additionally have to match the background in terms of spatial scale to prevent detection has not been specifically tested to our knowledge. This is despite evidence that there are significant differences in the spatial frequency of conspicuous and cryptic animal body patterns (Cheney et al, 2014;Godfrey et al, 1987). Interestingly, Cott (1940) did not make any predictions about the spatial characteristics of optimal disruptive colouration in animal body patterns.…”