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Changeable camouflage: how well can flounder resemble the colour and spatial scale of substrates in their natural habitats?

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ABSTRACT

Flounder change colour and pattern for camouflage. We used a spectrometer to measure reflectance spectra and a digital camera to capture body patterns of two flounder species camouflaged on four natural backgrounds of different spatial scale (sand, small gravel, large gravel and rocks). We quantified the degree of spectral match between flounder and background relative to the situation of perfect camouflage in which flounder and background were assumed to have identical spectral distribution. Computations were carried out for three biologically relevant observers: monochromatic squid, dichromatic crab and trichromatic guitarfish. Our computations present a new approach to analysing datasets with multiple spectra that have large variance. Furthermore, to investigate the spatial match between flounder and background, images of flounder patterns were analysed using a custom program originally developed to study cuttlefish camouflage. Our results show that all flounder and background spectra fall within the same colour gamut and that, in terms of different observer visual systems, flounder matched most substrates in luminance and colour contrast. Flounder matched the spatial scales of all substrates except for rocks. We discuss findings in terms of flounder biology; furthermore, we discuss our methodology in light of hyperspectral technologies that combine high-resolution spectral and spatial imaging.

No MeSH data available.


All individual spectral measurements of S. aquosus (left plots) and substrates (right plots), as well as an image showing a section of flounder and substrate. Grey lines are individual spectral measurements; heavy black line is the average. (a) Flounder on sand substrate, (b) flounder on small gravel substrate, (c) flounder on large gravel substrate, (d) flounder on rock substrate. (e) We computed the SAM metric for each substrate–flounder combination for ideal and actual scenarios and compared the similarity of these distributions (see Material and methods). Overall SAM values are low, indicating similar spectral shapes between the animal and the background spectra. Asterisk indicates that, using the Wilcoxon rank sum test at the 5% significance level, data come from distributions with equal medians.
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RSOS160824F3: All individual spectral measurements of S. aquosus (left plots) and substrates (right plots), as well as an image showing a section of flounder and substrate. Grey lines are individual spectral measurements; heavy black line is the average. (a) Flounder on sand substrate, (b) flounder on small gravel substrate, (c) flounder on large gravel substrate, (d) flounder on rock substrate. (e) We computed the SAM metric for each substrate–flounder combination for ideal and actual scenarios and compared the similarity of these distributions (see Material and methods). Overall SAM values are low, indicating similar spectral shapes between the animal and the background spectra. Asterisk indicates that, using the Wilcoxon rank sum test at the 5% significance level, data come from distributions with equal medians.

Mentions: We took spectral measurements of the fish as well as the adjacent substrate in the manner illustrated in figure 1a. We positioned our measurement points to capture as much of the variation as possible. (1) Fish measurements. Spectral measurements for each fish were taken in 20 locations along the anterior–posterior line, starting just behind the head, finishing just before the fin. To ensure we characterized the entire spectral signature of each flounder, we then took measurements of specific areas on the fish's body that are similar to the ‘morphological markers’ of Saidel [15]. These were often not located on the anterior–posterior line we routinely measured. Four such areas were identified: (i) white spot, (ii) area around white spot (or fish ‘background’), (iii) black spot, and (iv) area around black spot (figure 1b,c). Five measurements were taken of each area, for a total of 20 additional measurements. These areas could be shown by the fish with higher or lower intensity. For example, the white spots could vary between being large and conspicuous and being almost absent (i.e. taking on the colour of the surrounding body area). Similarly, the area around a black spot could be as black as the black spot itself; however, it could also take on the beige colour of the fish ‘background’ shade. We measured spectra from all areas, and grouped all of the fish spectra for a total of 40 measurements. (2) Substrate measurements. Ten spectral measurements were taken in a line in front of the head followed by 10 measurements behind the tail fin (figure 1a). We followed the anterior–posterior line of the fish when taking these substrate measurements. Additionally, we took 10 measurements of the substrate in a curve that was equidistant from the dorsal fin. In total, we took 30 measurements of the substrate. Figures 2 and 3 show all spectral data taken from one representative flounder on all four substrates for Paralichthys dentatus and Scophthalmus aquosus, respectively.Figure 1.


Changeable camouflage: how well can flounder resemble the colour and spatial scale of substrates in their natural habitats?
All individual spectral measurements of S. aquosus (left plots) and substrates (right plots), as well as an image showing a section of flounder and substrate. Grey lines are individual spectral measurements; heavy black line is the average. (a) Flounder on sand substrate, (b) flounder on small gravel substrate, (c) flounder on large gravel substrate, (d) flounder on rock substrate. (e) We computed the SAM metric for each substrate–flounder combination for ideal and actual scenarios and compared the similarity of these distributions (see Material and methods). Overall SAM values are low, indicating similar spectral shapes between the animal and the background spectra. Asterisk indicates that, using the Wilcoxon rank sum test at the 5% significance level, data come from distributions with equal medians.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5383827&req=5

RSOS160824F3: All individual spectral measurements of S. aquosus (left plots) and substrates (right plots), as well as an image showing a section of flounder and substrate. Grey lines are individual spectral measurements; heavy black line is the average. (a) Flounder on sand substrate, (b) flounder on small gravel substrate, (c) flounder on large gravel substrate, (d) flounder on rock substrate. (e) We computed the SAM metric for each substrate–flounder combination for ideal and actual scenarios and compared the similarity of these distributions (see Material and methods). Overall SAM values are low, indicating similar spectral shapes between the animal and the background spectra. Asterisk indicates that, using the Wilcoxon rank sum test at the 5% significance level, data come from distributions with equal medians.
Mentions: We took spectral measurements of the fish as well as the adjacent substrate in the manner illustrated in figure 1a. We positioned our measurement points to capture as much of the variation as possible. (1) Fish measurements. Spectral measurements for each fish were taken in 20 locations along the anterior–posterior line, starting just behind the head, finishing just before the fin. To ensure we characterized the entire spectral signature of each flounder, we then took measurements of specific areas on the fish's body that are similar to the ‘morphological markers’ of Saidel [15]. These were often not located on the anterior–posterior line we routinely measured. Four such areas were identified: (i) white spot, (ii) area around white spot (or fish ‘background’), (iii) black spot, and (iv) area around black spot (figure 1b,c). Five measurements were taken of each area, for a total of 20 additional measurements. These areas could be shown by the fish with higher or lower intensity. For example, the white spots could vary between being large and conspicuous and being almost absent (i.e. taking on the colour of the surrounding body area). Similarly, the area around a black spot could be as black as the black spot itself; however, it could also take on the beige colour of the fish ‘background’ shade. We measured spectra from all areas, and grouped all of the fish spectra for a total of 40 measurements. (2) Substrate measurements. Ten spectral measurements were taken in a line in front of the head followed by 10 measurements behind the tail fin (figure 1a). We followed the anterior–posterior line of the fish when taking these substrate measurements. Additionally, we took 10 measurements of the substrate in a curve that was equidistant from the dorsal fin. In total, we took 30 measurements of the substrate. Figures 2 and 3 show all spectral data taken from one representative flounder on all four substrates for Paralichthys dentatus and Scophthalmus aquosus, respectively.Figure 1.

View Article: PubMed Central - PubMed

ABSTRACT

Flounder change colour and pattern for camouflage. We used a spectrometer to measure reflectance spectra and a digital camera to capture body patterns of two flounder species camouflaged on four natural backgrounds of different spatial scale (sand, small gravel, large gravel and rocks). We quantified the degree of spectral match between flounder and background relative to the situation of perfect camouflage in which flounder and background were assumed to have identical spectral distribution. Computations were carried out for three biologically relevant observers: monochromatic squid, dichromatic crab and trichromatic guitarfish. Our computations present a new approach to analysing datasets with multiple spectra that have large variance. Furthermore, to investigate the spatial match between flounder and background, images of flounder patterns were analysed using a custom program originally developed to study cuttlefish camouflage. Our results show that all flounder and background spectra fall within the same colour gamut and that, in terms of different observer visual systems, flounder matched most substrates in luminance and colour contrast. Flounder matched the spatial scales of all substrates except for rocks. We discuss findings in terms of flounder biology; furthermore, we discuss our methodology in light of hyperspectral technologies that combine high-resolution spectral and spatial imaging.

No MeSH data available.