Quantification of cuttlefish (Sepia officinalis) camouflage: a study of color and luminance using in situ spectrometry

Abstract

Cephalopods are renowned for their ability to adaptively camouflage on diverse backgrounds.Sepia officinalis camouflage body patterns have been characterized spectrally in the laboratory but not in the field due to the challenges of dynamic natural light fields and the difficulty of using spectrophotometric instruments underwater. To assess cuttlefish color match in their natural habitats, we studied the spectral properties of S. officinalis and their backgrounds on the Aegean coast of Turkey using point-by-point in situ spectrometry. Fifteen spectrometry datasets were collected from seven cuttlefish; radiance spectra from animal body components and surrounding substrates were measured at depths shallower than 5m. We quantified luminance and color contrast of cuttlefish components and background substrates in the eyes of hypothetical di-and trichromatic fish predators. Additionally, we converted radiance spectra to sRGB color space to simulate their in situ appearance to a human observer. Within the range of natural colors at our study site, cuttlefish closely matched the substrate spectra in a variety of body patterns.Theoretical calculations showed that this effect might be more pronounced at greater depths. We also showed that a non-biological method ("Spectral Angle Mapper"), commonly used for spectral shape similarity assessment in the field of remote sensing, shows moderate correlation to biological measures of color contrast. This performance is comparable to that of a traditional measure of spectral shape similarity, hue and chroma. This study is among the first to quantify color matching of camouflaged cuttlefish in the wild.3

Spectral discrimination in color blind animals via chromatic aberration and pupil shape

“…The most definitive recent evidence for color matching in a laboratory setting used (14) a hyperspectral imager in conjunction with spectral angle mapping to show that cuttlefish varied their spectral reflectance (chromatic properties) to maintain excellent spectral matches to a diversity of natural backgrounds and interestingly, maintained poorer matches in brightness (luminance). These studies (14,(17)(18)(19)(20) corroborate the earlier result by Kühn (21): cephalopods vary their spectral reflectance by active control over their chromatophores in response to natural backgrounds rather than simply varying their luminance.…”

“…Contemporary laboratory and field observations (17)(18)(19)(20) show that octopus and cuttlefish produce high-fidelity color matches to natural backgrounds (Fig. 1).…”

“…Some had claimed (12) that these organisms simply match the brightness and spatial scale of patterns in their environment, tricking the human visual system without actually requiring a color match. Numerous studies (14,(17)(18)(19)(20)(21) show, however, that cuttlefish and octopus actively vary their spectral reflectance in response to background color rather than simply modulating their luminance.…”

A computational approach to the quantification of animal camouflage

“…Here we give an example for case (1): capture of the colors of the skin of a camouflaged animal exactly the way an observer in situ would have seen them. For this application, we surveyed a dive site and built a database of substrate reflectance and light spectra (Akkaynak et al 2013). Novel (raw) images taken at the same site were white balanced to match the white point of the ambient light.…”

“…It is often useful to assess the similarity (or, dissimilarity) of two spectra without referencing a biological visual system (Akkaynak et al 2013). In this paper, the mathematical dissimilarity between the pure and recorded version of a spectrum is used as an objective measure of contamination, which is quantified using the Spectral Angle Mapper (SAM) metric (Yuhas et al 1992):…”