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Chromatophore organs in cephalopod skin are known to produce ultra-fast changes in appearance for camouflage and communication. Light-scattering pigment granules within chromatocytes have been presumed to be the sole source of coloration in these complex organs. We report the discovery of structural coloration emanating in precise register with expanded pigmented chromatocytes. Concurrently, using an annotated squid chromatophore proteome together with microscopy, we identify a likely biochemical component of this reflective coloration as reflectin proteins distributed in sheath cells that envelop each chromatocyte. Additionally, within the chromatocytes, where the pigment resides in nanostructured granules, we find the lens protein Ω- crystallin interfacing tightly with pigment molecules. These findings offer fresh perspectives on the intricate biophotonic interplay between pigmentary and structural coloration elements tightly co-located within the same dynamic flexible organ - a feature that may help inspire the development of new classes of engineered materials that change color and pattern.

Sound is a widely available cue in aquatic environments and is used by many marine animals for vital behaviors. Most research has focused on marine vertebrates. Relatively little is known about sound detection in marine invertebrates despite their abundance and importance in marine environments. Cephalopods are a key taxon in many ecosystems, but their behavioral interactions relative to acoustic stimuli have seldom been studied. Here we review current knowledge regarding (1) the frequency ranges and sound levels that generate behavioral responses and (2) the types of behavioral responses and their biological relevance.

Octopus, squid, cuttlefish, and other cephalopods exhibit exceptional capabilities for visually adapting to or differentiating from the coloration and texture of their surroundings, for the purpose of concealment, communication, predation, and reproduction. Long-standing interest in and emerging understanding of the underlying ultrastructure, physiological control, and photonic interactions has recently led to efforts in the construction of artificial systems that have key attributes found in the skins of these organisms. Despite several promising options in active materials for mimicking biological color tuning, existing routes to integrated systems do not include critical capabilities in distributed sensing and actuation. Research described here represents progress in this direction, demonstrated through the construction, experimental study, and computational modeling of materials, device elements, and integration schemes for cephalopod-inspired flexible sheets that can autonomously sense and adapt to the coloration of their surroundings. These systems combine high-performance, multiplexed arrays of actuators and photodetectors in laminated, multilayer configurations on flexible substrates, with overlaid arrangements of pixelated, color-changing elements. The concepts provide realistic routes to thin sheets that can be conformally wrapped onto solid objects to modulate their visual appearance, with potential relevance to consumer, industrial, and military applications.

Adaptive reflective surfaces have been a challenge for both electronic paper (e-paper) and biological organisms. Multiple colours, contrast, polarization, reflectance, diffusivity and texture must all be controlled simultaneously without optical losses in order to fully replicate the appearance of natural surfaces and vividly communicate information. This review merges the frontiers of knowledge for both biological adaptive coloration, with a focus on cephalopods, and synthetic reflective e-paper within a consistent framework of scientific metrics. Currently, the highest performance approach for both nature and technology uses colourant transposition. Three outcomes are envisioned from this review: reflective display engineers may gain new insights from millions of years of natural selection and evolution; biologists will benefit from understanding the types of mechanisms, characterization and metrics used in synthetic reflective e-paper; all scientists will gain a clearer picture of the long-term prospects for capabilities such as adaptive concealment and signaling.

Polarization sensitivity is documented in a range of marine animals. The variety of tasks for which animals can use this sensitivity, and the range over which they do so, are confined by the visual systems of these animals and by the propagation of the polarization information in the aquatic environment. We examine the environmental physical constraints in an attempt to reveal the depth, range and other limitations to the use of polarization sensitivity by marine animals. In clear oceanic waters, navigation that is based on the polarization pattern of the sky appears to be limited to shallow waters, while solar-based navigation is possible down to 200-400 m. When combined with intensity difference, polarization sensitivity allows an increase in target detection range by 70-80% with an upper limit of 15 m for large-eyed animals. This distance will be significantly smaller for small animals, such as plankton, and in turbid waters. Polarization-contrast detection, which is relevant to object detection and communication, is strongly affected by water conditions and in clear waters its range limit may reach 15 m as well. We show that polarization sensitivity may also serve for target distance estimation, when examining point source bioluminescent objects in the photic mesopelagic depth range.

Cephalopods (squid, cuttlefish and octopus) are probably best known for their ability to change color and pattern for camouflage and communication. This is made possible by their complex skin, which contains pigmented chromatophore organs and structural light reflectors (iridophores and leucophores). Iridophores create colorful and linearly polarized reflective patterns. Equally interesting, the photoreceptors of cephalopod eyes are arranged in a way to give these animals the ability to detect the linear polarization of incoming light. The capacity to detect polarized light may have a variety of functions, such as prey detection, navigation, orientation and contrast enhancement. Because the skin of cephalopods can produce polarized reflective patterns, it has been postulated that cephalopods could communicate intraspecifically through this visual system. The term 'hidden' or 'private' communication channel has been given to this concept because many cephalopod predators may not be able to see their polarized reflective patterns. We review the evidence for polarization vision as well as polarization signaling in some cephalopod species and provide examples that tend to support the notion--currently unproven--that some cephalopods communicate using polarized light signals.

Octopus, squid and cuttlefish are renowned for rapid adaptive coloration that is used for a wide range of communication and camouflage. Structural coloration plays a key role in augmenting the skin patterning that is produced largely by neurally controlled pigmented chromatophore organs. While most iridescence and white scattering is produced by passive reflectance or diffusion, some iridophores in squid are actively controlled via a unique cholinergic, non-synaptic neural system. We review the recent anatomical and experimental evidence regarding the mechanisms of reflection and diffusion of light by the different cell types (iridophores and leucophores) of various cephalopod species. The structures that are responsible for the optical effects of some iridophores and leucophores have recently been shown to be proteins. Optical interactions with the overlying pigmented chromatophores are complex, and the recent measurements are presented and synthesized. Polarized light reflected from iridophores can be passed through the chromatophores, thus enabling the use of a discrete communication channel, because cephalopods are especially sensitive to polarized light. We illustrate how structural coloration contributes to the overall appearance of the cephalopods during intra- and interspecific behavioural interactions including camouflage.

Individual cuttlefish, octopus and squid have the versatile capability to use body patterns for background matching and disruptive coloration. We define--qualitatively and quantitatively--the chief characteristics of the three major body pattern types used for camouflage by cephalopods: uniform and mottle patterns for background matching, and disruptive patterns that primarily enhance disruptiveness but aid background matching as well. There is great variation within each of the three body pattern types, but by defining their chief characteristics we lay the groundwork to test camouflage concepts by correlating background statistics with those of the body pattern. We describe at least three ways in which background matching can be achieved in cephalopods. Disruptive patterns in cuttlefish possess all four of the basic components of 'disruptiveness', supporting Cott's hypotheses, and we provide field examples of disruptive coloration in which the body pattern contrast exceeds that of the immediate surrounds. Based upon laboratory testing as well as thousands of images of camouflaged cephalopods in the field (a sample is provided on a web archive), we note that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Mottle and disruptive patterns are frequently mixed, suggesting that background matching and disruptive mechanisms are often used in the same pattern.

A mathematical model of multiple layer skin coloration in cephalopods, a class of aquatic animals, is presented. The model incorporates diffuse and specular reflection from both pigment and structural photonic components found in the skin of these animals. Specific physical processes of this coloration are identified and modeled utilizing available biological materials data. Several examples of combination spectra are calculated to illustrate multiple layer and incident light effects as well as the potentially rich repertoire of color schemes available to these animals. A detailed understanding of the physical principles underlying cephalopod coloration is expected to yield insights into their possible functions.

We have developed a mathematical model of skin coloration in cephalopods, a class of aquatic animals. Cephalopods utilize neurological and physiological control of various skin components to achieve active camouflage and communication. Specific physical processes of this coloration are identified and modeled, utilizing available biological materials data, to simulate active spectral changes in pigment-bearing organs and structural iridescent cells. Excellent agreement with in vitro measurements of squid skin is obtained. A detailed understanding of the physical principles underlying cephalopod coloration is expected to yield insights into the behavioral ecology of these animals.