Neural control of tuneable skin iridescence in squid

Direct Digital Photonic Patterning of Hydrogen-Bonded Cholesteric Liquid-Crystalline Films

A modular tool box for photoresponsive cholesteric liquid crystals based on hydrogen-bonded assemblies is reported. By employing 3-azopyridines as photoswitch in cholesteric liquid-crystalline thin films, a fast and reversible blue shift is observed upon irradiation, allowing tuning of the structural color over the whole visible electromagnetic spectrum. Investigations of the materials via X-ray diffraction studies indicate that the blue shift is attributed to the photoinduced destruction of smectic clusters in the cholesteric phase, resulting in a contraction of the helical structure. Unprecedently, the use of a stereolithography 3D printer (SLA) allowed direct transfer of digital information into a multicolor photonic pattern, an important step toward photonic imaging and data storage.

Centrifugation-Assisted Growth of Single-Crystalline Grains in Microcapsules

Colloidal crystals have been tailored in a format of microspheres to use them as a building block to construct macroscopic photonic surfaces. However, the polycrystalline grains grown from the spherical surface usually exhibit low reflectivity. Although single-crystalline microspheres have been produced, it is difficult to control the crystal orientation. Here, we design spherical microcapsules with density anisotropy that contain single-crystalline grains along the heavy side. The microcapsules spontaneously align to have a heavy side down under the action of gravity and display a bright and uniform reflection color from the entire surface of the grains. Key to the success is the use of gentle centrifugal force to initiate nucleation and grow single-crystalline grains from the heavy side through depletion attraction. The microcapsules have density anisotropy due to the heterogeneity of the shell thickness, which causes them to self-align under centrifugation. At the same time, particles are accumulated on the heavy side, which produces many tiny grains on the heavy side immediately after the centrifugation. With controlled depletion attraction among particles, only a few grains survive during postincubation through Ostwald ripening, and one or a few giant single-crystalline grains are finally produced along the heavy side of each microcapsule.

Multilayered optofluidics for sustainable buildings

Indoor climate control is among the most energy-intensive activities conducted by humans. A building facade that can achieve versatile climate control directly, through independent and multifunctional optical reconfigurations, could significantly reduce this energy footprint, and its development represents a pertinent unmet challenge toward global sustainability. Drawing from optically adaptive multilayer skins within biological organisms, we report a multilayered millifluidic interface for achieving a comprehensive suite of independent optical responses in buildings. We digitally control the flow of aqueous solutions within confined milliscale channels, demonstrating independent command over total transmitted light intensity (95% modulation between 250 and 2,500 nm), near-infrared-selective absorption (70% modulation between 740 and 2,500 nm), and dispersion (scattering). This combinatorial optical tunability enables configurable optimization of the amount, wavelength, and position of transmitted solar radiation within buildings over time, resulting in annual modeled energy reductions of more than 43% over existing technologies. Our scalable "optofluidic" platform, leveraging a versatile range of aqueous chemistries, may represent a general solution for the climate control of buildings.

Molecular characterization of cell types in the squid Loligo vulgaris

Cephalopods are set apart from other mollusks by their advanced behavioral abilities and the complexity of their nervous systems. Because of the great evolutionary distance that separates vertebrates from cephalopods, it is evident that higher cognitive features have evolved separately in these clades despite the similarities that they share. Alongside their complex behavioral abilities, cephalopods have evolved specialized cells and tissues, such as the chromatophores for camouflage or suckers to grasp prey. Despite significant progress in genome and transcriptome sequencing, the molecular identities of cell types in cephalopods remain largely unknown. We here combine single-cell transcriptomics with in situ gene expression analysis to uncover cell type diversity in the European squid Loligo vulgaris. We describe cell types that are conserved with other phyla such as neurons, muscles, or connective tissues but also cephalopod-specific cells, such as chromatophores or sucker cells. Moreover, we investigate major components of the squid nervous system including progenitor and developing cells, differentiated cells of the brain and optic lobes, as well as sensory systems of the head. Our study provides a molecular assessment for conserved and novel cell types in cephalopods and a framework for mapping the nervous system of L. vulgaris.

Designing photonic microparticles with droplet microfluidics

Photonic materials with a periodic change of refractive index show unique optical properties through wavelength-selective diffraction and modulation of the optical density of state, which is promising for various optical applications. In particular, photonic structures have been produced in the format of microparticles using emulsion templates to achieve advanced properties and applications beyond those of a conventional film format. Photonic microparticles can be used as a building block to construct macroscopic photonic materials, and the individual microparticles can serve as miniaturized photonic devices. Droplet microfluidics enables the production of emulsion drops with a controlled size, composition, and configuration that serve as the optimal confining geometry for designing photonic microparticles. This feature article reviews the recent progress and current state of the art in the field of photonic microparticles, covering all aspects of microfluidic production methods, microparticle geometries, optical properties, and applications. Two distinct bottom-up approaches based on colloidal assembly and liquid crystals are, respectively, discussed and compared.

At the Intersection of Natural Structural Coloration and Bioengineering

Most of us get inspired by and interact with the world around us based on visual cues such as the colors and patterns that we see. In nature, coloration takes three primary forms: pigmentary coloration, structural coloration, and bioluminescence. Typically, pigmentary and structural coloration are used by animals and plants for their survival; however, few organisms are able to capture the nearly instantaneous and visually astounding display that cephalopods (e.g., octopi, squid, and cuttlefish) exhibit. Notably, the structural coloration of these cephalopods critically relies on a unique family of proteins known as reflectins. As a result, there is growing interest in characterizing the structure and function of such optically-active proteins (e.g., reflectins) and to leverage these materials across a broad range of disciplines, including bioengineering. In this review, I begin by briefly introducing pigmentary and structural coloration in animals and plants as well as highlighting the extraordinary appearance-changing capabilities of cephalopods. Next, I outline recent advances in the characterization and utilization of reflectins for photonic technologies and and discuss general strategies and limitations for the structural and optical characterization of proteins. Finally, I explore future directions of study for optically-active proteins and their potential applications. Altogether, this review aims to bring together an interdisciplinary group of researchers who can resolve the fundamental questions regarding the structure, function, and self-assembly of optically-active protein-based materials.

In situ measurements of reef squid polarization patterns using two-dimensional polarization data mapped onto three-dimensional tessellated surfaces

Reef squids belong to a group reputed for polarization sensitivity, yet polarization patterns of reef squid have not been quantified in situ. To quantify polarization patterns from video polarimetric data, we developed a protocol to map two-dimensional polarization data onto squid-shaped three-dimensional tessellated surfaces. This protocol provided a robust data container used to investigate three-dimensional regions-of-interest, producing data lineouts derived from the squid's geometry. This protocol also extracted polarimeter and squid body orientations and the solar heading from polarization images. When averaged over the solar heading, the ventral midline gave a low degree of polarization (2.4 ± 5.3%), and the area between the ventral and flank midlines had higher values (9.0 ± 5.3%). These averaged data had a large discontinuity in the angle of polarization (AoP) at the mantle's ventral midline (64 ± 55°), with larger discontinuities measured on individual squid. Ray-tracing calculations demonstrated that the AoP pattern was not related to the squid's surface-normal geometry. However, the AoP followed virtual striation axes on the squid's surface oriented 24° to the squid's long axis, similar in angle (27°) to the striations of birefringent collagen fibres documented in other squid species' skin.

Bioinspired Dynamic Camouflage from Colloidal Nanocrystals Embedded Electrochromics

Camouflage is often seen in animals, and it presents in both passive and active forms. For instance, the wings of Closterocerus coffeellae exhibit distinct appearances against different backgrounds, while the chameleon actively changes its skin colors to morph into the environment. Herein, we report an artificial skin-like optoelectronic device that enables actively changing appearances and passively morphing into the environment by manipulating light-matter interactions with electrochromic polymers and photonic colloid nanocrystals. To construct the new electrochromic device, highly reflective, yet transmissive photonic nanocrystals are introduced into the gel electrolyte and sandwiched between the layers of electrochromic polymers and ion storage materials. Through voltage-controlled color switching of electrochromic polymers from colored state to bleached state, the degree of light absorbance, transmittance, and reflectance can be finely balanced and precisely modulated with the device. A broad synthesized color gamut and angle-dependent visual effects can be realized on this electronic skin-like device.

Flashing spots on the dorsal trunk of hardyhead silverside fish

A large number of living creatures are able to use ambient light effectively in biological signalling. Atherinomorus lacunosus, a teleost fish has alignments of circular spots on its dorsal trunk. The spot consists of iridophores, whose diameters are approximately 7-10 µm. The iridophore contains guanine crystals with diameters of 1-3 µm. Here, it is found that more than one spot with a diameter of approximately 0.1 mm causes a rhythmic flashing of light when viewed under white light. The typical light flash has a pulse width of approximately one second. When a pulsed train of flashes appears, the flash repeats at a typical frequency of 0.5-1 Hz. The observed phenomenon is one example of the evidence for the existence of rapid colour changing teleost fish.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Light organ photosensitivity in deep-sea shrimp may suggest a novel role in counterillumination

Extraocular photoreception, the ability to detect and respond to light outside of the eye, has not been previously described in deep-sea invertebrates. Here, we investigate photosensitivity in the bioluminescent light organs (photophores) of deep-sea shrimp, an autogenic system in which the organism possesses the substrates and enzymes to produce light. Through the integration of transcriptomics, in situ hybridization and immunohistochemistry we find evidence for the expression of opsins and phototransduction genes known to play a role in light detection in most animals. Subsequent shipboard light exposure experiments showed ultrastructural changes in the photophore similar to those seen in crustacean eyes, providing further evidence that photophores are light sensitive. In many deep-sea species, it has long been documented that photophores emit light to aid in counterillumination - a dynamic form of camouflage that requires adjusting the organ's light intensity to "hide" their silhouettes from predators below. However, it remains a mystery how animals fine-tune their photophore luminescence to match the intensity of downwelling light. Photophore photosensitivity allows us to reconsider the organ's role in counterillumination - not only in light emission but also light detection and regulation.

A draft genome sequence of the elusive giant squid, Architeuthis dux

BACKGROUND

The giant squid (Architeuthis dux; Steenstrup, 1857) is an enigmatic giant mollusc with a circumglobal distribution in the deep ocean, except in the high Arctic and Antarctic waters. The elusiveness of the species makes it difficult to study. Thus, having a genome assembled for this deep-sea-dwelling species will allow several pending evolutionary questions to be unlocked.

FINDINGS

We present a draft genome assembly that includes 200 Gb of Illumina reads, 4 Gb of Moleculo synthetic long reads, and 108 Gb of Chicago libraries, with a final size matching the estimated genome size of 2.7 Gb, and a scaffold N50 of 4.8 Mb. We also present an alternative assembly including 27 Gb raw reads generated using the Pacific Biosciences platform. In addition, we sequenced the proteome of the same individual and RNA from 3 different tissue types from 3 other species of squid (Onychoteuthis banksii, Dosidicus gigas, and Sthenoteuthis oualaniensis) to assist genome annotation. We annotated 33,406 protein-coding genes supported by evidence, and the genome completeness estimated by BUSCO reached 92%. Repetitive regions cover 49.17% of the genome.

CONCLUSIONS

This annotated draft genome of A. dux provides a critical resource to investigate the unique traits of this species, including its gigantism and key adaptations to deep-sea environments.

Cellulose-Based Microparticles for Magnetically Controlled Optical Modulation and Sensing

Responsive materials with birefringent optical properties have been exploited for the manipulation of light in several modern electronic devices. While electrical fields are often utilized to achieve optical modulation, magnetic stimuli may offer an enticing complementary approach for controlling and manipulating light remotely. Here, the synthesis and characterization of magnetically responsive birefringent microparticles with unusual magneto-optical properties are reported. These functional microparticles are prepared via a microfluidic emulsification process, in which water-based droplets are generated in a flow-focusing device and stretched into anisotropic shapes before conversion into particles via photopolymerization. Birefringence properties are achieved by aligning cellulose nanocrystals within the microparticles during droplet stretching, whereas magnetic responsiveness results from the addition of superparamagnetic nanoparticles to the initial droplet template. When suspended in a fluid, the microparticles can be controllably manipulated via an external magnetic field to result in unique magneto-optical coupling effects. Using a remotely actuated magnetic field coupled to a polarized optical microscope, these microparticles can be employed to convert magnetic into optical signals or to estimate the viscosity of the suspending fluid through magnetically driven microrheology.

The Current State of Cephalopod Science and Perspectives on the Most Critical Challenges Ahead From Three Early-Career Researchers

Here, three researchers who have recently embarked on careers in cephalopod biology discuss the current state of the field and offer their hopes for the future. Seven major topics are explored: genetics, aquaculture, climate change, welfare, behavior, cognition, and neurobiology. Recent developments in each of these fields are reviewed and the potential of emerging technologies to address specific gaps in knowledge about cephalopods are discussed. Throughout, the authors highlight specific challenges that merit particular focus in the near-term. This review and prospectus is also intended to suggest some concrete near-term goals to cephalopod researchers and inspire those working outside the field to consider the revelatory potential of these remarkable creatures.

Adaptive infrared-reflecting systems inspired by cephalopods

Materials and systems that statically reflect radiation in the infrared region of the electromagnetic spectrum underpin the performance of many entrenched technologies, including building insulation, energy-conserving windows, spacecraft components, electronics shielding, container packaging, protective clothing, and camouflage platforms. The development of their adaptive variants, in which the infrared-reflecting properties dynamically change in response to external stimuli, has emerged as an important unmet scientific challenge. By drawing inspiration from cephalopod skin, we developed adaptive infrared-reflecting platforms that feature a simple actuation mechanism, low working temperature, tunable spectral range, weak angular dependence, fast response, stability to repeated cycling, amenability to patterning and multiplexing, autonomous operation, robust mechanical properties, and straightforward manufacturability. Our findings may open opportunities for infrared camouflage and other technologies that regulate infrared radiation.

Cephalopod-Derived Biopolymers for Ionic and Protonic Transistors

Cephalopods (e.g., squid, octopuses, and cuttlefish) have long fascinated scientists and the general public alike due to their complex behavioral characteristics and remarkable camouflage abilities. As such, these animals are explored as model systems in neuroscience and represent a well-known commercial resource. Herein, selected literature examples related to the electrical properties of cephalopod-derived biopolymers (eumelanins, chitosans, and reflectins) and to the use of these materials in voltage-gated devices (i.e., transistors) are highlighted. Moreover, some potential future directions and challenges in this area are described, with the aim of inspiring additional research effort on ionic and protonic transistors from cephalopod-derived biopolymers.

Polarisation vision: overcoming challenges of working with a property of light we barely see

In recent years, the study of polarisation vision in animals has seen numerous breakthroughs, not just in terms of what is known about the function of this sensory ability, but also in the experimental methods by which polarisation can be controlled, presented and measured. Once thought to be limited to only a few animal species, polarisation sensitivity is now known to be widespread across many taxonomic groups, and advances in experimental techniques are, in part, responsible for these discoveries. Nevertheless, its study remains challenging, perhaps because of our own poor sensitivity to the polarisation of light, but equally as a result of the slow spread of new practices and methodological innovations within the field. In this review, we introduce the most important steps in designing and calibrating polarised stimuli, within the broader context of areas of current research and the applications of new techniques to key questions. Our aim is to provide a constructive guide to help researchers, particularly those with no background in the physics of polarisation, to design robust experiments that are free from confounding factors.

Neural Control of Dynamic 3-Dimensional Skin Papillae for Cuttlefish Camouflage

The color and pattern changing abilities of octopus, squid, and cuttlefish via chromatophore neuromuscular organs are unparalleled. Cuttlefish and octopuses also have a unique muscular hydrostat system in their skin. When this system is expressed, dermal bumps called papillae disrupt body shape and imitate the fine texture of surrounding objects, yet the control system is unknown. Here we report for papillae: (1) the motoneurons and the neurotransmitters that control activation and relaxation, (2) a physiologically fast expression and retraction system, and (3) a complex of smooth and striated muscles that enables long-term expression of papillae through sustained tension in the absence of neural input. The neural circuits controlling acute shape-shifting skin papillae in cuttlefish show homology to the iridescence circuits in squids. The sustained tension in papillary muscles for long-term camouflage utilizes muscle heterogeneity and points toward the existence of a "catch-like" mechanism that would reduce the necessary energy expenditure.

White reflection from cuttlefish skin leucophores

The highly diverse and changeable body patterns of cephalopods require the production of whiteness of varying degrees of brightness for their large repertoire of communication and camouflage behaviors. Leucophores are structural reflectors that produce whiteness in cephalopods; they are dermal aggregates of numerous leucocytes containing spherical leucosomes ranging in diameter from 200-2000 nm. In Sepia officinalis leucophores, leucocytes always occur in various combinations with iridocytes, cells containing plates that function as Bragg stacks to reflect light of particular wavelengths. Both spheres and plates contain the high-refractive-index protein reflectin. Four leucophore skin-patterning components were investigated morphologically and with spectrometry. In descending order of brightness they are: white fin spots, White zebra bands, White square, and White head bar. Different densities, thicknesses and proportions of leucocytes and iridocytes were correlated with the relative brightness measurements of the skin. That is, White fin spots and White zebra bands had leucocytes of the highest density, the greatest number of reflective cell layers, and the highest proportion of leucocytes to iridocytes. In contrast, the White square and White head bar had the lowest density of reflective cells, fewer cell layers and the lowest ratios of leucocytes to iridocytes. Leucophores are white in white light, yet reflect whatever colors are in the available light field: e.g. red in red light, green in green light, etc. Leucophores are physiologically passive, thus their ultrastructure alone is capable of diffusing all ambient wavelengths in all directions, regardless of the angle of incident light. However, the specific optical contributions of spherical leucosomes versus the associated plate-like iridosomes in producing whiteness versus brightness are yet to be determined. This study reveals complex morphological arrangements that produce white structural coloration for different brightnesses of skin by differentially combining spheres and plates.

Cyclable Condensation and Hierarchical Assembly of Metastable Reflectin Proteins, the Drivers of Tunable Biophotonics

Reversible changes in the phosphorylation of reflectin proteins have been shown to drive the tunability of color and brightness of light reflected from specialized cells in the skin of squids and related cephalopods. We show here, using dynamic light scattering, electron microscopy, and fluorescence analyses, that reversible titration of the excess positive charges of the reflectins, comparable with that produced by phosphorylation, is sufficient to drive the reversible condensation and hierarchical assembly of these proteins. The results suggest a two-stage process in which charge neutralization first triggers condensation, resulting in the emergence of previously cryptic structures that subsequently mediate reversible, hierarchical assembly. The extent to which cyclability is seen in the in vitro formation and disassembly of complexes estimated to contain several thousand reflectin molecules suggests that intrinsic sequence- and structure-determined specificity governs the reversible condensation and assembly of the reflectins and that these processes are therefore sufficient to produce the reversible changes in refractive index, thickness, and spacing of the reflectin-containing subcellular Bragg lamellae to change the brightness and color of reflected light. This molecular mechanism points to the metastability of reflectins as the centrally important design principle governing biophotonic tunability in this system.

An Unexpected Diversity of Photoreceptor Classes in the Longfin Squid, Doryteuthis pealeii

Cephalopods are famous for their ability to change color and pattern rapidly for signaling and camouflage. They have keen eyes and remarkable vision, made possible by photoreceptors in their retinas. External to the eyes, photoreceptors also exist in parolfactory vesicles and some light organs, where they function using a rhodopsin protein that is identical to that expressed in the retina. Furthermore, dermal chromatophore organs contain rhodopsin and other components of phototransduction (including retinochrome, a photoisomerase first found in the retina), suggesting that they are photoreceptive. In this study, we used a modified whole-mount immunohistochemical technique to explore rhodopsin and retinochrome expression in a number of tissues and organs in the longfin squid, Doryteuthis pealeii. We found that fin central muscles, hair cells (epithelial primary sensory neurons), arm axial ganglia, and sucker peduncle nerves all express rhodopsin and retinochrome proteins. Our findings indicate that these animals possess an unexpected diversity of extraocular photoreceptors and suggest that extraocular photoreception using visual opsins and visual phototransduction machinery is far more widespread throughout cephalopod tissues than previously recognized.

Structural colour in Chondrus crispus

The marine world is incredibly rich in brilliant and intense colours. Photonic structures are found in many different species and provide extremely complex optical responses that cannot be achieved solely by pigments. In this study we examine the cuticular structure of the red alga Chondrus crispus (Irish Moss) using anatomical and optical approaches. We experimentally measure the optical response of the multilayer structure in the cuticle. Using finite-difference time-domain modelling, we demonstrate conclusively for the first time that the dimensions and organisation of lamellae are responsible for the blue structural colouration on the surface of the fronds. Comparison of material along the apical-basal axis of the frond demonstrates that structural colour is confined to the tips of the thalli and show definitively that a lack of structural colour elsewhere corresponds with a reduction in the number of lamellae and the regularity of their ordering. Moreover, by studying the optical response for different hydration conditions, we demonstrate that the cuticular structure is highly porous and that the presence of water plays a critical role in its ability to act as a structural light reflector.

Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells

The reversible assembly of reflectin proteins drives dynamic iridescence in cephalopods. Squid dynamically tune the intensity and colors of iridescence generated by constructive interference from intracellular Bragg reflectors in specialized skin cells called iridocytes. Analysis of the tissue specificity of reflectin subtypes reveals that tunability is correlated with the presence of one specific reflectin sequence. Differential phosphorylation and dephosphorylation of the reflectins in response to activation by acetylcholine, as well as differences in their tissue-specific and subcellular spatial distributions, further support the suggestion of different roles for the different reflectin subtypes.

Expression of squid iridescence depends on environmental luminance and peripheral ganglion control

Squid display impressive changes in body coloration that are afforded by two types of dynamic skin elements: structural iridophores (which produce iridescence) and pigmented chromatophores. Both color elements are neurally controlled, but nothing is known about the iridescence circuit, or the environmental cues, that elicit iridescence expression. To tackle this knowledge gap, we performed denervation, electrical stimulation and behavioral experiments using the long-fin squid, Doryteuthis pealeii. We show that while the pigmentary and iridescence circuits originate in the brain, they are wired differently in the periphery: (1) the iridescence signals are routed through a peripheral center called the stellate ganglion and (2) the iridescence motor neurons likely originate within this ganglion (as revealed by nerve fluorescence dye fills). Cutting the inputs to the stellate ganglion that descend from the brain shifts highly reflective iridophores into a transparent state. Taken together, these findings suggest that although brain commands are necessary for expression of iridescence, integration with peripheral information in the stellate ganglion could modulate the final output. We also demonstrate that squid change their iridescence brightness in response to environmental luminance; such changes are robust but slow (minutes to hours). The squid's ability to alter its iridescence levels may improve camouflage under different lighting intensities.

Experimental determination of refractive index of condensed reflectin in squid iridocytes

Loliginid squid dynamically tune the structural iridescence of cells in their skin for active camouflage and communication. Bragg reflectors in these cells consist of membrane-bound lamellae periodically alternating with low refractive index extracellular spaces; neuronal signalling induces condensation of the reflectin proteins that fill the lamellae, consequently triggering the expulsion of water. This causes an increase in refractive index within the lamellae, activating reflectance, with the change in lamellar thickness and spacing progressively shifting the wavelength of reflected light. We used micro-spectrophotometry to measure the functionally relevant refractive index of the high-index lamellae of the Bragg reflectors containing the condensed reflectins in chemically fixed dermal iridocytes of the squid, Doryteuthis opalescens. Our high-magnification imaging spectrometer allowed us to obtain normalized spectra of optically distinct sections of the individual, subcellular, multi-layer Bragg stacks. Replacement of the extracellular fluid with liquids of increasing refractive index allowed us to measure the reflectivity of the Bragg stacks as it decreased progressively to 0 when the refractive index of the extracellular medium exactly matched that of the reflectin-filled lamellae, thus allowing us to directly measure the refractive index of the reflectin-filled lamellae as ncondensed lamellae ≈ 1.44. The measured value of the physiologically relevant ncondensed lamellae from these bright iridocytes falls within the range of values that we recently determined by an independent optical method and is significantly lower than values previously reported for dehydrated and air-dried reflectin films. We propose that this directly measured value for the refractive index of the squid's Bragg lamellae containing the condensed reflectins is most appropriate for calculations of reflectivity in similar reflectin-based high-index layers in other molluscs.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons

The engineering of optical and acoustic material functionalities via construction of ordered local and global architectures on various length scales commensurate with and well below the characteristic length scales of photons and phonons in the material is an indispensable and powerful means to develop novel materials. In the current mature status of photonics, polymers hold a pivotal role in various application areas such as light-emission, sensing, energy, and displays, with exclusive advantages despite their relatively low dielectric constants. Moreover, in the nascent field of phononics, polymers are expected to be a superior material platform due to the ability for readily fabricated complex polymer structures possessing a wide range of mechanical behaviors, complete phononic bandgaps, and resonant architectures. In this review, polymer-centric photonic and phononic crystals and metamaterials are highlighted, and basic concepts, fabrication techniques, selected functional polymers, applications, and emerging ideas are introduced.

Optical parameters of the tunable Bragg reflectors in squid

Cephalopods (e.g. octopus, squid and cuttlefish) dynamically tune the colour and brightness of their skin for camouflage and communication using specialized skin cells called iridocytes. We use high-resolution microspectrophotometry to investigate individual tunable Bragg structures (consisting of alternating reflectin protein-containing, high-refractive index lamellae and low-refractive index inter-lamellar spaces) in live and chemically fixed iridocytes of the California market squid, Doryteuthis opalescens. This subcellular, single-stack microspectrophotometry allows for spectral normalization, permitting use of a transfer-matrix model of Bragg reflectance to calculate all the parameters of the Bragg stack-the refractive indices, dimensions and numbers of the lamellae and inter-lamellar spaces. Results of the fitting analyses show that eight or nine pairs of low- and high-index layers typically contribute to the observed reflectivity in live cells, whereas six or seven pairs of low- and high-index layers typically contribute to the reflectivity in chemically fixed cells. The reflectin-containing, high-index lamellae of live cells have a refractive index proportional to the peak reflectivity, with an average of 1.405 ± 0.012 and a maximum around 1.44, while the reflectin-containing lamellae in fixed tissue have a refractive index of 1.413 ± 0.015 suggesting a slight increase of refractive index in the process of fixation. As expected, incremental changes in refractive index contribute to the greatest incremental changes in reflectivity for those Bragg stacks with the most layers. The excursions in dimensions required to tune the measured reflected wavelength from 675 (red) to 425 nm (blue) are a decrease from ca 150 to 80 nm for the high-index lamellae and from ca 120 to 50 nm for the low-index inter-lamellar spaces. Fixation-induced dimensional changes also are quantified, leading us to suggest that further microspectrophotometric analyses of this iridocyte system can be used as a model system to quantify the effects of various methods of tissue fixation. The microspectrophotometry technique described can be expected to provide deeper insights into the molecular and physical mechanisms governing other biophotonically active cells and structures.

Membrane invaginations facilitate reversible water flux driving tunable iridescence in a dynamic biophotonic system

Squids have used their tunable iridescence for camouflage and communication for millions of years; materials scientists have more recently looked to them for inspiration to develop new "biologically inspired" adaptive optics. Iridocyte cells produce iridescence through constructive interference of light with intracellular Bragg reflectors. The cell's dynamic control over the apparent lattice constant and dielectric contrast of these multilayer stacks yields the corresponding optical control of brightness and color across the visible spectrum. Here, we resolve remaining uncertainties in iridocyte cell structure and determine how this unusual morphology enables the cell's tunable reflectance. We show that the plasma membrane periodically invaginates deep into the iridocyte to form a potential Bragg reflector consisting of an array of narrow, parallel channels that segregate the resulting high refractive index, cytoplasmic protein-containing lamellae from the low-index channels that are continuous with the extracellular space. In response to control by a neurotransmitter, the iridocytes reversibly imbibe or expel water commensurate with changes in reflection intensity and wavelength. These results allow us to propose a comprehensive mechanism of adaptive iridescence in these cells from stimulation to color production. Applications of these findings may contribute to the development of unique classes of tunable photonic materials.

Immunohistochemical localization of two types of choline acetyltransferase in neurons and sensory cells of the octopus arm

Cholinergic structures in the arm of the cephalopod Octopus vulgaris were studied by immunohistochemistry using specific antisera for two types (common and peripheral) of acetylcholine synthetic enzyme choline acetyltransferase (ChAT): antiserum raised against the rat common type ChAT (cChAT), which is cross-reactive with molluscan cChAT, and antiserum raised against the rat peripheral type ChAT (pChAT), which has been used to delineate peripheral cholinergic structures in vertebrates, but not previously in invertebrates. Western blot analysis of octopus extracts revealed a single pChAT-positive band, suggesting that pChAT antiserum is cross-reactive with an octopus counterpart of rat pChAT. In immunohistochemistry, only neuronal structures of the octopus arm were stained by cChAT and pChAT antisera, although the pattern of distribution clearly differed between the two antisera. cChAT-positive varicose nerve fibers were observed in both the cerebrobrachial tract and neuropil of the axial nerve cord, while pChAT-positive varicose fibers were detected only in the neuropil of the axial nerve cord. After epitope retrieval, pChAT-positive neuronal cells and their processes became visible in all ganglia of the arm, including the axial and intramuscular nerve cords, and in ganglia of suckers. Moreover, pChAT-positive structures also became detectable in nerve fibers connecting the different ganglia, in smooth nerve fibers among muscle layers and dermal connective tissues, and in sensory cells of the suckers. These results suggest that the octopus arm has two types of cholinergic nerves: cChAT-positive nerves from brain ganglia and pChAT-positive nerves that are intrinsic to the arm.

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

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 5 m. 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.