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

Dynamic skin behaviors in cephalopods

The coleoid cephalopods (cuttlefish, octopus, and squid) are a group of soft-bodied mollusks that exhibit a wealth of complex behaviors, including dynamic camouflage, object mimicry, skin-based visual communication, and dynamic body patterns during sleep. Many of these behaviors are visually driven and engage the animals' color changing skin, a pixelated display that is directly controlled by neurons projecting from the brain. Thus, cephalopod skin provides a direct readout of neural activity in the brain. During camouflage, cephalopods recreate on their skin an approximation of what they see, providing a window into perceptual processes in the brain. Additionally, cephalopods communicate their internal state during social encounters using innate skin patterns, and create waves of pigmentation on their skin during periods of arousal. Thus, by leveraging the visual displays of cephalopods, we can gain insight into how the external world is represented in the brain and how this representation is transformed into a recapitulation of the world on the skin. Here, we describe the rich skin behaviors of the coleoid cephalopods, what is known about cephalopod neuroanatomy, and how advancements in gene editing, machine learning, optical imaging, and electrophysiological tools may provide an opportunity to explore the neural bases of these fascinating behaviors.

Toward an Understanding of Octopus Arm Motor Control

Octopuses have the extraordinary ability to control eight prehensile arms with hundreds of suckers. With these highly flexible limbs, they engage in a wide variety of tasks, including hunting, grooming, and exploring their environment. The neural circuitry generating these movements engages every division of the octopus nervous system, from the nerve cords of the arms to the supraesophegeal brain. In this review, the current knowledge on the neural control of octopus arm movements is discussed, highlighting open questions and areas for further study.

Neural control of cephalopod camouflage

In Die Another Day, James Bond receives an Aston Martin that can render itself invisible by dynamically reproducing the surroundings on the car's "polymer skin". In what is widely regarded as the worst Bond movie ever, the invisible car scene is cited as the moment the plot plunges into the truly absurd. But what if nature had actually invented such a technology, and did so hundreds of millions of years ago? The coleoid cephalopods - octopus, cuttlefish and squid - are living examples of dynamic camouflage. Their skin is covered with a high-resolution array of 'cellular pixels' (chromatophores) that are controlled by the brain. To disappear into their surroundings, cephalopods recreate an approximation of their environment on their skin by activating different combinations of colored chromatophores. However, unlike the fictional Bond car, whose surface is coated in tiny cameras to detect the environment, cephalopods don't see the world with their skin. Instead, the visual world is detected by the eyes, processed in the brain, and then used to activate motor commands that direct the skin's camouflage pattern. Thus, cephalopod skin patterns are an external manifestation of their internal perception of the world. How do cephalopods approximate the world with their skin? What can this teach us about how brains work? And which neurobiological tools will be needed to uncover the neural basis of camouflage?

Wake-like skin patterning and neural activity during octopus sleep

While sleeping, many vertebrate groups alternate between at least two sleep stages: rapid eye movement and slow wave sleep1-4, in part characterized by wake-like and synchronous brain activity, respectively. Here we delineate neural and behavioural correlates of two stages of sleep in octopuses, marine invertebrates that evolutionarily diverged from vertebrates roughly 550 million years ago (ref. 5) and have independently evolved large brains and behavioural sophistication. 'Quiet' sleep in octopuses is rhythmically interrupted by approximately 60-s bouts of pronounced body movements and rapid changes in skin patterning and texture6. We show that these bouts are homeostatically regulated, rapidly reversible and come with increased arousal threshold, representing a distinct 'active' sleep stage. Computational analysis of active sleep skin patterning reveals diverse dynamics through a set of patterns conserved across octopuses and strongly resembling those seen while awake. High-density electrophysiological recordings from the central brain reveal that the local field potential (LFP) activity during active sleep resembles that of waking. LFP activity differs across brain regions, with the strongest activity during active sleep seen in the superior frontal and vertical lobes, anatomically connected regions associated with learning and memory function7-10. During quiet sleep, these regions are relatively silent but generate LFP oscillations resembling mammalian sleep spindles11,12 in frequency and duration. The range of similarities with vertebrates indicates that aspects of two-stage sleep in octopuses may represent convergent features of complex cognition.

The dynamics of pattern matching in camouflaging cuttlefish

Many cephalopods escape detection using camouflage¹. This behaviour relies on a visual assessment of the surroundings, on an interpretation of visual-texture statistics^2-4 and on matching these statistics using millions of skin chromatophores that are controlled by motoneurons located in the brain^5-7. Analysis of cuttlefish images proposed that camouflage patterns are low dimensional and categorizable into three pattern classes, built from a small repertoire of components^8-11. Behavioural experiments also indicated that, although camouflage requires vision, its execution does not require feedback^5,12,13, suggesting that motion within skin-pattern space is stereotyped and lacks the possibility of correction. Here, using quantitative methods¹⁴, we studied camouflage in the cuttlefish Sepia officinalis as behavioural motion towards background matching in skin-pattern space. An analysis of hundreds of thousands of images over natural and artificial backgrounds revealed that the space of skin patterns is high-dimensional and that pattern matching is not stereotyped-each search meanders through skin-pattern space, decelerating and accelerating repeatedly before stabilizing. Chromatophores could be grouped into pattern components on the basis of their covariation during camouflaging. These components varied in shapes and sizes, and overlay one another. However, their identities varied even across transitions between identical skin-pattern pairs, indicating flexibility of implementation and absence of stereotypy. Components could also be differentiated by their sensitivity to spatial frequency. Finally, we compared camouflage to blanching, a skin-lightening reaction to threatening stimuli. Pattern motion during blanching was direct and fast, consistent with open-loop motion in low-dimensional pattern space, in contrast to that observed during camouflage.

A brain atlas for the camouflaging dwarf cuttlefish, Sepia bandensis

The coleoid cephalopods (cuttlefish, octopus, and squid) are a group of soft-bodied marine mollusks that exhibit an array of interesting biological phenomena, including dynamic camouflage, complex social behaviors, prehensile regenerating arms, and large brains capable of learning, memory, and problem-solving.^{1,2,3,4,5,6,7,8,9,10} The dwarf cuttlefish, Sepia bandensis, is a promising model cephalopod species due to its small size, substantial egg production, short generation time, and dynamic social and camouflage behaviors.¹¹ Cuttlefish dynamically camouflage to their surroundings by changing the color, pattern, and texture of their skin. Camouflage is optically driven and is achieved by expanding and contracting hundreds of thousands of pigment-filled saccules (chromatophores) in the skin, which are controlled by motor neurons emanating from the brain. We generated a dwarf cuttlefish brain atlas using magnetic resonance imaging (MRI), deep learning, and histology, and we built an interactive web tool (https://www.cuttlebase.org/) to host the data. Guided by observations in other cephalopods,^{12,13,14,15,16,17,18,19,20} we identified 32 brain lobes, including two large optic lobes (75% the total volume of the brain), chromatophore lobes whose motor neurons directly innervate the chromatophores of the color-changing skin, and a vertical lobe that has been implicated in learning and memory. The brain largely conforms to the anatomy observed in other Sepia species and provides a valuable tool for exploring the neural basis of behavior in the experimentally facile dwarf cuttlefish.

A Stretchable Fiber with Tunable Stiffness for Programmable Shape Change of Soft Robots

All soft robots require the same functionality, that is, controlling the shape of a structure made from soft materials. However, existing approaches for shape control of soft robots are primarily dominated by modular pneumatic actuators, which require multichambers and complex flow control components. Nature shows exciting examples of manipulation (shape change) in animals, such as worms, using a single-chambered soft body and programmable stiffness changes in the skin; controlling the spatial distribution of changes in stiffness enables achieving complex shape evolutions. However, such stiffness control requires a drastic membrane stiffness contrast between stiffened and nonstiffened states. Generally, this is extremely challenging to accomplish in stretchable materials. Inspired by longitudinal muscle fibers in the skin of worms, we developed a new concept for fabricating a hybrid fiber with tunable stiffness, that is, a fiber comprising both stiff and soft parts connected in a series. A substantial change in membrane stiffness was then observed by the locking/unlocking of the soft part. Our proposed hybrid fiber cyclically produced a membrane stiffness contrast of more than 100 × in less than 6 s using an input power of 3 W. A network of these hybrid fibers with tunable stiffness could manipulate a single-chambered soft body in multiple directions and transform it into a complex shape by selectively varying the stiffness at different locations.

Color-Tunable Fluorescent Hierarchical Nanoassemblies with Concentration-Encoded Emission

Cephalopods possess a dynamic coloration behavior to change their iridescence due to the concentration-induced optical properties of chromatophores and hierarchical assembly of reflectin. However, cephalopods rarely have iridescence in the darkfield. It would be interesting to develop color-tunable fluorescent hierarchical nanoassemblies with concentration-encoded emission. Herein, to construct the bioavailable fluorophore with dynamic coloration properties, a histidine-rich peptide is designed, which can self-assemble into hierarchical nanoassemblies stabilized by hydrogen bonds and π-π stacking interactions. The peptidyl nanoassemblies emit fluorescent iridescence, encompassing the blue to orange region due to the assembly-induced emission. The fluorescence of histidine-rich peptides is color-tunable and reversible, which can be dynamically controlled in a concentration-encoded mode. Due to the coloration ability of histidine-rich peptides, fluorescent polychromatic human cells are developed, highlighting its potential role as a fluorescent candidate for future applications such as bioimaging, implantable light-emitting diodes, and photochromic camouflage.

Cuttlefish camouflage: Blending in by matching background features

Cuttlefish are masters of camouflage and show a remarkable ability to hide in plain sight. A new study reveals how these animals translate visual information about their surroundings into effective camouflage patterns.

Embryonic development of the camouflaging dwarf cuttlefish, Sepia bandensis

BACKGROUND

The dwarf cuttlefish Sepia bandensis, a camouflaging cephalopod from the Indo-Pacific, is a promising new model organism for neuroscience, developmental biology, and evolutionary studies. Cuttlefish dynamically camouflage to their surroundings by altering the color, pattern, and texture of their skin. The skin's "pixels" (chromatophores) are controlled by motor neurons projecting from the brain. Thus, camouflage is a visible representation of neural activity. In addition to camouflage, the dwarf cuttlefish uses dynamic skin patterns for social communication. Despite more than 500 million years of evolutionary separation, cuttlefish and vertebrates converged to form limbs, camera-type eyes and a closed circulatory system. Moreover, cuttlefish have a striking ability to regenerate their limbs. Interrogation of these unique biological features will benefit from the development of a new set of tools. Dwarf cuttlefish reach sexual maturity in 4 months, they lay dozens of eggs over their 9-month lifespan, and the embryos develop to hatching in 1 month.

RESULTS

Here, we describe methods to culture dwarf cuttlefish embryos in vitro and define 25 stages of cuttlefish development.

CONCLUSION

This staging series serves as a foundation for future technologies that can be used to address a myriad of developmental, neurobiological, and evolutionary questions.

Dynamic Courtship Signals and Mate Preferences in Sepia plangon

Communication in cuttlefish includes rapid changes in skin coloration and texture, body posture and movements, and potentially polarized signals. The dynamic displays are fundamental for mate choice and agonistic behavior. We analyzed the reproductive behavior of the mourning cuttlefish Sepia plangon in the laboratory. Mate preference was analyzed via choice assays (n = 33) under three sex ratios, 1 male (M): 1 female (F), 2M:1F, and 1M:2F. We evaluated the effect of modifying polarized light from the arms stripes and ambient light with polarized and unpolarized barriers between the cuttlefish. Additionally, to assess whether a particular trait was a determinant for mating, we used 3D printed cuttlefish dummies. The dummies had different sets of visual signals: two sizes (60 or 90 mm mantle length), raised or dropped arms, high or low contrast body coloration, and polarized or unpolarized filters to simulate the arms stripes. Frequency and duration (s) of courtship displays, mating, and agonistic behaviors were analyzed with GLM and ANOVAs. The behaviors, body patterns, and their components were integrated into an ethogram to describe the reproductive behavior of S. plangon. We identified 18 body patterns, 57 body patterns components, and three reproductive behaviors (mating, courtship, and mate guarding). Only sex ratio had a significant effect on courtship frequency, and the male courtship success rate was 80%. Five small (ML < 80 mm) males showed the dual-lateral display to access mates while avoiding fights with large males; this behavior is characteristic of male "sneaker" cuttlefish. Winner males showed up to 17 body patterns and 33 components, whereas loser males only showed 12 patterns and 24 components. We identified 32 combinations of body patterns and components that tended to occur in a specific order and were relevant for mating success in males. Cuttlefish were visually aware of the 3D-printed dummies; however, they did not start mating or agonistic behavior toward the dummies. Our findings suggest that in S. plangon, the dynamic courtship displays with specific sequences of visual signals, and the sex ratio are critical for mate choice and mating success.

Histamine and histidine decarboxylase in the olfactory system and brain of the common cuttlefish Sepia officinalis (Linnaeus, 1758)

Cephalopods are radically different from any other invertebrate. Their molluscan heritage, innovative nervous system, and specialized behaviors create a unique blend of characteristics that are sometimes reminiscent of vertebrate features. For example, despite differences in the organization and development of their nervous systems, both vertebrates and cephalopods use many of the same neurotransmitters. One neurotransmitter, histamine (HA), has been well studied in both vertebrates and invertebrates, including molluscs. While HA was previously suggested to be present in the cephalopod central nervous system (CNS), Scaros, Croll, and Baratte only recently described the localization of HA in the olfactory system of the cuttlefish Sepia officinalis. Here, we describe the location of HA using an anti-HA antibody and a probe for histidine decarboxylase (HDC), a synthetic enzyme for HA. We extended previous descriptions of HA in the olfactory organ, nerve, and lobe, and describe HDC staining in the same regions. We found HDC-positive cell populations throughout the CNS, including the optic gland and the peduncle, optic, dorso-lateral, basal, subvertical, frontal, magnocellular, and buccal lobes. The distribution of HA in the olfactory system of S. officinalis is similar to the presence of HA in the chemosensory organs of gastropods but is different than the sensory systems in vertebrates or arthropods. However, HA's widespread abundance throughout the rest of the CNS of Sepia is a similarity shared with gastropods, vertebrates, and arthropods. Its widespread use with differing functions across Animalia provokes questions regarding the evolutionary history and adaptability of HA as a transmitter.

Neural pathways in the pallial nerve and arm nerve cord revealed by neurobiotin backfilling in the cephalopod mollusk Octopus vulgaris

Here, we report the findings after application of neurobiotin tracing to pallial and stellar nerves in the mantle of the cephalopod mollusk Octopus vulgaris and to the axial nerve cord in its arm. Neurobiotin backfilling is a known technique in other molluscs, but it is applied to octopus for the first time to be best of our knowledge. Different neural tracing techniques have been carried out in cephalopods to study the intricate neural connectivity of their nervous system, but mapping the nervous connections in this taxon is still incomplete, mainly due to the absence of a reliable tracing method allowing whole-mount imaging. In our experiments, neurobiotin backfilling allowed: (1) imaging of large/thick samples (larger than 2 mm) through optical clearing; (2) additional application of immunohistochemistry on the backfilled tissues, allowing identification of neural structures by coupling of a specific antibody. This work opens a series of future studies aimed to the identification of the neural diagram and connectome of octopus nervous system.

Elucidating the control and development of skin patterning in cuttlefish

Few animals provide as objective a readout of their perceptual state as camouflaging cephalopods. Their skin display system includes an extensive array of pigment cells (chromatophores), each activated by radial muscles controlled by motoneurons. If one could track the individual expansion states of the chromatophores, one would obtain a quantitative description—and potentially even, a neural description by proxy— of the perceptual state of the animal in real time. We developed computational and analytical methods to achieve this in behaving animals, quantifying the state of tens of thousands of chromatophores at sixty frames per second, single-cell resolution, and over weeks. We could infer a statistical hierarchy of motor control, reveal an underlying low-dimensional structure to pattern dynamics, and uncover rules governing skin pattern development. This approach provides an objective description of complex perceptual behaviour, and powerful means to uncover organizational principles underlying neural systems function, dynamics, and morphogenesis.

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.