TY - JOUR
AU1 - Bagge, Laura, E
AB - Abstract The “superpower” of invisibility is a reality and a necessity for many animals that live in featureless environments like the open ocean, where there is nowhere to hide. How do animals achieve invisibility? Many animals match their color patterns to their background, but this strategy is limited when the background scene is dynamic. Transparency allows organisms to match any background all the time. However, it is challenging for an organism to maintain transparency across its entire body volume. To be transparent, tissues must minimize light scattering, both at the surface and within. Until recently, it has been unclear how clear animals with complex bodies (such as many crustaceans with hard cuticles, thick muscles, and other internal organs) minimize such light scattering. This is especially challenging in an environment where light can come from many directions: reflections from downwelling sunlight and bioluminescent searchlights from predators. This review summarizes several recent discoveries of multiple unique adaptations for minimizing light scattering both on the exterior cuticle surface and throughout the body volume of transparent crustaceans, as well as the potential tradeoffs and challenges associated with transparent camouflage. Introduction Transparency is the transmission of light across the visible spectrum without appreciable absorption or scattering such that an object or organism appears completely see-through. It is a common camouflage strategy in the open ocean. Intuitively, we understand the benefits of avoiding detection by having a clear body, but the physical basis of that transparency and the challenges associated with maintaining a transparent body are poorly understood. While most living materials do not absorb light unless they contain pigments, the absence of pigment is not sufficient to make an object or organism transparent. Light scattering must also be minimized (reviewed in Johnsen 2001). When light moves between materials with different refractive indices, the light scatters rather than passing through unaffected (Benedek 1971). Whole-body transparency is not a common strategy on land, in part due to problems such as increased surface reflections that occur due to a large refractive index mismatch (i.e., the lower refractive index of air as compared to water and biological tissues) that ruin invisibility (reviewed in Johnsen 2001 and 2014). In addition, land organisms have a higher vulnerability to ultraviolet radiation damage than marine organisms. For these reasons, terrestrial examples of transparency are generally confined to one part of an animal, such as the transparent wings of insects, or the transparent ventral bodies of glass frogs (Schwalm and McNulty 1980). However, whole-body transparency is found in species from numerous animal phyla that inhabit featureless, marine pelagic environments and is also found, though less commonly, in benthic species inhabiting reefs and sea grass beds (Johnsen 2001,, 2014; Bagge et al. 2017; see Fig. 1 for examples from two phyla). Water is close in refractive index to many biological materials, yet there is still enough of a mismatch that certain materials (such as chitinous crustacean cuticles) can be considered effective reflectors or scatterers of light (Bagge et al. 2016). Additionally, within a functioning cell, many components such as nuclei, mitochondria, and lipids have different refractive indices that are roughly proportional to their densities. It is because biological tissues vary in density and refractive index across different spatial scales that many animals are opaque (Johnsen and Widder 1999; Johnsen 2014). Fig. 1 Open in new tabDownload slide Transparent ocean animals from Phylum Arthropoda and Phylum Mollusca. Top from left to right: a hyperiid amphipod, Phronima sp.; a two-spot octopus larva, Octopus bimaculoides. Bottom from left to right: an anemone shrimp, Periclimenes yucatanicus; a pteropod, Corolla sp. Note that photographs were taken ex situ with two or more xenon flashes mounted to the side of the holding tank to maximize the visibility of these transparent animals. Fig. 1 Open in new tabDownload slide Transparent ocean animals from Phylum Arthropoda and Phylum Mollusca. Top from left to right: a hyperiid amphipod, Phronima sp.; a two-spot octopus larva, Octopus bimaculoides. Bottom from left to right: an anemone shrimp, Periclimenes yucatanicus; a pteropod, Corolla sp. Note that photographs were taken ex situ with two or more xenon flashes mounted to the side of the holding tank to maximize the visibility of these transparent animals. Several empirical studies have measured the degree of transparency in organisms (Greze 1963; Chapman 1976; Johnsen and Widder 1998, 2001; Gagnon et al. 2007), and additional studies have shown that transparency functions as a successful defense against visual predation in terrestrial (Arias et al. 2019) and in aquatic environments (Zaret and Kerfoot 1975; Kerfoot 1981; Brownell 1985; Langsdale 1993; Thetmeyer and Kils 1995; Utne-Palm 1999; Tsuda et al. 1998; Feller and Cronin 2014). However, it is important to remember that transparency does not equal invisibility under all conditions. Animals that have modified their bodies in some way to become transparent are not necessarily able to maintain their transparency in different environments or under different conditions of physiological stress (Mackie and Mackie 1967; Bhandiwad et al. 2011; Bagge et al. 2017). Maintaining transparency is critical for remaining camouflaged, but the mechanisms underlying whole-body transparency as well as the potential metabolic or fitness-related tradeoffs of being transparent versus opaque have not been well-characterized. This review will cover recently discovered adaptations in clear crustaceans that have been shown to minimize both surface light scattering (Bagge et al. 2016) and internal light scattering (Bagge et al. 2017). Additionally, I will discuss why studying transparent organisms and making comparisons with closely related opaque organisms will increase our understanding of the puzzling problem of the physical basis of transparency in whole organisms and can help inspire biomimetic applications. Challenges associated with minimizing light scattering at the surface Anyone who has ever shone a flashlight on a glass window at night or experienced glare from a pair of eyeglasses is familiar with the concept that glass can be transparent but also visible due to light reflections. Transparent animals also reflect light, and so many fish use their photophores as a kind of bioluminescent searchlight that could reveal the position of these reflective prey (Fig. 2). Fig. 2 Open in new tabDownload slide A dragonfish uses photophores underneath its eyes to shine bioluminescent searchlights into the dark to look for reflections from transparent prey. Photograph of dragonfish by Sönke Johnsen used with permission, and combined with photograph of Phronima sp. by Laura Bagge to illustrate the concept of reflections breaking camouflage. Fig. 2 Open in new tabDownload slide A dragonfish uses photophores underneath its eyes to shine bioluminescent searchlights into the dark to look for reflections from transparent prey. Photograph of dragonfish by Sönke Johnsen used with permission, and combined with photograph of Phronima sp. by Laura Bagge to illustrate the concept of reflections breaking camouflage. How does a marine organism avoid reflecting light? Some species of cephalopods remain transparent most of the time which minimizes their silhouette, but in the presence of blue bioluminescent light, these cephalopods expand pigmented chromatophores in their skin which absorbs that light and minimizes the amount of light that can reflected back to the eyes of a predator (Zylinski and Johnsen 2011). For animals lacking the dynamic chromatophores of cephalopods, avoiding reflections means eliminating the refractive index difference between the surface and surrounding water (Bagge et al. 2016; Cronin 2016). There are multiple ways to manufacture an antireflective or anti-glare coating (reviewed in Han et al. 2019). One of the first examples of antireflection discovered in nature was on the corneas of compound eyes of moths (Bernhard and Miller 1962). These insect eyes were covered with a dense ordered array of nanoprotuberances, called nipple arrays (Bernhard and Miller 1962). These subwavelength structures, subsequently found on the transparent wings of certain insects (e.g. Yoshida et al. 1996), have a geometry such that the fractional area of the chitinous cuticle increases as light approaches the surface of the insect, resulting in a gradient in the refractive index and less reflected light (Stavenga et al. 2006). The exact amount of reflectance depends on the indices of both the surrounding medium and the surface of the animal as well as the angle and wavelength of the incident light (Han et al. 2019). As previously mentioned, in the open ocean environment, crustacean surfaces would be expected to scatter downwelling sunlight in addition to scattering direct bioluminescent light because these surfaces are composed of chitin that has a higher refractive index than the surrounding seawater (Stoddart et al. 2006). However, when Bagge et al. (2016) examined the surfaces of seven different species of hyperiid amphipods, a type of transparent pelagic crustacean, using scanning electron microscopy (SEM), all hyperiids examined had unusual antireflective surface layers. One species, Cystisoma sp., possessed ordered arrays of nanoprotuberances on its legs that appeared very similar to the antireflective corneal nipple arrays in insects. Additionally, there were previously undescribed dense monolayers of subwavelength spheres found on the dorsal surfaces of all species of hyperiids examined that ranged in size from 50 nm to ∼320 nm (Fig. 3; Bagge et al. 2016). Optical modeling using transfer matrix methods confirmed that the observed nanostructures and nanospheres reduced overall surface reflectance by two-fold to more than 100-fold, meaning overall reflectance was <0.1% in some cases (Bagge et al. 2016). Fig. 3 Open in new tabDownload slide Size distributions ± SE of the nanospheres associated with each of the seven examined hyperiid species. Photographs of each respresentative species of hyperiid amphipod is provided on each bar for reference. Photographs by Karen Osborn, used with permission. Fig. 3 Open in new tabDownload slide Size distributions ± SE of the nanospheres associated with each of the seven examined hyperiid species. Photographs of each respresentative species of hyperiid amphipod is provided on each bar for reference. Photographs by Karen Osborn, used with permission. The diversity of surface coatings found in the seven species of hyperiids, from nanoprotuberances to monolayers of spheres, raises additional questions about the composition of the coatings and how these coatings are made. Phronima sp. had spheres that showed planes of division as well as fimbrae attaching the spheres to the cuticle surface, which led Bagge et al. (2016) to hypothesize that the monolayers of spheres could be a monoculture of coccoid bacteria that may be part of an adaptive symbiosis. However, it is important to note that while all species of hyperiids examined possessed some kind of spherical or bumpy layer, there were variations in size and appearance between species that suggested that the monolayers of spheres could be made from either living or non-living material (Bagge et al. 2016). For example, Leptocotis sp. had variations in the sizes and spacings of the spheres on its cuticle surface (Fig. 4A) and an additional species, Oxycephalus sp. (not included in Bagge et al. 2016) was found to possess an unusual layer (Fig. 4B) that could be a feature of the cuticle itself (i.e., similar to the nanoprotuberances observed in Cystisoma sp.) or that could be a thick biofilm. Fig. 4 Open in new tabDownload slide Scanning electron micrographs of the cuticular surfaces of hyperiid amphipods. (A) Leptocotis sp. dorsal surface, (B) Oxycephalus sp. dorsal surface, (C) Cystisoma sp. appendages with a rod-shaped bacteria in the middle of the micrograph for further size reference, (D) Cystisoma sp. ventral surface. Scale bars equal 1 μm. Fig. 4 Open in new tabDownload slide Scanning electron micrographs of the cuticular surfaces of hyperiid amphipods. (A) Leptocotis sp. dorsal surface, (B) Oxycephalus sp. dorsal surface, (C) Cystisoma sp. appendages with a rod-shaped bacteria in the middle of the micrograph for further size reference, (D) Cystisoma sp. ventral surface. Scale bars equal 1 μm. Many questions remain unanswered regarding the optical consequences of the variations observed in the surface layers and regarding the challenges of making such layers. For example, if the layer is bacterial, are there any metabolic costs involved in recruiting or hosting such bacteria on its cuticle surface? If the layer is something the hyperiid excretes, are there any physiological costs as a result of forming this layer? Hyperiids, like most crustaceans, molt their shells, and it is unknown how they may re-acquire the antireflective monolayers after a molt, or whether there is a period of enhanced vulnerability to predation. Additionally, many questions remain about why only one kind of hyperiid, Cystisoma sp., possessed cuticular nanoprotuberances (only found on the legs and ventral surface) that resembled the nipple array strategy found on transparent wings and eyes of insects (Fig. 4C). Cystisoma sp. was the largest hyperiid examined (>100 mm in length), with appendages that had a large surface area and many angles and edges that could increase the chance of the animal being seen (Bagge et al. 2016). It makes sense that an antireflective layer would be found on an area with the largest surface area: volume ratio (Cronin 2016). Additional SEMs from one Cystisoma specimen revealed that the appendages had the longest (∼200 nm in height) and densest nanoprotuberances found anywhere on the surface (Fig. 4C). At the junction with the ventral surface of the animal’s abdomen, the nanoprotuberances became shorter (<100 nm in height) and less dense (Fig. 4D). Variations in height of nanoprotuberances are known to help with broadband and omnidirectional antireflection (Siddique et al. 2015). It remains an open question as to why nanoprotuberances were not found on any other part of the body of Cystisoma or found at all on any other species of hyperiid examined. Hyperiids are the first confirmed group of marine organisms that possess ultrastructural surface features that can function in antireflection (Bagge et al. 2016). Coleoid cephalopods were found to have microprojections on their eyes, but researchers have suggested that the low refractive index (1.40) and small heights (<60 nm) would not be effective at enhancing crypsis (Talbot et al. 2012). We need to conduct a broader survey of midwater animals and any possible associated surface nanostructures before we can begin to draw conclusions about potential antireflective functions and tradeoffs to this strategy. Presumably, there might be fitness-related costs involved in producing a cuticle with these nanostructures. The potential challenges of making such an ordered array, and the challenges of keeping it clean and functional, are of great interest to researchers who manufacture antireflective coatings for a variety of functions from solar cells to eyeglasses (Han et al. 2019). Challenges associated with minimizing light scattering throughout the interior The role of scattering from internal surfaces and structures was not considered in the previously mentioned study of antireflective layers (Bagge et al. 2016) but we know that transparency requires minimizing scattering both externally and internally. Studies of the transparent corneas and lenses of the eyes of vertebrates have provided insights about how certain tissues are modified for transparency by having a homogenous refractive index (Hart and Farrell 1969; Benedek 1971; Twersky 1975; Farrell and McCally 1976; Freund et al. 1986; Costello et al. 2007, 2008, 2010, 2012; Gilliland et al. 2004, 2008; Marsili et al. 2004; Metlapally et al. 2008). Corneas and lenses, however, are specialized structures within an organism that lack the organelles and cellular machinery required for life at the whole-body level. Whole-body transparency appears to be an actively maintained process rather than a static physical state. For example, the siphonophore (Hippopodius hippopus) becomes opaque when touched because proteins precipitate in its mesoglea that cause an increase in scattering (Mackie 1996). Opacity also increases in other transparent animals because of physiological or environmental stressors (Bhandiwad and Johnsen 2011; Bagge et al. 2017), but the mechanism or mechanisms underlying these changes are only beginning to be understood. Clear ghost shrimp in the genus Palaemonetes experience a reversible loss in their transparency due to environmental stressors experienced over a tidal cycle—increased temperature or increased salinity result in increased opacity (Bhandiwad and Johnsen 2011). Bhandiwad and Johnsen (2011) hypothesized that the loss in transparency was related to the pooling of fluid (e.g., hemolymph) in the intramuscular space; having regions of low-index fluid between the regions of high-index muscle would lead to an increase in light scattering. They based this hypothesis on confocal microscopy results which showed disorganized opaque muscle tissue with large spacings between the muscle fibers, and they calculated that having areas of pooled fluid at least every 20 μm throughout the entire 2.4 mm thick body of the shrimp would cause an ∼30% loss in transparency (Bhandiwad and Johnsen 2011). Another species of shrimp, Ancylomenes pedersoni, was found to temporarily become opaque after performing multiple tail-flips, contractions of the abdominal muscles for the purpose of escape via rapid backward propulsion (Bagge et al. 2017). After as few as three tail flips, the previously transparent abdominal muscle in these shrimp became cloudy. Eliciting additional tail flips to the point of exhaustion resulted in complete opacity, though the original transparency returned after 20–60 min of inactivity. No clear relationship between number of tail flips and amount of time required to recover transparency was found (Fig. 5). Building from Bhandiwad’s and Johnsen’s (2011) hypothesis, Bagge et al. (2017) proposed that the increased opacity was likely due to changes in blood flow associated with exercise (wider blood vessels and more open vessels) that would increase the number of scattering interfaces (between the low refractive index hemolymph and high refractive index muscles) in the interior of a transparent shrimp’s body. To test whether this was a possible mechanism for the observed increased opacity, pre- and post-exercise perfusion was measured by injecting a dye (Alexa Fluor 594-labeled wheat germ agglutinin; Molecular Probes in FSW [1 mg L−1 mL]) into the shrimp’s pericardial cavity that labeled sarcolemmal surfaces and endothelial cells in contact with hemolymph (Fig. 6A, B). As expected, more hemolymph perfused through the abdominal tissue post-exercise in correlation with increased opacity (Fig, 6D), presumably owing to more capillaries opening (Fig, 6B). Increased blood flow was further correlated with increased opacity in additional experimental conditions, including exposure to both increasing and decreasing salinities, wound healing response to a perforated cuticle, and injection of a vasodilator. All these experiments of physiological perturbation support the hypothesis that increases of hemolymph perfusion to the shrimp abdomen provides a mechanism for transparency disruption (Bagge et al. 2017). Fig. 5 Open in new tabDownload slide No obvious correlation was found between the number of tail flips and the amount of time required for recovery and return to original levels of transparency in A. pedersoni. Fig. 5 Open in new tabDownload slide No obvious correlation was found between the number of tail flips and the amount of time required for recovery and return to original levels of transparency in A. pedersoni. Fig. 6 Open in new tabDownload slide Cross-sections of abdominal muscle from shrimp (nuclei shown in blue, areas where perfusion occurred shown in red) with the corresponding photograph of each shrimp. (Top left) The control shrimp (no exercise) showed no evidence of staining, indicating no perfusion, around in any vessels or around the muscle fibers. (Bottom left) The abdomen of the control shrimp remained transparent. (Top right) The experimental shrimp (tail-flip/exercise) had staining in the capillaries and around individual fibers indicating that hemolymph perfused these areas. (Bottom right) The abdomen of the exercised shrimp became opaque. Reproduced from Bagge et al. (2017). Fig. 6 Open in new tabDownload slide Cross-sections of abdominal muscle from shrimp (nuclei shown in blue, areas where perfusion occurred shown in red) with the corresponding photograph of each shrimp. (Top left) The control shrimp (no exercise) showed no evidence of staining, indicating no perfusion, around in any vessels or around the muscle fibers. (Bottom left) The abdomen of the control shrimp remained transparent. (Top right) The experimental shrimp (tail-flip/exercise) had staining in the capillaries and around individual fibers indicating that hemolymph perfused these areas. (Bottom right) The abdomen of the exercised shrimp became opaque. Reproduced from Bagge et al. (2017). While Bagge et al. (2017) were not able to rule out additional mechanisms, such as protein precipitation or other cellular changes that may also increase light scattering, this study developed a simplified model that shows how increasing blood-flow around individual muscle fibers can increase the number of optical interfaces within a shrimp’s tissue and increase the opacity. This is the first documented example of a tradeoff strategy where an animal diverts blood flow to maintain camouflage. Limiting or controlling a shrimp’s abdominal blood flow may lead to pulsatile oxygen delivery, which could be a problem leading to tissue damage from reactive oxygen species. This fact that transparent shrimp at rest have little to no evidence of perfusion of their abdominal musculature (unlike an opaque shrimp Lysmata pederseni, which had more perfusion even at rest) indicates that these transparent shrimps may have significantly reduced performance in order to maintain their transparency. Tail-flipping is a common predator-avoidance strategy in many crustaceans (Jimenez et al. 2008). Contraction is anaerobic during tail-flipping because energy requirements exceed aerobic capacity. The hydrolysis of arginine phosphate initially powers contractions, and adenosine triphosphate for additional contractions is supplied by anaerobic glycogenolysis, which results in the accumulation of lactate and depletion of glycogen (England and Baldwin 1983). Metabolic recovery must occur before crustaceans can continue with more high-force contractions (England and Baldwin 1983). In large muscle fibers, metabolic recovery is thought to be diffusion limited, though anaerobic glycogenolysis has been shown to speed up the replenishment of arginine phosphate (reviewed in Jimenez et al. 2008). If transparent crustaceans have larger muscle fibers than opaque crustaceans or other modifications for increased transparency such as fewer mitochondria, this raises the question of whether they are more easily exhausted or have longer recovery periods than an opaque shrimp. Thus, one trade-off to transparency may be a prolonged recovery time from exercise during which the animal is vulnerable because it is opaque and not able to undergo another escape response. Muscle physiology and ultrastructural studies of the muscle of transparent versus opaque shrimps and other organisms are currently underway in order to answer this question about morphological modifications for transparency that may have fitness-related consequences. Future directions in the study of transparent camouflage Microscopy techniques to characterize morphology associated with physiological states One of the biggest challenges to carrying out research into the physical basis of transparency is that the nano-sized structures we wish to characterize can often only be measured after subjecting the tissue to many processes that have the potential to introduce artifacts. Fixation and dehydration of the tissue samples in preparation for traditional electron microscopy can cause shrinkage of the tissue, with one study finding a 25–45% decrease in thickness of lamellae and interlamellar spacings in squid iridocytes prepared for transmission electron microscopy (Ghoshal et al. 2013). Therefore, preserving specimens in a closer-to-native state than can be achieved with traditional dehydration methods is extremely important, especially when trying to determine optical function and light scattering based on measurements smaller than half a wavelength of light. Recent advances in electron detection and image processing now allow for an unprecedented level of resolution by cryoelectron microscopy (Cryo-EM) that is beginning to rival that of X-ray crystallography (Xiaou-chen et al. 2015). Cryo-EM is a method for imaging frozen-hydrated specimens at cryogenic temperatures by electron microscopy. Specimens remain in their native state without the need for dyes or fixatives, allowing for the study of fine cellular structures at a molecular resolution. These advances are critical to being able to investigate the ultrastructural arrangements of stuctures in transparent species to obtain accurate measurements of the sizes of gaps that may influence light scattering. For example, Cryo-EM should be performed on opacified tissue of A. pedersoni shrimp (studied in Bagge et al. 2017) to gain insight into whether there are other changes besides increased hemolymph perfusion. Another recent advance in technology is the use of computed tomography (CT) to explore morphology. Specifically, one technique is called Diffusible Iodine-based Contrast-Enhanced Computed Tomography, or DICECT, which is a non-destructive way of visualizing the 3D anatomy of animals by using a contrast-enhancing agent (iodine) to distinguish between soft tissues (Gignac et al. 2016). For the study of transparent animals, this technique offers a fast and non-destructive way to explore the internal anatomy from a diversity of phyla. Using this technique could help us to finally answer questions about whether most transparent animals have reduced internal structures to minimize scattering or have modified ultrastructures. While most DICECT methods do not currently allow for observations of individual myofibrils, and the equipment used in Bagge et al.’s (2017) study could only resolve down to 8 μm (Fig. 7), rapid improvements in imaging capabilities as well as modifications to the iodine staining protocols now allow us to non-destructively resolve features down to 1 μm. Finally, DICECT methods could also be used to observe how anatomy changes according to different physiological states. Experiments could be performed exposing animals to different environmental conditions, such as increased temperature or acidity, and then any changes could be quickly quantified using DICECT, which may yield new insights into how exactly animals maintain their transparency and how it can be disrupted. Fig. 7 Open in new tabDownload slide 3D reconstruction from a micro-CT scan of A. pedersoni, virtually dissected at a transverse plane through the posterior abdomen, composed of muscle (lighter areas) and blood vessels and hemolymph spaces (dark areas). The slight variations in the gray color where there is the appearance of multiple circles on the image demonstrate that individual muscle fibers surrounded by hemolymph can be resolved. Notable muscular and vascular features are labeled: da, dorsal artery; fm, flexor muscles; hs, hemolymph spaces; aa, abdominal artery. Scale bar equals 1 mm. Fig. 7 Open in new tabDownload slide 3D reconstruction from a micro-CT scan of A. pedersoni, virtually dissected at a transverse plane through the posterior abdomen, composed of muscle (lighter areas) and blood vessels and hemolymph spaces (dark areas). The slight variations in the gray color where there is the appearance of multiple circles on the image demonstrate that individual muscle fibers surrounded by hemolymph can be resolved. Notable muscular and vascular features are labeled: da, dorsal artery; fm, flexor muscles; hs, hemolymph spaces; aa, abdominal artery. Scale bar equals 1 mm. Biochemical modification for transparency In addition to the ultrastructural features already reviewed here, animals may make their bodies transparent using biochemical modifications. Clear organisms may solve the problem of light scattering by using high-index “clearing” agents to become transparent in the same way that the urea-containing reagent called Scale (Hama et al. 2011) has been used to clear mice brains while still preserving the volume and structure of the tissue. Two approaches that remain untested include: (1) a clearing agent could raise the refractive index of the body fluids to approximate that of protein, thus minimizing internal light scattering and thus increasing transparency. Alternatively, (2) another clearing strategy for matching refractive indices of internal components may be the insertion of specialized low-index lipids into cell membranes, which would lower scattering by causing the membrane to more closely match the index of the surrounding cytoplasm. Future research could begin to address these questions, starting by using one of the largest clear crustaceans, Cystisoma sp. before testing additional gelatinous organisms such as salps. The optical properties of the intact animal could be measured using special imaging tools such as a spectropolarimeter to measure percent reflectance and polarization, and an ellipsometer (a technique that uses polarized light to characterize thin film and bulk materials) could be used to measure refractive indices. A visible density difference (Schlieren) effect when the body fluid from Cystisoma comes into contact with seawater is preliminary evidence that suggests that a transparent animal’s body fluid may have a higher refractive index than seawater, but these observations need to be confirmed. Regardless of refractive index, determining the biochemical composition of the fluid and determining whether it is filled with few or many components will be useful for understanding more about the physiology and camouflage strategies of this transparent organism. The second biochemical hypothesis, that low-index lipids inserted into cell membranes may help with transparency by matching the refractive index of surrounding fluid, could be tested by extracting membrane lipids from transparent species and using gas chromatography to characterize membrane lipid composition. Lipid molecules can be formed either endogenously or from dietary lipids; thus, aspects of an organism’s diet (e.g., the ratio of omega-3 to omega-6 fatty acids) may be critically important in determining refractive index and thus may affect the maintenance of transparency. Identifying the lipids, their refractive indices, and their temperature-dependent phase characteristics will be important for understanding maintenance of transparency under changing environmental conditions. Future analyses will be informed by the above characterization work. The next steps include performing a separation of the protein and carbohydrate components of both the body fluids and transparent tissues from the representative species, and then completing more in-depth analyses using proteomics approaches and the molecular biology tools as well as materials characterization. Conclusions Once we have gathered more information about the ultrastructure, refractive index, and biochemical composition of transparent tissues, the next steps include experiments measuring how these factors affect overall visibility of the animal in the water column; given the current realities of climate change, it is especially important that we better understand the effects of increased temperature and acidity on transparent physiology. By uncovering the mechanisms of transparent camouflage and how it may be disrupted, we can then create modeled predictions that relate temperature and pCo2-induced stress, camouflage capabilities, and ultimate food web interactions in the open ocean. Beyond the basic biology, the broader impacts of this work are primarily in biomimetics, the technological imitation of biological traits. Research on transparent animals from their outer surface to their interior muscle may inspire new technological innovations for ways to increase transparency of materials. Gaining insight into how animals structure their bodies out of different components to remain transparent has far-reaching implications, especially in regards to treating conditions like cataracts, and in bioimaging in medicine. For example, unlocking the mystery of how these crustaceans and other organisms make their whole bodies transparent could have applications in diagnosing diseases where it is necessary to image deep into a tissue without destroying it. Ultimately, the study of transparency is an interdisciplinary endeavor that will require expertise in biology, chemistry, physics, and even nanoengineering to learn how organisms have made their bodies transparent and how we can use that information for various applications in medicine, imaging systems, and materials science. From the symposium “Adaptation and Evolution of Biological Materials” presented at the annual meeting of the Society for Integrative and Comparative Biology, January 3–7, 2019 at Tampa, Florida. Acknowledgments The author thanks Mason Dean and Robert Campbell for organizing the “Adaptation and Evolution of Biological Materials’ symposium and for the opportunity to present her work. She also thanks her collaborators Sönke Johnsen, Karen Osborn, and Steve Kinsey. She thanks the editor and two anonymous reviewers for their suggestions that improved this manuscript. Funding This work is contributed in association with the ‘Adaptation and Evolution of Biological Materials’ symposium at SICB 2019, which was generously supported by numerous SICB Divisions, the American Microscopy Society, the Company of Biologists, the journal Bioinspiration and Biomimetics, Micro Photonics Inc., Overleaf and Thermo Fisher Scientific. References Arias M , Mappes J , Desbois C , Gordon S , McClure M , Elias M , Nokelainen O , Gomez D. 2019 . Transparency reduces predator detection in mimetic clearwing butterflies . Funct Ecol 00 : 1 – 10 . (https://doi.org/10.1111/1365-2435.13315) WorldCat Bagge LE , Osborn KJ , Johnsen S. 2016 . Nanostructures and monolayers of spheres reduce surface reflections in hyperiid amphipods . Curr Biol 26 : 3071 – 6 . 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TI - Not As Clear As It May Appear: Challenges Associated with Transparent Camouflage in the Ocean
JF - Integrative and Comparative Biology
DO - 10.1093/icb/icz066
DA - 2019-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/not-as-clear-as-it-may-appear-challenges-associated-with-transparent-Xi63L0dYxB
SP - 1653
VL - 59
IS - 6
DP - DeepDyve
ER -