I recently watched The Darkest Hour, a film about invisible aliens that invade and terrorize Earth. While the movie itself leaves much to be desired, the concept of invisible aliens is one that I find noteworthy. As we search for alien lifeforms, could we be missing them due to their invisibility?
While invisibility has been a frequent theme in science fiction, often presented with a feeling of artificiality, nature has actually perfected it a number of times. The Darkest Hour reminded me that “invisible” animals, those that are comprised to a large degree of transparent tissues, are common in nature and pose a fundamental and intriguing question. How is it possible for us to see through a living structure? While we often take transparency for granted (e.g., glass), it is a physical and biological enigma worthy of both research for practical applications, and also simply marveling at its’ beauty and complexity. Today’s post is intended to explore this beauty and complexity in further detail.
Transparent organisms are relatively common in nature, being found primarily in aquatic and marine environments (Johnsen, 2000) and are found commonly within the comb jellies, jellyfish, arrow worms, and crustaceans (Johnsen, 2001). Transparent tissues are uncommon for terrestrial/aerial invertebrates, though do occur in some butterflies and moths as clear panes on their wings (Johnsen 2001). There exist fish from six families with transparent tissue, most famously the glass catfish popular in home aquaria, though many larval fish are also transparent (Johnsen, 2001). Transparency is almost unknown in amphibians, reptiles, bird or mammals, though there is a frog with transparent skin on its ventral surface. This transparency is not always a permanent feature in an organism; many larval fish switch from transparency to pigmented and opaque tissues, sometimes within 24 hours, during metamorphosis into juveniles (Johnsen 2001). Some squid can switch between transparent and pigmented tissue depending upon the light environment (Zylinski and Johnsen, 2011).
Neither is tissue transparency confined to animals. Plants also show some transparent tissues. Some plants have transparent “windows” in the leaves (termed heterobaric leaves) allowing light to enter the leaves. Within the leaves, specialized tissue may then act as optic cables or light guides to transfer the light to more internal parts of the plant (Nikolopoulos et al., 2002). For example, some desert plants use heterobaric leaves and light guides to transfer light to its photosynthetic machinery underground; this strategy is thought to minimize water loss from and heat load to, the sensitive areas of the plant (Nikolopoulos et al., 2002).
Clearly transparent tissue is widespread and has independently evolved a number of times.
The widespread nature of this phenomenon, unique qualities, and all round “cool” factor makes this a fascinating subject. It does, however, beg the question, “how does it work?” Developing living tissues that are transparent is not a trivial problem. Unlike a static pane of glass, these living cells must move, grow and reproduce. Moreover, more than one structure must be made invisible. For example, in the glassfish, not only must the muscle be made transparent, but also the skin, mesentery and associated tissues to achieve true transparency. In these fish, only the bones and gut remain opaque.
The short answer to “How does it work?” is… No one knows. Certain principles are known which tell us something of the mechanism, but how it is possible for a living tissue to be transparent remains (pardon the pun), unclear. For an object to be transparent to light it must not absorb or scatter (including reflecting) a significant portion of the incoming light (Johnsen and Widder, 1999). Absorption of light by biological molecules is not a widespread phenomenon (Johnsen and Widder, 1999), though obviously it is necessary for photosynthesis, light detection/vision, and as protection against ultraviolet radiation. The greater obstacle to transparency is scattering/reflection of light. There are a few ways to minimize scattering.
First, if the molecule under consideration is less than one-half the wavelength of the incoming light it will not interact with it. Individual cellular components may be this size (for which an electron microscope is required, specifically because they don’t interact with visible light), but structures the size of macromolecules, cells and tissues will scatter light. This is an inherent physical barrier to transparency
A second approach is to minimize the thickness of the body so that there is little material for the light to interact with. By extreme narrowing of the body, for example the larva of the eel, the body approaches transparency. This is because absorption and scattering are exponentially proportional to path length (Johnsen and Widder, 1999), meaning that the greater the path length through the material, the greater the scattering. By reducing body width to only a few millimeters, and elongating or deepening the body in order to maintain volume, the animal can approach transparency.
By definition, to achieve transparency, or near transparency, the refractive index of the organism must approximate the refractive index of the surrounding medium, either water or air. The clearest model of this is provided by the analogy of a water droplet in air (highly visible due to differences in refractive index between air and water), but invisible in water (due to no difference in refractive index). At first blush this implies that an organism would need to be made up of one material (which is clearly vastly incorrect) or multiple materials but all with the same refractive index (which we also know to be incorrect) (Johnsen and Widder, 1999). However, a third alternative exists. If the average refractive index discontinuity is less than one half the wavelength of the light, the surface will not scatter light. This is due to destructive interference resulting from overlapping light waves canceling each other out (Johnsen, 2000).
There is evidence of this strategy of destructive interference to reduce scattering. Some organisms have microscopic bumps on their surface. These bumps are dome-like and, because they are broad at their base and narrow at the top, the base has a refractive index similar to the underlying material while the narrow part (less than one-half wavelength of light in diameter), has a refractive index more similar to the water. As the bumps are smooth the resulting refractive index is an “average” between the base and the summit of the bump and so the refractive index is reduced (Johnsen and Widder, 1999; Johnsen, 2000; 2001). This is not transparent however, only a reduction of reflection. Further, this method, if it did accomplish transparency, would only create a transparent skin; all of the underlying tissues would still be opaque and revealed.
A second example of the use of destructive interference, this time to truly create transparency, is that of the mammalian eye. The cornea of the eye is composed of collagen fibers (high refractive index) embedded in a low refractive index medium. Without some method of reducing the refractive index discontinuity of the two to less than one half of the wavelength of light, the cornea would be opaque. That method is that the collagen fibers are small and ordered within the matrix. The distribution and orderly arrangement of the collagen fibers create the required destructive interference resulting in transparency (Johnsen and Widder 1999; Johnsen, 2000). Indeed, the cause of cataracts is the breakdown of the ordered arrangement of collagen, reducing transparency of the cornea. Of course, the structure of the cornea and lens of the eye are very simple relative to a whole organism. And these components of the eye through which light must pass are made up of atypical cells which lack much of the machinery of life (e.g., nuclei, mitochondria) and instead are dependent upon adjacent cells for maintenance (Johnsen, 2001). That simplification allows the transparency.
The above methods of minimizing scattering to reduce reflection or promote transparency are extra-cellular mechanisms. There is no currently known method of making cellular components transparent.
Now, given these physical challenges to transparency – very small size, distribution and orientation of structures, requirement to match surrounding medium – the development of transparent tissue is indeed amazing. These tissues cannot meet these challenges in the accepted ways because they are living, moving structures. For that reason, the way in which light interacts with them is constantly changing. Further, and intriguingly, transparent animals become opaque rapidly after death (Johnsen 2000) implying that transparency is a function of the living tissue.
The concept of invisibility (or transparency) to avoid detection has been used extensively in fiction. For example a “cloaking device” of the Romulans in Star Trek, or the invisibility cloak of Harry Potter are two that you are likely familiar to you. Or think of the invisibility device that rendered the alien in Predator nearly invisible.
Understanding how tissues are made transparent in nature will not provide a “cloak of invisibility” as the object under the cloak would need to be transparent as well; if the object can be made transparent there is no need for a cloak. Rather, perhaps invisibility will be developed via a solution that infuses the body as described in The Invisible Man by H.G. Wells. Science fiction only?
Perhaps, but in 2011 Japanese scientists developed a solution, called Scale, which turns non-living tissue transparent. This has proven to be very useful for visualizing structures deeper within an object as the body is made transparent. However, this is on non-living material and may simply be an advanced bleaching process.
Transparent tissues provide us with an aspect of nature worth deep reflection. Given our current understanding of the universe, we cannot begin to truly describe how these transparent tissues are possible. Yet there they are. Concentrated research on the mechanisms of these tissues would likely not only lead to understanding how they work, but also contribute greatly to the discipline of optics and other fields as well. Furthermore, as we search the universe for intelligent life beyond our own, perhaps we should consider that they may indeed be transparent or have developed technologies that would make them invisible to us. Perhaps this may explain why we have yet to find conclusive evidence of their existence at all. It is certainly a possibility.
I personally am intrigued by invisibility, and these organisms described above present us with a fascinating and exotic challenge – to see that which cannot be seen.
Johnsen, S., & Widder, E. (1999). The Physical Basis of Transparency in Biological Tissue: Ultrastructure and the Minimization of Light Scattering Journal of Theoretical Biology, 199 (2), 181-198 DOI: 10.1006/jtbi.1999.0948
Johnsen, S. (2000). Transparent Animals Scientific American, 282 (2), 80-89 DOI: 10.1038/scientificamerican0200-80
Johnsen, S. (2001). Hidden in Plain Sight: The Ecology and Physiology of Organismal Transparency Biological Bulletin, 201 (3) DOI: 10.2307/1543609
Zylinski, S., & Johnsen, S. (2011). Mesopelagic Cephalopods Switch between Transparency and Pigmentation to Optimize Camouflage in the Deep Current Biology, 21 (22), 1937-1941 DOI: 10.1016/j.cub.2011.10.014
Nikolopoulos, D. (2002). The Relationship between Anatomy and Photosynthetic Performance of Heterobaric Leaves PLANT PHYSIOLOGY, 129 (1), 235-243 DOI: 10.1104/pp.010943