Inner Nature: Transparent Animals

By Vidya Rajan, Columnist, The Times

Recently I came across this quiz clue: What links barreleye fish, ghost shrimp, glass frogs, golden tortoise beetles, jellyfish and sea angels?[1] Besides the obvious fact that they are all animals, there were a few options I considered: 1. They are aquatic (but the beetles are not); 2. They are invertebrates (but frogs are not) and 3. They are transparent (and that is the correct answer.) But then I wondered: How does transparency work in the different refractive mediums of water and air? What are the benefits of being transparent other than camouflage? Then too, this list only skims is the the barrel. They include glass-winged butterflies, surgeonfish, icefish, Enope and glass squids, glass octopuses, European eels and probably many animals that have so far successfully evaded prying human eyes and equipment. In this article, I will examine why and how some animals have evolved to be transparent.

The obvious answer is that it is beneficial for survival – either to eat or to escape being eaten. Most of these transparent animals live in the open ocean – the pelagic zone. There are no plants and no reefs and nowhere to hide except in the water itself. If an aquatic animal is transparent, it makes a most excellent camouflage, called crypsis. The animal can then escape being seen by predators and therefore escape being eaten. But transparency also helps a predator hide in plain sight of its prey. The comb jelly eats small fish and crustaceans by disabling them with visible stinging cells shaped like fish or shrimp which lure its prey closer. When the prey tries to eat what it thinks is food, the ‘invisible’ comb jelly stings them and consumes them.

Opacity is conferred by light scattering off an object. Animal bodies are constructed of proteins, lipids and water which all have different refractive indices. You can check the effect by putting a few teaspoons of oil in water and shaking hard – the previously separate and transparent liquids will disperse and the mixture will appear cloudy. This happens because the little oil bubbles in water cause light to scatter unevenly. Achieving transparency is not an easy task because blood vessels and organs have functions that make invisibility difficult in animals larger than a few millimeters long, because they carry blood or food material. Most transparent aquatic animals’ bodies are filled with a jelly-like substance which is buoyant and provides resistance to the crushing pressures of deep water. But the lack of strong musculature leaves these animals bobbing along with water currents rather than powering their own movement. To address the problem of blood, bones and organs being hard to render transparent, natural selection has shaped the visible organs to look like small universal prey, such as fish or shrimp, which can even help the animal obtain food as seen in the comb jelly’s case. The transparent icefish has a different adaptation. It lives in the cold, oxygen-rich waters around Antarctica. It has lost its scales altogether, has transparent flesh, and its blood lacks hemoglobin, and so the animal is see-through. To compensate for the lack of hemoglobin, the icefish has larger vessels with more blood plasma which moves at a high flow rate to supply its body with oxygen. This is combined with a low metabolic rate and larger mitochondria for producing energy. But there is a downside – it can never leave the cold waters around Antarctica, and would perish if these waters warmed up even a little because – if you remember Henry’s Law – warm waters contain less dissolved gases than cold waters.

Transparency in aquatic animals is largely seen in animals that inhabit, or venture up to feed in, the photic zones, and is achieved by the clever application of the physics of light: light must pass straight through; be reflected back to the observer (as by a mirror); or the light waves must cancel when being scattered. For light to pass unimpeded through the body, the refractive index of the animal must match the refractive index of the medium, so that there is no bending when light passes from one medium to another. This is achievable in an aquatic medium by the use of a transparent, slimy, gelatinous material and is achieved by fragile animals like jellyfish (Figure 1, left). Such transparency is also partly seen in the deeper-living barreleye fish, which looks up into the photic zone for the silhouettes of prey. Its head is transparent, and its eyes can look forward or straight up through its own transparent head. The rest of its body is opaque but contains bioluminescent organs which provide camouflage when the fish is observed from above or below. (Figure 1, middle). When light is reflected straight back, it blends the animal with the light of the background, and crucially helps to hide the opaque digestive system full of food. This is the case with silvery fish whose scales reflect light back to the observer and is particularly effective when the fish is schooling, which also breaks up individual outlines (Figure 1, right). Finally, when light is scattered, one out of two things can happen: the scattered light’s waves may interfere and cancel, or they may not. In the case of cancellation of waves, the effect is like light passing straight through the animal. If they do not, opacity results.

On land, it is difficult to balance the need for support to counter gravity with the lack of support provided by gelatinous material. To have any hope of moving, animals on land need either a hydrostatic skeleton made of fluid filled tubes that provide rigidity, like a hose filled with water; an exoskeleton made of a strong structural material such as chitin found in arthropods; or an internal skeletons made of bone, like we have. The challenge that adding support poses is met by minimizing pigmentation or by providing a perfectly regular arrangement of molecules which allows light to pass straight through the gaps in the stacks. The former is seen in animals such as the glasswing butterfly, the golden tortoise beetle, and the latter is seen in the cornea of the mammalian eye.

Think how easy surgery would be if the skin and underlying tissues could be made transparent. Scientists built a transparent zebra fish which they called ‘Casper’ back in 2007 scientists upended the world of transparency research by making a see-through mouse using the common yellow food dye tartrazine (FD&C Yellow 5).[8] By rubbing it on a mouse’s skin after its hair was removed, the researchers found that it caused the skin to become transparent enough to see blood vessels and superficial organs. The mechanism was alluded to above – the dye reduces scattering and allows the light to penetrate further into the skin.[9] Next time you eat Cheetos, colored with this dye, check your fingers. Maybe you’ll be able to see right through them!*

*Note that safe concentrations have not been established, nor has this product been tested on human skin. Maybe try rubbing F&DC Yellow 5 on a piece of chicken breast instead.

 

References:

[1]. D, L. (2017, January 16). 36 Transparent Animals That Are Hard To Believe Actually Exist. Bored Panda. https://www.boredpanda.com/transparent-animals/

[2]. https://www.facebook.com/OceanPortal (2023). Transparent sea jelly. [online] Smithsonian Ocean. Available at: https://ocean.si.edu/ocean-life/invertebrates/transparent-sea-jelly

[3]. Theearthfeed.com. (2025). Apple-Support assistance. [online] Available at: https://theearthfeed.com/barreleye-fish/

[4]. Rasmussen, C. (2023). Discover 8 Spectacular Fish Found in Jordan. [online] A-Z Animals. Available at: https://a-z-animals.com/blog/discover-8-spectacular-fish-found-in-jordan/

[5]. Image credit: David Tiller – Own work, CC BY-SA 3.0. From: Wikimedia.org. (2022). File:Greta oto.jpg – Wikimedia Commons. [online] Available at: https://commons.wikimedia.org/w/index.php?curid=4762150

[6]. Sr, T.S. (2016). Golden Tortoise Beetle. [online] Our Breathing Planet. Available at: https://www.ourbreathingplanet.com/golden-tortoise-beetle/

[7]. Wikipedia Contributors (2019). Cornea. [online] Wikipedia. Available at: https://en.wikipedia.org/wiki/Cornea

[8]. Rowlands, C.J. and Gorecki, J. (2024). Turning tissues temporarily transparent. Science, 385(6713), pp.1046–1047. doi: https://doi.org/10.1126/science.adr7935

[9]. Keck, C.H.C., Schmidt, E.L., Zhao, S., Liu, Z., Zhang, L.-Y., Cui, M., Chen, X., Wang, C., Cui, H., Brongersma, M.L. and Hong, G. (2025). Achieving transient and reversible optical transparency in live mice with tartrazine. Nature Protocols. doi: https://doi.org/10.1038/s41596-025-01187-z

[10]. White, R.M., Sessa, A., Burke, C., Bowman, T., LeBlanc, J., Ceol, C., Bourque, C., Dovey, M., Goessling, W., Burns, C.E. and Zon, L.I. (2008). Transparent Adult Zebrafish as a Tool for In Vivo Transplantation Analysis. Cell Stem Cell, [online] 2(2), pp.183–189. doi: https://doi.org/10.1016/j.stem.2007.11.002

[11]. Young, L.J. (2024). Scientists Make Living Mice’s Skin Transparent with Simple Food Dye. [online] Scientific American. Available at: https://www.scientificamerican.com/article/scientists-make-living-mices-skin-transparent-with-simple-food-dye/