Transparent Animals – Nature's Invisibility Cloaks
Two thousand years before J.R.R. Tolkien’s Ring of Power1 enabled its wearer to vanish, the Greek philosopher Plato described a magic ring that granted invisibility. No mean parlor trick, invisibility also conferred power to prevail against one’s competitors.2Transparent animals benefit similarly in their struggle to survive.
Now you see me, and now you don’t
Animals are complex, material beings, and even the tiniest among them can stub their figurative toes against other objects – so how can they blend in with their environment to fool predators and prey alike?
Camouflage – the mimicry4 of surrounding colors, shapes, and patterns to avoid being seen by predators or prey – is a common and effective strategy used by many animals, from insects to octopuses to large mammals. In essence, the viewer is tricked into misperceiving the hidden animal.
Not to cast aspersions on camouflage, but it only scratches the surface of deception.
Transparency similarly veils an animal but is literally a deeper ploy. The entire body – surface and inner tissues – must work together to become clear. There isn’t just one pill, one strategy, that always works.
Reflections on wearing the magic ring
In science fiction, fantasy, and mythology, shape-shifting is the ability to physically transform from one shape to another, like Sirius Black changing into a dog in the Harry Potter books. Transparency might be thought of as light-shifting – the shape doesn’t change, but rather how the body interacts with light changes, like when a watery mirage appears above the hot surface of a highway.
The mirage arises from refractive differences between the layers of hot air on the road. The greater the differences, the more likely there will be a mirage. Reduce these differences, say by a cooler day, and there is no mirage. Aspiring transparent animals can use a similar strategy by reducing the refractive difference between themselves and the surrounding water. (I say “surrounding water” because it is very difficult for terrestrial animals to reduce the difference between themselves and the surrounding air.)
Salps, comb jellies, and jellyfish are all aquatic animals that mainly consist of water – a jellyfish is 95% water compared to a human’s meager 60%. The refractive difference between a water-full animal and the surrounding water is minimal and produces a less visible outline of the animal, like the mirage that vanishes as the day cools off. If it were made entirely of water, it would vanish altogether. But a 100% water creature is impossible, so animals augment this watery strategy with other mechanisms to enhance their transparency.
“Simplicity is the ultimate sophistication” – Leonardo da Vinci
Internal organs can also announce an animal’s presence by scattering light. Consider a handful of table salt. A single salt crystal is transparent, yet in a pile, the jumble of irregular shapes and different refractive indexes becomes opaque. And this is child’s play compared to the complexity of a living animal’s organs. The more uniform and simple an organ becomes, the less light will be scattered.
However, even the most transparent animals may have gonads (reproductive organs) and a gastrointestinal tract replete with the last meal – the “cross” in the jellyfish picture above is part of the digestive tract, and many species have eyes. These can interfere with concealment, meaning there are also practical limitations: one can eliminate only so many organs before a living thing ceases to exist.
The eyes always reveal what’s hidden below the surface
As an aside, a biologist might quibble that Griffin, the protagonist of HG Wells’ novel, The Invisible Man, couldn’t be utterly invisible and able to see. Vision requires photoreceptors that absorb light, and this absorption would appear as dark spots, betraying Griffin.
Validating the biologist’s quibble, many transparent animals have prominent dark spots marking their eyes.
If it’s so great, why aren’t we all see-through?
Transparency isn’t available to all aquatic animals. It is more prevalent in small juveniles, providing much-needed protection from larger predators. As they mature or grow, they become more opaque; the 50 cm long adult American Eel is not “glassy” like its smaller, 1-2 cm larva.
Tiny animals like the larval eel have small, simple organs. They can acquire oxygen directly from the water through their skin, bypassing the need for hemoglobin, the red blood, oxygen-carrying pigment. However, as they grow, their oxygen demand outstrips this simple mechanism, and they begin to produce hemoglobin. As they gain this light-absorbing pigment, they lose transparency.
Ultimately, the combined development of more prominent, complex organs and the requirement for hemoglobin effectively shut the door to transparency. Aquatic animals like dolphins, porpoises, and seals are warm-blooded, have inherently high oxygen demands, and never even get to the transparency starting gate.
But I'm as transparent as glass!
Can an animal be large, simple, and transparent?
Ignoring the increased oxygen requirements and complex organ structure of larger animals, there are physical constraints on size and transparency. Materials like glass darken as they become thicker. No substance is perfectly transparent – there is always some light loss, even if not noticeable to a casual observer.
At left is the word TRANSPARENT photographed through ½ inch of glass (above) versus 32 inches of glass (below). The loss of transparency is striking.
An animal is no different; a meter-thick jellyfish will lose its transparency and become more visible because of this property. Salps can form long chains, and yard-long comb jellies exist. However, they are usually a centimeter thick at most. If you could convince the salps to line up and wait for you to look through them like a pirate with a spy-glass, you couldn’t read TRANSPARENT either.
An anti-invisibility strategy
Vision is an extraordinary ability that, for some animals, extends into domains we can only imagine. Many arthropods can detect the polarization of light. We can get a glimpse of this ability when wearing polarized sunglasses; otherwise, we are blind to it.6
Some tissues like muscle have an intrinsic property that polarizes light so that they shine like a beacon if you’re one of those polarization-detecting predators.
The bright areas in the photograph to the left show the polarization produced by an aquatic Dobsonfly larva’s muscles.
The tiny water flea, Daphnia, also polarizes light and is preyed upon by other animals like transparent phantom midges, which are, in turn, eaten by fishes who can detect polarized light.
A waterflea as seen by a predator with the ability to detect polarized light.
Squids, themselves partly transparent as juveniles, use their polarization sensitivity to enhance the capture of smaller transparent prey.
This 1 mm squid embryo, still in its egg, was found on a Maine beach. The reddish-purple spots are its polarized-light sensitive eyes.
It’s a squid-eat-fish-eat-midge-eat-Daphnia world. Perhaps Plato’s tale of invisibility and intrigue isn’t a far-fetched analogy for these animals.
Are terrestrial animals missing out altogether?
One of those exceptions is found in the wings of many flying insects that are transparent by virtue of having little or no circulation and being only a few cells thick.7
However, whether this is an actual you-can’t-see-me strategy or simply the insect’s default condition is unclear. After all, adding colored pigments or scales costs resources to the animal. These insect add-ons are often used for camouflage, mate selection, and predator avoidance.8
Even humans can participate in a limited but essential way. Transparency of our corneas, lenses, and the aqueous and vitreous humors that fill our eyes is critical for our vision.
Not surprisingly, the characteristics that favor transparency in small aquatic vertebrates and invertebrates also feature prominently in our eyes: The tissues are mostly water.9 The distances are short since the diameter of the average eye is only about 2.4 cm. There are no interfering organs between our corneas and retinas, and there is no jumble of refractive indexes among the eye’s tissues.
“Everything has beauty, but not everyone sees it.” – Confucius
There is an ironic twist of convergent evolution revealed by our ability to use the transparent tissue in our eyes to help look for other transparent animals. If lucky, you may glimpse a tiny shimmering Dobsonfly larva in a stream or an undulating comb jelly in the ocean. On the other hand, if you see nothing, look again. Someone may be hiding in plain view.
1 Notably, it is the Ring of Power, not the Ring of InvisiblitySee The Hobbit and The Lord of the Rings trilogy.
2 The wearer of Plato’s ring came to power by murdering a king and marrying his widow. You can read the story of intrigue at https://en.wikipedia.org/wiki/Ring_of_Gyges.
3 A Praying Mantis is to the right of the photograph’s center.
4 Four types of camouflage are described: mimicry, disguise, and concealing or disruptive coloration.
5 Jellyfish have nerve nets that control movement and feeding behavior, similar to sea urchins (see my blog https://forestbarkdollweil.com/an-urchin-in-hand/).
6 Haidinger’s brushes are faint human visual phenomena consistent with a limited ability to detect polarization.
7 There are even one-cell thick plants that vie to wear the magic ring.
8 The scales on moths’ wings can help them slip from spider webs and avoid predation by confounding bats’ echolocation systems.
9 The liquid and gelatinous components – aqueous and vitreous humors – of our eyes contain 98% water, a percentage that a jellyfish would covet.
Special thanks for Zoe Weil’s editorial expertise and permission to use her photographs.