“Is there such a thing as an invisible animal? In the sea, yes. Thousands! millions! All the larvae, all the little nauplii and tornarias, all the microscopic things, the jelly-fish. In the sea there are more things invisible than visible! I never thought of that before. And in the ponds too! All those little pond-life things—specks of colourless translucent jelly! But in air? No!… If a man was made of glass he would still be visible.”
–H.G. Wells, “The Invisible Man”.
I have written about invisible animals before, and all the ways in which one can become invisible, of which transparency is only one. I’ve spent some time thinking about transparent animals, from the Glass Frog of Central America:
to the Glass Squid of the deep oceans:
The transparency of the frog is obvious as a means of camouflage, but it is less certain in the case of the squid. Does its transparency serve to make it invisible? Or is there simply so little available light that producing pigments of any sort is wasteful? A great number of deep-ocean animals are transparent, including the Phronima, a type of amphipod with a glass-like exoskeleton, and the sea cucumbers which make up 90% of the complex animals on the abyssal plain. But the depths are not the only dark places on Earth; in the subterranean grottoes live the “troglobites,” animals adapted to the life in the sub-basement of the world:
The Alabama Cave Shrimp:
The Transparent Cave Crayfish:
And the Glass Goby:
Where these animals live, there is not even a stray photon bouncing off the stalactites, and so even the term “invisible” is inherently useless. There’s no such thing as “visible” there. To make an admittedly silly pop culture reference, I’m reminded of the character Invisible Boy from the 1999 film Mystery Men. On a team of quirky superheroes with dubious “powers,” Invisible Boy’s abilities are the most useless: He can only turn invisible when no one’s looking. The majority of “invisible” animals have the same superpower: their transparency is just a by-product of another adaptation, because where they live, nobody could see them even if they were day-glo orange.
“Visibility depends on the action of the visible bodies on light. Either a body absorbs light, or it reflects or refracts it, or does all these things. If it neither reflects nor refracts nor absorbs light, it cannot of itself be visible. You see an opaque red box, for instance, because the colour absorbs some of the light and reflects the rest, all the red part of the light, to you. If it did not absorb any particular part of the light, but reflected it all, then it would be a shining white box… A glass box would not be so brilliant, not so clearly visible, as a diamond box, because there would be less refraction and reflection. See that? From certain points of view you would see quite clearly through it… And if you put a sheet of common white glass in water… it would vanish almost altogether, because light passing from water to glass is only slightly refracted or reflected or indeed affected in any way.”
One of the most beautiful of the glass animals is the Crocodile Icefish, of the order Channichthyidae. They are aptly named, for they live only in Antarctic waters, where the ambient temperature is below 32 degrees Fahrenheit. While there are many transparent animals in the world, the icefish has done what no other vertebrate has been able to do: make its blood invisible.
In The Invisible Man, H.G. Well’s titular antagonist, an albino by the name of Griffin, manages to make himself see-thru by filling the interstices of his cells with fluid, lowering its refractive index and rendering his cell walls clear, the way you can see through opaque paper by dipping it in oil. The problem, he found, was with his blood. Hemoglobin, the protein binding agent which carries oxygen, also happens to be what makes red blood red. An albino may lack melanin, but the pinkness in their eyes and skin proves they haven’t lost their hemoglobin pigment. In fact, if anyone were to lose their hemoglobin, they would suffocate within seconds. So one of the Invisible Man’s last challenges during his transformation was to make his blood invisible, and Wells, never one to get extremely technical, doesn’t explain how. Perhaps he should have met the icefish.
The ancestor of the icefish, a perch, retained the hemoglobin gene, like every other vertebrate — this much we know from the icefish’s genome. But in the frigid waters of the Antarctic, where dissolved oxygen is higher than any other place on Earth, there is so much oxygen there is no need for hemoglobin to carry it. Also absent in the icefish is myoglobin, the red oxygen-carrying protein in muscles, making its flesh and heart invisible. Of course, the icefish had to evolve many more adaptations to balance out the loss of hemoglobin and myoglobin — larger blood volume and larger hearts; a glass-brittle, demineralized skeleton; a slowed metabolism and a lower demand for oxygen in general — so it seems unlikely that the trade-off was to its advantage. Its capacity to carry dissolved oxygen in its blood is less than 10% of that of its closest red-blooded relatives, which means it is confined to the coldest, most oxygen-rich waters: a prison of ice. Geneticists have found no evolutionary upside to the icefish’s loss of hemoglobin and myoglobin, and tests have confirmed that adding myoglobin to its bloodstream enhances its efficiency. Invisibility, then, seems to be a by-product of a necessarily and tolerable sacrifice in order to exploit a habitat no other fish could. To survive in the frigid environment as an ectothermic creature, it had to give something up, and the hemoglobin was an acceptable gene to part with.
It also suggests that the first step to vertebrate invisibility, oh you mad scientists, is to find a substitute to hemoglobin and myoglobin — to somehow make oxygen more accessible to our cells. It seems like the key to invisibility may lay, as it does with so many superpowers, with proper breathing technique.