“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.”