(Picture from here.)
Some things are just weird.
The little gem to the left here is the Barreleye Fish, Macropinna microstoma. The green spheres are the eyes. The things that look like Sad Sack eyes are in fact nostrils.
The fish has to watch up to find food and look for predators but, as can be seen, the mouth is below where the eyes can look-- or, below where the could look if the head wasn't transparent.
Transparency has evolved over an array of organisms, mostly pelagic. Worms, medusae and fish and other groups all have transparent representatives.
Transparency derives from a low refractive index between animal and environment coupled with a lack of opacity. Transparency, then, must be a joining of several complimentary factors:
- A similarity in the refractive index between the internal chemical environment of the animal and the external environment.
- A physical adaptation of the placement and function of organs in the transparent component such that external material doesn't get in the way. (The "I can see what you have for breakfast." problem.)
- A selective pressure towards transparency once the opportunity to select for it is presented.
Given the above it's no surprise that transparency is almost exclusively found in aquatic animals. The refractive index of cellular fluids is close to that of water since it's mostly water anyway. Consider ice outside of water. It's visible even when transparent by its reflectivity.
Besides, it's hard to achieve transparency when you need bones.
The selective advantages of transparency aren't as obvious as one might think. Most transparent animals are found deep in the sea where there is light available-- not surprising. After all, if there is no light what is the advantage? In the deep ocean there is a marked shift away from transparency to red or black pigmentation. This also makes sense since any light at this depth is generated by biological entities themselves and tends to lack red wavelengths. Consequently, red or black both will absorb the light rendering the pigmented fish essentially invisible.
So transparent species live where there is enough light for it to be an advantage. However, there are a number of animals that live right at the surface. One would think transparency would be an advantage since much of the predation would come from below the animal where any pigment would show as a shadow against the light. However, transparency right at the surface is quite rare.
For evolution to work to favor a trait the rudiments of that trait have to available prior to selection. Consequently, Barrel Bob, above, had to have some transparent qualities on his forehead for selection to favor opening a window to the world.
For selection on Barrel Bob to occur not only does he have to have the rudiments of transparency at hand, his eyes have to be receptive to the frequency of the photons permitted by that transparency.
Transparency as well as opacity depend on what is meant by "visible" light.
Thinking about transparency opens up all sorts of other perceptual ideas.
In the articles I read on transparency it was presumed that reduction of visibility was the selective advantage. The best camouflage is to be invisible, right? But there's another approach to camouflage: disruptive camouflage. This approach has been explored by the military. The idea is not to blend in but to disrupt the perception by the observer. It is also one of the hypotheses behind a zebra's stripes: the striping causes the predator to confuse the individual zebra to blend into with other zebras making it difficult to single out a particular prey animal. I wonder if the distortions inherent in transparency act as a disruptive camouflage.
To a creature that sees in x-rays we would be transparent. In point of fact, such a creature would not be able to perceive opacity as a selective quality-- even though we would. In our creature's home biological environment, transparency would be the norm rather than the exception. There might be an advantage to depositing lead in the skin-- an X-ray based predator might not be able to detect weakness and therefore be less inclined to attack. Internal organ camouflage might be developed-- putting bones (or opacities that appear to be bones) where bones shouldn't be to fool the attacker into thinking the prey was stronger than it was or which end to attack. There are insects and lizards that have "eye spots" on their hind end, presumably to fool predators into attacking the wrong thing.
In my first novel, Caliban Landing, the aliens saw by radio waves. I had presumed they would see surfaces differently and not see inside of things. However, photon penetration is dependent of frequency. Microwaves, as we know, penetrate biological material. That's how microwaves cook a steak or trigger an MRI.
All biological organisms evolve between what the environment presents and what mechanisms in their repertoire are available to them. Had animals on earth pigments that absorbed X-rays and were X-rays readily available, some animal would have evolved a means to use them.
Barrel Bob uses transparency to get around a tricky anatomical problem. He wants to see up. His eyes are badly place for it. So if he sees through his head he can see what he wants to eat. Presumably, in his history, he was eating fine looking down but at some point food above became the better food source.
Barrel Bob selected for a different point of view.
The article that was largely the source for this blog entry is here.