Why are glassfish transparent?

Why are glassfish transparent? By this I’m not asking the evolutionary purpose of their transparency, but rather how it is that one is able to see through them, or at least through their tails. It must be both their skin and their muscles which are transparent. How is it that they can have transparent muscles when every other fish flesh I’ve seen has been opaque white, grey, pink, orange, or red? Do all these other fishes have pigmented muscles, and if so, what’s the purpose of putting the pigment on the inside where no one can see it? On the other hand if it’s not pigment but something else (like fat or blood), how can glassfishes get away with not having it? Or are transparent muscles in fact quite common in the fish world, but nobody realizes it because their skin is opaque? It occurred to me that glassfish flesh might be transparent mostly because it’s very small and thin, but I’ve seen lots of other raw fish their size (say, sardines, sprats, and anchovies) with opaque flesh.

Oh, and is there any chance we could genetically engineer other transparent animals? Like maybe a transparent horse or housecat?

The pink and orange muscles certainly contain a pigment, one of the porphyrin pigments, the same chemical group that makes your blood and muscles red. In humans the red pigment is myoglobin, not sure what it is in fish.

However the porphyrin pigments are only needed in fish that engage in constant active swimming, such as those that live in fats flowing streams. The pigment serves as an oxygen buffer between the blood and the muscle, allowing the muscle to function indefinitely without tiring. Most fish get by just fine with white meat that lacks the reddish pigment, even though that means the muscle is only useful for short bursts of activity.

As for why the pigment is on the inside, it’s an accident of evolution. The porphyrin pigments probably originally served a purpose as actual light gathering pigments, and they are still the core unit of cholorphyll in plants. An “accident” of their structure also made them ideally suited to ion transfer, so they got co-opted. The fact that they are pigments is just an evolutionary relic, not an integral part of their function in oxygen transport.

With the exception of the pigments and highly calcified bone, virtually no organic material is actually opaque. That might seem untrue, but the fact is that material such as muscle, fat and so forth are nearly perfectly translucent. If you don’t believe me, find someone with fair skin, get them to hold their fingers out and tightly together as if saluting, and then shine a torch through their fingers. Because their is no muscle in the finger, and hence no myoglobin pigment, almost all the light will pass through. It will be scattered to hell and totally incoherent, but it will all pass through and come out somewhere in the other side.
So to make a transparent animal, you don’t need to change much in the gross actual structure. What you need to do is modify the microscopic structure so that the tissue has the same refractive index as water. That way instead of just being translucent and scattering light through the tissue, the light remains coherent. There are lots of ways that animals do that. Many coating their tissues with microscopic crenelations slightly smaller than the wavelength of the light so that the light changes direction gradually rather than having an abrupt refraction. Another trick is to flood the tissue with large amounts of transparent oil with a refractive index close to that of water, that way the light never experiences any change in refractive index when it enters the animal or moves between tissue types. There are other tricks, though I forget most of them.

The first problem is that you can’t make a transparent land animal that works. Biological material with a refractive index close to water is easy to come by. Material with the same RI as air is almost impossible. Glass itself is perfectly transparent, but it doesn’t “work” in the sense that a glass object is invisible on land. Glass is invisible in water but it sticks out like dogs balls on land.

The second problem with these organisms is that they are too damn thick. Even if your tissues are within 1% of the RI of air and each other, once light has passed through more than an inch of so of it, the refraction becomes large enough to make the object noticeably translucent.

The third problem is that tetrapods are highly dependent on calcified bone and haemomoglobin infusion of the tissue. That means that the blood and bone will never be invisible no matter how hard you try, so neither will the tissues infused with blood.

In theory you could make a cat that is semi transparent, with transparent skin for instance, but that would be about the limit of it.

Thanks for the very informative reply, Blake!

Wait a minute - of course there’s muscle in your fingers. Isn’t there?

Nope, just tendons which are connected to muscles in the palm of your hand and the wrist. Look at thesepicturesfor example.

Wow, that’s a lotta new info in one thread. An enormous “thank you” to Blake - I’ve had glass catfishbefore, and you can friggin’ read through them. It’s the damndest thing, and I always wondered the hows and whys.

But it never occurred to me that there is no muscle tissue in fingers, just tendons. (Ligaments, too? Or only tendons?)

Yes, ligaments as well as tendons. Collateral ligaments on either side of each joint keep your fingers bending in only one plane, as opposed to flopping from side to side.