Image: Courtesy of Larry Madin, Woods Hole Oceanographic Institution
Nature Blows Our Mind!
Jaymi started our Nature Blows My Mind! series by looking into the mesmerizing colors of sea slugs, the amazing structure of shark skin and the incredible math at behind the patterns of sunflowers, but lately I've been pondering that some animals are translucent. Take jellyfish, for example. How are they real? What is that jelly made of, anyway? If you've ever looked through the skin of a translucent organism and wondered, "what's up with that?" join me as I embark on learning about why humans make better doors than windows.
What Does It Mean for An Animal to be Translucent?
Okay, first things first, instead of jumping in to explain what these translucent creatures are doing, I think it's helpful to explain what they are not doing. When we see anything with a color (most things) what we're really seeing is the color that each object is reflecting back to us. So, for example, a green ball is not really green. The ball lacks the ability to absorb the color green, so that part of the light spectrum is reflected back into the world and perceived by our eyes as being of that color. If you think that is neat, consider this: What color is a mirror? Whoa! I know, right? You might be thinking it's just the color of whatever objects it is reflecting, but turns out mirrors contain one particular color more than others. Watch this smart video explaining the answer and you'll gain a better grasp of the way color works.
So, to recap, translucent parts are not absorbing light or reflecting color back to us. Instead, those parts are allowing light to pass through it and we're then seeing the color bouncing off the objects behind the translucent animal, like the Greta oto or Glasswing Butterfly, for example.
So the next question is why are they like this?
Why Are Some Animals Transparent?
There are far more organisms that feature transparency in aquatic environments than on land, according to this paper (pdf) on the Greta oto butterfly, so let's just focus on those organisms first.
According to the Wikipedia page on jellyfish, the body of a jellyfish consist of more than 95% water. Humans are 70% water. Water is clear and jellyfish are more water than humans, so if the human form had 15% more water would we be transparent, too? Not exactly. There's something more important going on here. The jelly part of jellyfish is made of a gelatinous material called mesoglea. Wikipedia explains that the mesoglea "is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut."
Humans, as well as other land and water-based animals also have different epidermis layers and gut linings and we're still not transparent, so what's going on?
The Evolutionary Benefit of Transparency in Animals
MapofLife.org is a resource that tracks and compiles research about evolution in nature and they have a good explanation about the role of transparency.
We are nearly all very familiar with transparent tissues, because without them our eyes would not work. The lens and cornea depend critically on the crystallin proteins, an excellent example of molecular convergence. Transparency is, however, much more widespread. It is useful as a means of camouflage in habitats where an escape from visual predators is otherwise impossible, such as the open ocean, which lacks structures to match or hide behind.
Ah! So the reason more land-based animals aren't transparent, is because it just doesn't make sense in our environment. Fascinating!
I'm only touching on the surface of all this and there's more to the story regarding the evolutionary benefits of transparency, so click through for more info. Map of Life goes on to discuss the interesting fact that certain species of different types of animals - jellyfish, squid, crustaceans - benefit from their evolved transparency, so do read on for more there.
What blows your mind? Tell us in the comments or on Twitter using the hashtag #NatureBlowsMyMind and we may write about your suggestion later in the series. For more Nature Blows My Mind! check out our tag page.