Mysterious ticking sound leads scientists to creature with green bones in Colombia

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This article from the Miami Herald talks about the discovery of a particular green frog Nymphargus pijao in the village of Pijao, Columbia. It is one of a species of glassfrogs. No, they aren't made of glass, but parts of their bodies are as clear as glass or nearly so. This one has a transparent stomach.

Photo from the article

Skin is thought to have evolved to protect the body of animals, from heat loss or from the effects of sunlight, as well as to provide some sort of distinctive coloring to identify the animal as male or female, or to camouflage it against predator attacks. Why would it be clear, and how does this happen? What other animals are transparent? And is it just their skin that is see-through?

First of all, what is the difference between transparent and translucent? Transparent objects allow light to pass through them completely; the light isn't reflected, absorbed, scattered or refracted (bent). But translucent ones only partially permit light to pass through.  Glass, swimming goggles, and clean water are transparent, but jelly, lampshades, and stained glass are translucent, for example.

Here are some examples of animals that are either transparent or translucent.

• Transparent Amazonian fish (Cyanogaster noctivaga)
• Golden tortoise beetle (Charidotella sexpunctata)
• Sea angels (Gymnosomata)
• Barton Springs salamander (Eurycea sosorum)
• Translucent tadpoles
• Larval squid
• Glasswinged butterfly (Greta oto), close-up dragonfly
• Transparent juvenile surgeonfish
• European eel larvae
• Juvenile octopus
• Translucent snail
• Transparent immortal jellyfish
• Juvenile cowfish
• Ghost shrimp
• Deep sea anglerfish
• Japanese icefish (shirauo), Antarctic ice fish
• Translucent pharaoh ants (Monomorium pharaonis)
• Yellow amycine jumping spider

If you look at the list, you will notice right away that most of the animals live in water. It has been easier for evolution to generate these clear creatures because water doesn’t scatter or refract (bend) light as much as air does. Being transparent in water is a great advantage to hide from predators. Put a glass in water , and you’ll notice how difficult it is to see it compared to being in air. (Some magicians rely on this quality to hide objects in plain view.)

Also, many of the examples are for young creatures (tadpoles, juveniles, larvae). Being see-through is a great defense against predators, especially when they are small and defenseless. If they can't see you very well, they can't eat you. But even adults benefit from this invisibility, too.

Sometimes the whole animal is transparent or translucent, but other times only part of it is, like the Nymphargus pilao frog above, or the Amazonian transparent fish Cyanogaster noctivaga (below).

photo from SciNews

The environment can cause changes so that the animal loses transparency. Ghost shrimp respond to an increase in temperature or salt concentration, and their blood-like material travels to the spaces between muscles, which shows up as white color. It can also happen with old age or when they are about to molt.

clear (left) and white (right) ghost shrimp

So, what is it about the cells through thick bodies of animals that allow light to pass through? Cells normally don't have many pigments in them (like melanin in skin). Nails and lenses of the eye have material called crystallin in the cells to make them clear. Organic molecules in cells have no pigment at all (which is why you see bottles of alcohol or benzene that are clear liquids).

Larvae of eel surgeonfish store energy in the form of a sugary chemical called GAG, which is mostly water, so light passed through easily.

Eel larva from Wikipedia

In the Antarctic icefish, there are no red blood cells. In the blood and other tissues is an anti-freeze protein to prevent the body from collecting ice, and this also reduces the scattering of light.

Photo from The Blood Project

Some mollusks like squids have direct control over cells. Nerve signals sent to pigment cells called iridophores cause a protein inside called reflectin to clump together, and enough of that changes how thick the cell membrane is. When it is thin enough, it stops reflecting light and lets light pass through. Scientists have genetically changed human cells in the lab to do this, too, and it will hopefully help watch certain processes more easily inside cells.

For glasswing butterflies, you can see how the cells are arranged to permit light to pass through. The fuzzy feeling of a wing comes from the little hairs you see below in picture b. Between the hairs at a higher magnification, you can see cell structures called nanopillars which point upward. The left picture is a view from the top, but the c picture is at an angle, so you can see how the nanopillars are arranged. That pattern lets light through. The colored parts of the wing are much more randomly arranged, and light is blocked.

Photos from article in Nature Communications

Nanopillar research is more than just cool stuff to read through a wing. Eye implants made of similar materials allow doctors to measure eye pressure in patients with glaucoma. Without the implants, the optical reader has to be held at a right angle to the eye, which is difficult. Nanopillar implants let the reader scan at any angle.

A good follow-up article on what makes animals cells transparent. "Tissue Transparency In Vivo"

Here's some Japanese research showing how chemicals added to brain tissues make them more transparent for better studying.