Today’s Wonder of the Day was inspired by Arvind from TX. Arvind Wonders, “Why are X-ray tetra and glass cat fish transparent?” Thanks for WONDERing with us, Arvind!

We were cleaning out the Wonderopolis fish tank the other day when we overheard an interesting conversation between two of the goldfish:

Goldfish #1: Hey Goldie! Who's that new fish over there?

Goldfish #2: Which fish are you talking about, Nugget?

Goldfish #1: That friendly one over there complimenting all the other fish…

Goldfish #2: Oh him. He just showed up today, but he isn't fooling me. I can see right through him.

Goldfish #1: Of course you can! He's an X-ray fish!

Goldfish #2: Well, I guess that explains a lot. He's got some backbone! I'll give him that.

We were surprised that a freshwater fish like Goldie was being so salty, but it's a small tank and we just chalked it up to her being territorial and a bit jealous. We did begin to WONDER about the new X-ray fish, though.

X-ray fish go by several names: X-ray Tetras, Golden Pristella Tetras, Water Goldfinches, and Pristella Maxillaris, according to scientists. They're small fish (usually two to three inches long) that can be found in large schools in the Amazon River and the coastal waters off South America.

Their name comes from the fact that their skin is usually translucent, allowing you to see their backbone and other internal structures clearly. Occasionally, they may also have a silver, yellow, or golden appearance.

Scientists believe the translucent skin of X-ray Tetras may have developed over time as a unique form of protection from predators. When swimming through dense vegetation amongst sparkling waters, their see-through skin makes them particularly difficult to spot.

In addition to their backbones, their skin also allows you to see a bony internal structure called the Weberian apparatus. This structure picks up sound waves and transmits them to the vertebrae, which gives X-ray Tetras a very keen sense of hearing.

X-ray Tetras were first described in the wild by American marine biologist Albert Ulrey in 1894. Since that time, they've become one of the most popular freshwater fish kept in aquariums around the world.

In addition to their unique, see-through skin, X-ray Tetras are known as peaceful fish that get along well with other fish in aquarium settings. They tend to be hardy fish that adapt well to different types of water.

If you choose to add X-ray Tetras to a home aquarium, experts recommend that you add several, since they prefer to live in schools. You should also avoid placing them with larger predatory fish.

Wonder What's Next?

Tomorrow’s Wonder of the Day examines how a dozen years can seem like forever!