Who Puts The Oooh In Shampoo Fish Scales?

Remember Prell shampoo? I know I'm dating myself on this one. In fact I'm not even certain Prell is still in existence. Being follicularly challenged, it's been quite some time since I strolled down the shampoo aisle. Be that as it may, Prell was/is a shampoo notable mostly because of its trademark viscosity (why this would matter beats me) as well as its opalescent green color. It truly was almost hypnotic to watch the immiscible shimmering colors in the shampoo move about in the tube. I recall that as a child my family had a seemingly endless supply of Prell on hand. It was the 70's, after all, and Prell did wonders for my parted-down-the-middle-and-feathered-back look, but that's a story for another time.

Prell got it's characteristic opalescent shimmer from the addition of crystalline guanine that was suspended in the viscous shampoo solution. Yes, that guanine...one of the five main nucleobases found in the nucleic acids DNA and RNA (the others being adenine, cytosine, thymine, and uracil.) When crystallized, guanine forms rhombic platelets composed of multiple, transparent layers that have an extraordinarily high index of refraction that partially reflects and transmits light from layer to layer, thus producing a pearly luster.

Since it makes for such appealing eye candy, it wasn't long before commercial applications became widespread. In the cosmetic industry, crystalline guanine is used as an additive to various products (including shampoos) where it provides the pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It also provides shimmering luster to eye shadow and nail polish.

So why am I taking the time to discuss Prell and crystallized guanine on an ocean blog? Well, I just finished a fascinating bit of news from Science Daily that describes a new report in the American Chemical Society's journal Crystal Growth and Design on how tiny crystals of guanine also provide the iridescent sparkle seen in fish skin that can help some fish blend in with shimmering water. A team of researchers in Israel has found that fish optimize the shape of these crystals, inhibiting the growth of guanine crystals into boxy prisms in favor of growing them into thin, stacked plates parallel to the fish's scales. This adaptive response creates an iridescent glow providing camouflage in light-dappled water surfaces. How fish are able to selectively inhibit biogenic crystal growth is still not understood.

The researchers looked at the skin of Japanese Koi. Japanese Koi are a highly prized ornamental, domesticated variety of the common carp closely related to goldfish. Koi naturally display an amazing variation in scale coloration and degree of scale iridescence. As a result of domestication and deliberate hybridization, there is a limitless level of variation possible in koi. Perhaps because of their variation and beauty, koi color and iridescence is no longer limited to fish scales but has made the transition to human skin as well.