Cool Critter: Spookfish
What would you do if you lived in a very dark place where vicious monsters lurked above and even nastier critters dwelled below? (Running away is not an option. Neither is crying.) Stumped? That’s okay. You’re not likely to face this dilemma in real life.
The brownsnout spookfish, however, deals with such issues on a daily basis. (That’s right, it’s called a spookfish.) There’s not much light 800-plus meters (more than 2,600 feet) below the ocean surface, but a whole lotta hungry critters still call this part of the ocean home.
How does a small-ish fish (up to 8 inches in length) with a funny name survive in these parts? Using light and mirrors.* First, the fish’s head is mostly see-through which allows it to capture lots of ambient light from the mostly dark waters around it. And then, there are its eyes.
Like most animals, the brownsnout spookfish has two eyes. But that’s pretty much where the similarities end. The brownsnout spookfish is the first vertebrate (animal with a spine) to use mirrors for eyes. Each eye has two parts. One part of the eye points upward. The other part points downward. With the smidgen of light that eeks in from the surface, the spookfish uses the upward-pointing part of the eye—which has a lens, just like any normal eye—to see any potential food or threats above it.
It uses the downward-facing portion of the eye to see flashes of bioluminescence below it. This is where the mirrors come in. Instead of using a lens, this part of the eye uses mirrors to focus directed light onto the retina. Lenses actually absorb some of the light that passes through them which would be rather silly when there’s so little light to begin with. Mirrors, on the other hand, reflect light, making them far more efficient in the low-light conditions the spookfish haunts.
Scientists suspect that the mirrors are made of guanine crystals. Wait—guanine? Does that ring a bell? It should. It’s one of the nucleobases in DNA. (Remember: Adenine bonds to thymine and cytosine bonds to guanine.) Crystaline guanine is the same thing that makes silvery fish iridescent. It’s also used to make cosmetics shiny or “pearly.”
Thanks to their super-special eyes, spookfish can spot predators from above and below—in the dark. But their tremendous visual abilities don’t help them hide in the wide-open sea. They’ve got another trick—called counter-illumination—that helps them hide from predators beneath them. All along the length of their bellies, spookfish have weakly glowing bioluminescent organs filled with symbiotic bioluminescent bacteria. From below, the spookfish’s glowing belly blends with the ambient light coming from the ocean’s surface to make the fish practically invisible.
*If you thought we meant “smoke and mirrors,” think again. We’re talking about underwater, people. There’s no smoke underwater.