Deep ocean teeming with glow-in-the-dark fish

You wanted to know

"Why do some fish glow in the dark?" asked a sixth-grader in Gregg Thompson's social studies class at Woodland Middle School in Gurnee.

We're all hooked by sultry summer nights swirling with shimmery fireflies. If you could sink deep in the dark region of the ocean at depths equaling eight football fields, you would see dozens of species of glimmery, glowing creatures at once.

The deep ocean is teeming with glow-in-the-dark fish, a trait called bioluminescence, which is common in 90 percent of deep ocean fish. Bioluminescence is extremely rare among fresh water fish.

On land, the pool is limited, including a land snail from Malaysia, a handful of centipedes, millipedes, nematodes and some fungi.

Matthew Davis, assistant professor of biology at St. Cloud State University in Minnesota, said some fish use their glow as camouflage; others hide behind the bright light they emit.

"Where it's really deep, there's no light," Davis said. "It's so deep that light is used for communication among lanternfish."

In his lab, Davis examines samples from bioluminescent fish to determine the evolutionary sources of the trait. His research concludes bioluminescence has evolved 27 times among marine animals.

"I analyze DNA to find common ancestry and sequence it. There's still so much to learn about the deep sea environment," he said.

Recently, Davis worked on a trawler off the shores of San Diego to sink nets and snag fish from the deep regions. His research has taken him to the oceans bordering Vietnam, Taiwan and the Gulf of Mexico.

Davis said although there are a few species in shallow coral reef environments, "The majority are in open ocean depths at around 1,000 meters in an area called the midwater environment."

Researchers also study the fish at eye level using a submersible, or by video using a remote operated underwater vehicle.

Luminescent fish first got their glow by eating luminescent bacteria, which formed colonies. The glow lured fish to the colony where they fed on the bacteria. Over millions of years, the glow became a part of the fish DNA.

As a result, some fish, such as lanternfish, glow on their own. Others, such as anglerfish, rely on symbiosis - a close relationship among species - with the luminescent bacteria to create the appearance of glowing light. There are about 1,500 species of bioluminescent fish.

Davis recorded podcasts for the Field Museum. Learn more about bioluminescent fish on Episode 3, "What the Fish? You Light Up My Life," found on his webpage, bathypterois.com/education.

Check it out

Warren-Newport Public Library in Gurnee suggests these titles on bioluminescent fish:

• "Bioluminescence: Nature and Science at Work," by Marc Zimmer

• "Glow: Animals with Their Own Night-Lights," by W.H. Beck

• "Cold Light: Creatures, Discoveries and Inventions that Glow," by Anita Sitarski

• "Fire in the Sea," by David A. McKee