The dark, cold depths of the ocean sparkle with trillions of glowing fish, report scientists in the current issue of PLOS ONE.
Creatures with names like lanternfish and dragonfish and ponyfish shine like fireflies, filling the ocean with countless sources of light.
The sheer volume of these glowing fish boggles the mind. The 250 species of lanternfish make up two-thirds of all deep sea fish by weight, according to Matthew Davis from St. Cloud State University, lead author on the new study. "They're among the most abundant vertebrates on the planet in terms of mass, but the average person doesn't know anything about them," Professor Davis told National Geographic.
Bioluminescence is hardly a new phenomenon. Scientists have found evidence of glowing fish as far back as the Jurassic era, when dinosaurs roamed the earth. Today, nearly 1,500 different species of fish produce light from their heads, glowing stripes on their bodies, bright dangling parts under their mouths or eyes, or even from their stomachs.
Some of these fish developed bioluminescence on their own, through cellular chemical reactions, while other species developed specialized organs to hold glowing bacteria.
Davis and his colleagues developed a family tree of ray-finned fish, which includes almost 99 percent of fish species. That helped them map out a startling discovery: Fish evolved luminescent capacities at least 27 different times.
On land, evolution happens most dramatically when species are isolated from each other, as on islands in narrow mountain valleys.
"In the ocean, there are no physical barriers to keep different groups from reproducing with one another," said John Sparks, study co-author and curator-in-charge of the American Museum of Natural History's Department of Ichthyology, in an interview with The Washington Post. "So you have to ask how reproductive isolation comes about."
The researchers argue that bioluminescence is key to much of the ocean's species diversity. They found that often, the number of species in a lineage expanded after that branch of the fish family tree evolved bioluminescence.
Some fish, like the deep-sea anglerfish featured in "Finding Nemo," keep the microbes in their rear fins, while ponyfish keep them in their throats, and control the amount of light they emit by evolving what National Geographic called "muscular shutters and translucent windows."
The most numerous bioluminescent fish, known as dragonfish, don't need help from glowing microbes – they produce their own light. The 420 dragonfish species, including the bristlemouth, which is the most common species in the world with a backbone, have long bodies, intimidating faces, and sharp teeth. Hundreds of trillions of dragonfish live in the deep ocean.
Self-glowing fish outnumber the fish that glow in partnership with bacteria, say the researchers, which Davis attributes to the greater control available to self-generated luminescence. Ponyfish must hide or reveal light they can't control, while lanternfish and dragonfish turn the light on and off through the use of different nerves.
The researchers speculate that the fish use their light to attract prey, hide from predators, and even communicate with one another. They are working on a deep-sea camera so they can observe the creatures interacting in their native environment.
"Right now, we have no idea how they flash," Dr. Sparks told The Washington Post. "And when something evolves so many times, you know it's obviously very important."