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A Blue View: Bristlemouth Fish

Imagine somewhere down in the jet-black depths of the ocean a creature with a mouth full of needlelike teeth lurks by the thousands.

Published December 15, 2015


Image via Wikicommons; U.S. Public Domain

Sounds like the stuff of nightmares, right? Turns out it’s the stuff of reality. It’s called the bristlemouth fish, and its population is actually well above the thousands.

The bristlemouth fish is small—its slender body just a few inches long—but what it lacks in size it makes up for in numbers. Scientists estimate the bristlemouth fish population is somewhere in the trillions or more. In fact, it’s the most abundant vertebrate on the planet.

In addition to its inconceivable numbers, the bristlemouth possesses another remarkable trait often found in deep-ocean dwellers: it glows. Rows of bioluminescent dots line the bristlemouth’s abdomen. Researchers surmise that they serve as a type of camouflage known as counter-illumination.

Little daylight reaches the deep-sea, but what does is enough to make fish appear as silhouettes to predators below. The bristlemouth’s bioluminescent dots help blur its silhouette.

So, why don’t we know more about the most abundant vertebrate on Earth? Until recently, humans had little means to access the depths where this species lives.

It wasn’t until the late 1800s, when the H.M.S Challenger dropped research nets three miles down into the ocean and pulled their contents to the surface, that this fish was first discovered. And it took until the 1930s for a scientist named William Beebe to descend in a submersible and observe this fish in its natural habitat.

Specialized trawling nets have been used in recent years to explore greater depths and oceanographers have even set out to map the density of life in the deep sea.

In any case, scientists are just scratching the surface. It’s estimated that millions of marine species are just waiting to be discovered.

Episode Transcript

If you had to guess the most common vertebrate on the planet, you might say deer or squirrels. Maybe mice or factory-farmed chicken? Even humans?

It's natural that our first guess would be another creature that lives like us. Human scientists are terrestrial. We live on land. We think Earth-first. Something familiar. Something we see all the time.

But it's not. The most abundant vertebrate on earth is a fish. A bony, little, deep-ocean fish just a few inches long. It emits light in the depths where it lives, capturing prey with the needlepoint jaws that earned it its name: bristlemouth. There are 13 species of this type of fish.

The bristlemouth fish has been relatively unknown to science—despite its truly mind-boggling population numbers. The figure for bristlemouths is estimated at "hundred of trillions—and perhaps quadrillions, or thousands of trillions." That's 15 zeros.

And scientists think that still might be an understatement.

By comparison, there are a mere 24 billion chickens on earth, approximately 14 billion rats and around 7 billion of us humans. And we think we're a big deal.

From Antarctica to Southern California to the Mediterranean and Indian Ocean—no matter where marine biologists sample in the deep ocean, they've hauled up bristlemouths. Lots of wriggling, snaggletooth bristlemouths, in shades of tan to transparent.

It’s not surprising, really. The sunless mesopelagic depths of the ocean are, in the words of New York Times science writer William Broad, "the Earth's main biosphere," but it still catches us by surprise, because we don’t live there.

If you’re a fish, the most action—and 95 percent of the biomass—happens between 650 and 3,000 feet below the ocean’s surface.

It's stunning that the most abundant vertebrate on our planet would be incognito. We've had blinders on, because bristlemouth fish thrive in an environment that, until recently, has been difficult for us to study. The first living bristlemouths were discovered in the 1930s.

Now that we have better dive equipment, sonar and increased deep-sea exploration technologies, we're getting better at knowing our most populous "neighbors" although there is still a lot that we don't know.

Bristlemouth are also called "lightfish" because they, like many deep-sea species, communicate, hunt and camouflage in the darkness by means of bioluminescent light, using specialized light-emitting organs. They hunt for small, shrimp-like creatures called copepods and, in turn, are hunted by fang-fish and dragon fish.

They change gender too, beginning life as male and transforming to a female.

Bristlemouth may also comprise the largest vertebrate migration in the ocean. They rise through the water column to surface waters to feed on drifting plankton by night, then swim deeper by day.

By doing so, they capture large quantities of carbon and release it into the deep waters much faster than any other organism. This feeding behavior transports organic matter, including carbon, from the surface to deep waters where it can be more readily used in deeper trophic food webs and biogeochemical cycles.

Incredible. This little deep-sea fish has had a big impact—we humans were the ones in the dark. We have a great deal to learn about the sea and especially about the denizens of its deep.

To learn more about the bristlemouth fish and how "deep life" is currently being surveyed and mapped, visit A Blue View is produced for WYPR by the National Aquarium whose mission is to inspire conservation of the world’s aquatic treasures. For the National Aquarium, I’m John Racanelli.

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