Bristlemouth

About the Bristlemouth

It ain’t easy being a small fish in the ocean twilight zone. There’s not too much food, and the place is full of hungry predators. That’s why so many twilight zone creatures tend to hedge their bets on having lots of babies. Although many won’t survive to adulthood, some will last long enough to bring up the next generation.

Bristlemouths take this “safety in numbers” strategy to the next level. They’re widely considered the most abundant animal on earth. Estimates, based on what scientists pull up in fine mesh nets, are in the quadrillions. If that’s hard to imagine, just think of thousands of trillions. (Is your mind blown yet?) The most abundant of this fishy family is the genus called Cyclothone, or “roundmouth” for its circular jaw filled with bristling fangs.

If there are so many of these fish, why haven’t we ever seen one? It might have something to do with their preferred hangout, typically a thousand meters (3,280 feet) below the ocean surface. Unlike many other fish in the ocean twilight zone, some bristlemouths don’t migrate to the surface at night to feed. Their physiology might have something to do with this decision: instead of the air-filled swim bladders that other fish use to travel up and down the water column, the bristlemouth’s swim bladder (if they have one) is filled with fat.

Whatever they’re doing to survive, it’s clearly working. Many species of bristlemouth start out life as males, and turn into females as they mature. Their lateral line—an organ that senses vibration in the water—helps the fish hunt and avoid predators. The bristlemouth’s dark skin helps them blend into their murky surroundings, while the glowing green dots along their bellies confuse predators that attack from below.