The most abundant vertebrate on Earth may be a finger-length deep-sea fish you've never seen: Cyclothone, estimated to number in the hundreds of trillions to quadrillions.
ILLUSTRATION · AIA finger-length, bioluminescent deep-sea fish called Cyclothone is considered the most abundant vertebrate genus on Earth, with an estimated hundreds of trillions to quadrillions of individuals.
Bristlemouth fishes of the genus Cyclothone are small, bioluminescent deep-sea fish, most about 1 to 3 inches long. They are considered the most abundant vertebrate genus on Earth.
A 2023 paper in Frontiers in Marine Science by A.N. Sarmiento-Lezcano and colleagues describes non-migratory Cyclothone as the most abundant vertebrates on Earth, and the Woods Hole Oceanographic Institution's profile puts estimates in the quadrillions.
An estimate, not a count
The published estimates of total numbers run from hundreds of trillions to quadrillions of individuals, roughly 10^14 to 10^16. That spread is itself the story: nobody can count a fish that lives hundreds of meters down in the dim twilight zone of the open ocean, so the figures come from sampling and extrapolation.
These estimates place this little-known genus among Earth's most abundant vertebrates.
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