Roughly 200 to 1,000 meters below the surface of the ocean sits a layer scientists call the twilight zone, or mesopelagic zone. Sunlight still reaches it, but only barely. It is dim, cold, and under enormous pressure. It is also, by some estimates, home to more fish biomass than the rest of the ocean combined.
What actually lives down there
The twilight zone is dominated by small creatures: lanternfish, bristlemouths, squid, and jellyfish. The bristlemouth alone is thought to be the most abundant vertebrate on Earth, numbering in the quadrillions. None of these animals are household names because almost none of them are commercially fished, and almost none of them ever come to the surface alive.
Why it matters to you
Every night, billions of these animals migrate toward the surface to feed, then sink back down before dawn. It is the largest animal migration on the planet, and it happens daily. In doing so they carry carbon from the surface down into the deep ocean, a process scientists call the biological pump. Without it, atmospheric CO2 levels would be substantially higher than they are today.
Why it is in the news
Several countries and companies are exploring commercial fishing of twilight-zone species for fishmeal and supplements, and deep-sea mining interests are eyeing the same waters. Marine biologists have warned that we know remarkably little about a zone that regulates so much of the planet’s carbon cycle. Removing biomass there could have consequences we cannot yet model.
The bottom line
The twilight zone is a good example of how the parts of Earth we understand least often do the most work. It is dark, it is distant, and it is quietly keeping the planet’s carbon budget in check.
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