Light in the Deep

Below about a thousand meters, the ocean has no sunlight at all. Not dim, not gloomy. None. For most of the living space on Earth, the dark is complete and permanent.

And yet it is not empty of light. In one long survey of open water off California, about three quarters of the animals seen made their own.

The chemistry is simple at heart. Typically, a molecule called luciferin and an enzyme called luciferase react with oxygen, and light comes out with almost no heat. Most of it is blue-green, the color that travels farthest through seawater.

What the creatures do with that light is where the wonder lives.

The anglerfish dangles a glowing lure in front of its mouth, lit by bacteria it hosts. Hatchetfish and some squid glow along their undersides to match the faint light filtering down from above, so a predator looking up sees no shadow, only more ocean. The vampire squid, instead of ink, releases a cloud of glowing mucus and slips away while the attacker chases the light. Some dragonfish make red light, which most deep-sea animals cannot see at all. It is a flashlight only they can use.

Closer to the surface, single-celled dinoflagellates flash blue whenever the water around them is disturbed. In a few bays, Mosquito Bay in Puerto Rico among them, every paddle stroke leaves a glowing trail, and a hand trailed over the side comes up dripping light. Off Japan, each spring, firefly squid rise toward the shore in the thousands and turn the water blue.

Psalm 104 looks at the sea and marvels at how many creatures fill it, large and small beyond counting. Long before anyone could descend into it, the psalmist was already amazed, and the amazement has only grown. In the book of Job, God asks whether a man has ever walked in the recesses of the deep. For most of history, the honest answer from all of humanity was no.

Yet the lights were burning down there the whole time. Signaling, hiding, hunting, searching for mates, in places no human could go.

That may be the most striking part. This much beauty did not wait for an audience. A sea full of living lanterns was made, and left burning, in water no one would visit for ages, until someone finally went down and saw it.