Glowing in the Dark Ocean

Sunlight reaches only the upper layers of the ocean. Below about a thousand meters lies a realm of permanent darkness, crushing pressure, and frigid cold. Yet this seemingly empty zone teems with life, and much of it makes its own light. The ability of an organism to produce light through a chemical reaction is called bioluminescence, and in the deep sea it is the rule rather than the exception.

Animals there use their glow for many purposes. Some anglerfish dangle a luminous lure to draw curious prey within reach of their jaws. Certain squid release a cloud of glowing fluid to confuse a predator, much as a shallow-water octopus releases ink. Others flash patterns to find a mate in the vast blackness. Because the deep ocean offers almost no places to hide, light has become both a weapon and a language.

A sudden burst can startle an attacker, while a steady glow can betray the swimmer that produces it, so each animal must balance the risk of being seen against the benefit of being noticed. Some species even tune the color and timing of their flashes so precisely that they amount to private codes, recognized only by their own kind.

Studying these creatures is difficult, since the delicate animals often die when hauled up through warmer, lower-pressure water, so researchers increasingly send cameras and robotic vehicles down to observe them where they live. Each expedition tends to return with footage of species no one has ever catalogued, a reminder of how much of our own planet remains genuinely unexplored.