For a human, the deep sea is as alien as deep space. Go down several hundred feet into the ocean and the world is dark blue. Another thousand feet and your surroundings have faded to a dim bluish-gray twilight. There is enough illumination for a person to see at that depth, but too little for photosynthesis. Descend through this twilight zone another thousand feet and it is eternal night.
The darkness is not truly dark, however, and the seeming emptiness is actually full of secret messages: About 80 to 90 percent of deep-sea animals use chemicals to create bioluminescent light, piercing the gloom with signals in blue and green, orange and yellow. When it comes to understanding who is flashing whom and what it all means, though, we might as well be trying to eavesdrop on an extraterrestrial conversation.
For decades marine biologists have gotten glimpses of this glittering life by casting nets and retrieving deep-sea organisms. More recently they have lowered cameras on cables and measured the bioluminescence on display beneath the waves. Using special diving suits and submersibles, they have even entered the habitat of deep-sea organisms, watching in awe as the water world lit up with bursts of color that sparkled like fireworks. From these studies researchers have been able to glean a few basic details about bioluminescence. They know that luminescent displays signal the best mates, point the way to food, and warn of danger. The bioluminescent hatchetfish, for instance, uses its light to hide from predators by mimicking sunlight filtering through the water; the shining tubeshoulder uses bioluminescence to startle predators.
