Deep below the ocean surface where sunlight cannot reach, organisms have adapted in astounding ways to deal with total darkness.
Not all darkness is created equal, however. Some fish live between 200-1,000 meters down into the ocean. This “twilight zone” is the transitional region of the sea where all remaining light is slowly swallowed by the abyss. In contrast, the fish below 1,000 meters are in the bathypelagic zone: here, they live in the total darkness of the true deep-sea.
Many species of fish in the twilight zone migrate to shallow water at night. If they were to enter shallow water during the day, they would be vulnerable to the predators there. When they go at night, though, their adaptations to the dark make them right at home. There is no proof that fish living below 1,000 meters migrate to shallow water. Since there isn't any light in their domain, they would have no daily visual cues for such movement.
Many deep-sea fish have a characteristic called bioluminescence, which means “living light.” This living light phenomenon is created by a colony of luminous bacteria in the organ, photophore, which creates light. Bioluminescence helps fish see food, distract predators, and lure prey. One way deep-sea fish distract predators is by squirting a luminous cloud to startle them. They also use bioluminescence to see prey by having lights near their eyes, much like a car has headlights. Finally, fish like the anglerfish have lures attached to their bodies with living light in them that act like built-in fishing poles.
The eyes of deep-sea fish vary in size depending on the zones that they inhabit. Twilight zone fish usually have big eyes, and some of them focus upward to see the shadows of prey made by down-coming light. On the other hand, in the bathypelagic zone, fishes’ eyes are usually tiny. In this lightless environment, the lateral line system – which picks up pressure waves created by swimming animals – is crucial and may be very large. The nasal organ is also highly developed in deep-sea creatures, but it's more developed in males than it is in females. This may indicate that mate selection involves female pheromones, researchers believe.
The deep sea holds many wonders that humans rarely see. This dark place may terrify some people, but for many animals, total darkness is a place called home.
[Source: Oceans and Islands]