The ocean can be divided from its surface to its depth into three zones based on the amount of light received. They are:
Because photosynthesis occurs here, more than 90 percent of all marine life lives in the sunlit zone.
The sunlit zones goes down about 600 feet. Many animals inhabit this zone. Most fish live in this zone. Plankton occurs in this zone. Plankton are free-floating aquatic organisms. They are usually microscopic and form the basis of the food chain in the ocean.
2. Twilight Zone: Only a small amount of light can penetrate the water at this depth. As the water becomes deeper, the pressure increase, too. Plants do not grow here. Only animals that have adapted to little light survive.
The twilight zone is also known as the disphotic zone.
Animals that live in the twilight zone include: lantern fish, rattalk fish, hatchet fish, viperfish, and mid-water jellyfish.
This murky part of the ocean begins at about 600 feet under the water and extends to the darkest part, which begins about 3000 feet down.
Some squid and fish can use their bodies to make light. These creatures are said to have bioluminescence.
(external link): Biolumenescence
3. Midnight Zone: Ninety percent of the ocean is in the midnight zone. It is entirely dark—there is no light. The water pressure is extreme. The temperature is near freezing.
The midnight zone is also called the aphotic zone.
What can live in the midnight zone? The living things found here live close to cracks in the Earth's crust. These cracks give off mineral-rich materials from the Earth itself. Special forms of bacteria utilize hydrogen sulfide from the cracks for energy to make food. All other living things in the midnight zone are nourished by these bacteria.
Living things in the midnight zone include: angler fish, tripod fish, sea cucumber, snipe eel, opposom shrimp, black swallower, and vampire squid.