An ocean is full of life, starting from the surface of the ocean to the deepest ocean bed. Several species of animals and plants are found here, ranging from the microscopic algae (living things which cannot be seen by naked eyes) to the largest creature, the blue whale. The ocean has five major life zones categorised on the basis of the level of light they receive from the sun. These life zones support life of organisms that are uniquely adapted to the specific marine ecosystem.
The Epipelagic or Light Zone: The top layer, which is nearest to the Earth’s surface, is called the epipelagic zone or the light zone. A lot of light penetrates in this zone. It is present up to a depth of 200m from the ocean surface. Due to sufficient sunlight, this zone supports photosynthesis. Most of the marine life exists in this zone, such as plankton, coral, bluefin tuna, jellyfish, etc.
A tuna fish has shiny scales on its body. These scales reflect the sunlight when the tuna swims swiftly and this movement, in turn, confuses the predator.
The Mesopelagic or Twilight Zone: The Mesopelagic zone, also called the twilight zone, is present at a depth of 200 to 1,000m in the ocean. There is no plant growth in this zone because very small amount of sunlight reaches here. As a result, this zone appears deep blue to black in colour. Animals that have adapted to the little light survive here. Huge sperm whales, comb jellies, sea pens, nautilus and a few large fish are inhabitants of the twilight zone.
Twilight zone fish are mostly small and luminous. The lantern fish is one such example. It has organs along its side that produce light.
The Bathypelagic or Dark Zone: The Bathypelagic zone is also called the dark or midnight zone as it receives no sunlight. It is present at a depth of 4,000m in the ocean. The animals living in this zone have huge mouths, sharp teeth and expandable stomachs, such as angler fish, tripod fish, fangtooth, snipe eel, opposom shrimp, black swallower and vampire squid. Plants and animals that die in the light and the twilight zone decompose and the debris falls downwards (marine snow) in the dark zone. It is a source of food for fish in the bathypelagic zone and the zones present below it.
The Abyssopelagic Zone: The bottom zone of the water is known as the abyssopelagic zone. This zone is mostly plain with sea mounts, trenches and valleys. This zone is very salty and cold (temperature = 2°C). It is present at a depth of 4000 to 6000m in the ocean. The pressure of water is very high in this zone (about 1,000 psi). Due to these extreme conditions, life for most animals is impossible. The fish in this zone have specialised jaws which allow them to drag their open mouths along the seafloor to find food, such as mussels, shrimp and microscopic organisms. Certain fish in this zone have bioluminescence.
The Hadalpelagic Zone: The Hadalpelagic zone is the deepest ocean zone, mostly in the form of trenches and canyons. The trenched hadal zone is mostly found in the Pacific Ocean. It is a total dark zone and found at a depth of about 6,000 to 11,000m. Tiny isopods, snailfish, sponges and sea cucumbers live in this zone They survive completely on the marine snow. These animals are adapted to the high water pressure.
Bioluminescent Organisms: These are the organisms which produce light through chemical reactions in their bodies.
