When most people look at the ocean all they see is just water and more water. But an interesting way of looking at our oceans is to divide the water into vertical zones. Oceans have three main zones; sunlit, twilight and midnight zones. The names of these zones as you might have already guessed depends on the amount of sunlight they receive. This article will look at the conditions of these three zones as well as the organisms that live there.

The first and top layer is called the sunlit zone or euphotic, which is actually derived from Greek which means "well lit". This term is used to describe the layer of water that is bathed in sunlight during the daytime. It has an average depth of about 200m however; the depth of this zone varies and is dependent on the clarity of the water. The temperature ranges from 3 to 40oC. Of course with all this light, it will come as no surprise to find plants and other organisms that depend on photosynthesis to survive in this zone. Photosynthesis is the process where plants take in carbon dioxide and convert it into food, giving off oxygen as a by-product. Oceans produce about 90% of the planet's gaseous oxygen supply via photosynthesis. So we can see how important our oceans are. The animals found there that use the sunlight for food are termed primary producers and are the first link in oceanic food chains. Of course where there is food, there are also animals around to eat it, such as jellyfish, seals, corals and fish and sharks. So that most of the marine life is found in this narrow zone.

Some plants that live in this area include free floating algae also generally known as seaweed. There is the red algae from which nori is made (that is the seaweed used to wrap sushi). Then there is green algae which include sea lettuce and the brown algae which include kelp. Plants found in this zone may also be submerged, like seagrasses and those that are partially submerged like the Red Mangrove whose roots are submerged in the shallow seafloor but the tree grows above the water.

A unique feature of animals that inhabit this zone is that some display countershading. When an animal has a light-coloured underside and dark-coloured upper parts, it is said to display countershading. This form of body colouration provides great protection for the animal because when viewed from above by a predator, it blends in with the darker waters below, but when viewed from below it blends into the light above the water. Besides serving as camouflage it also allows the animal to sneak up on prey, for example sharks.

The next zone is the twilight, mesopelagic or "middle sea" zone or disphotic zone. The latter term is derived from Greek and means "poorly lit", therefore, the colour of the water in this zone which may appear blue or black depends on the clarity of the water. This zone ranges from 200 to 1,000m in depth. However, sunlight does enter this zone but it is too faint and filtered by the previous zone to be used for photosynthesis, and as a result no plants are present and very few animals. This is not only due to the lack of plants, but because of conditions such as high pressures, as high as 1,470 psi (pounds per square inch) which increases with depth. It is also very cold with temperatures falling between 4 to 5oC. Also the amount of dissolved oxygen present in this zone is less than in the sunlit zone.

Like animals in the previous zone, twilight zone animals also have a distinguishing characteristic in that many of them are bioluminescent. This means that they are able to produce their own light by chemical reactions that occur in special organs. The light produced can range from blue-green to red, the latter of which is actually invisible to most deep-sea marine organisms which see mainly blues and greens. The light produced is used in a variety of ways by these organisms, for instance for counterillumination. Counterillumination can be used as a means of camouflage where bioluminescent animals that produce light in their undersides seem to disappear when viewed from above since it forms a dark silhouette against a black background. The animals found in this zone can be categorized as filter feeders and grazers as they feed on the plant matter and algae that fall from the upper zone. But, it also includes predators some of which can lure their prey, using bioluminescent barbells, that is fleshy projections that resemble small animals. However, some animals also migrate up and down between the zones to feed on animals in the sunlit zone. Another characteristic of the animals that live here is that many have large eyes, in order to deal with near darkness. They also have large jaws and teeth for example, viper fish and hatchet fish as well as being small, thin and dark in order so as to aid in camouflage.

The final and deepest zone is the midnight or photic zone which when translated from Greek means "no light". Of course as to be expected, the conditions in this zone are extreme and organisms here have to deal with extremely high pressures and near freezing temperatures.

This zone can be subdivided into the bathyal zone and the abyssal zone.

Depending on the overall depth, the bathyal zone can extend from 200 to 2,000m while the abyssal zone extends from 2,000m to the very bottom which on average is about 4,000m. In the bathyal zone one can find fish, crustaceans and squid that migrate to the twilight zone by day and back to the depths at night, while in the abyssal zone animals are streamlined so that they can move through the water more easily and thus conserve energy. However, the nutrient content is quite high as it is in this zone where all dead organisms fall and settle.

So you can see that the ocean is not just a lot of water, but a complex layered ecosystem which contains organisms that have characteristics, many of which are unique that allows them to be suited to the zone it occupies. Also each zone has unique conditions which as one goes deeper get more extreme.