Fauna of the Pelagic Zone
The pelagic zone is the area of the open ocean, which is home to the majority of marine life including phytoplankton, zooplankton and nekton (free swimming marine organisms). Most of these animals live in the surface waters, where food is abundant, visibility is good, and the temperature is not too cold.
The epipelagic zone extends from the surface to a depth of 660 feet (200 meters). The epipelagic zone receives sunlight, allowing phytoplankton to photosynthesize and flourish forming the basis of an extensive marine food web supporting many higher species, including large numbers of schooling prey fish such as sardines and anchovy. These in turn, are important for the survival of other marine species including predatory fish like the Atlantic salmon of the north, Atlantic bluefin tuna, and small sharks. Seabirds such as gannets, cormorants, and the Humboldt and African penguins are found in the Southern Atlantic Ocean; together with seals, and the largest predator in the ocean, the great white shark, a top predator that will feed opportunistically on whatever whets its appetite.
The mesopelagic zone extends from the epipelagic zone to a depth of 3280 feet (1000 meters) where sunlight cannot penetrate. This zone is dark and cold, and is home to predatory fish such as sharks, which generally have poor vision, but are equipped with finely tuned sensory organs to compensate. These include a keen sense of smell and hearing, combined with extra sensory devices such as the lateral line, and ampullae of Lorenzini, a unique electroreceptive device that enables them to detect prey by picking up the electric field emitted by all living organisms. Sharks are also able to reduce their metabolism to conserve energy in cold waters. The mesopelagic zone is also home to filter feeding fish that migrate upwards to feed on plankton in the upper layer at night, when they are less conspicuous to predators, and migrate back down to the safety of the dark depths by day.
The bathypelagic zone extends from the mesopelagic zone down to a depth of 13,000 feet (4000 meters), it is extremely dark and cold, with low oxygen levels, making it very difficult for animals to survive here unless they are specially adapted to survive these inhospitable conditions. Lantern fish are specially adapted to life in these dark depths by having large eyes that enable them to gather light in order to see in the dimly lit conditions, and photophores distributed over the surface of their body, which enable them to produce light through the chemical process of bioluminescence. This light is thought to aid in attracting prey on which it feeds.
The abyssopelagic or abyssal zone extends from the bathypelagic zone to the depths of the deep oceanic trenches reaching the ocean floor. The conditions are similar to the bathypelagic zone, but the pressure is extreme, and temperatures can reach near freezing, with the exception of thermal vents, where sea water is forced down fissures in the sea floor, where it is heated by molten magma in the Earth's mantle, causing extremely hot water, rich in minerals, to escape, heating up the surrounding water considerably. Thermal vents in the deep troughs of the Mid-Atlantic Ridge provide warm water, rich in minerals that allows animals, such as mussels, tube-worms, crabs and eelpout fish to survive. These animals have adapted to cope with the extremely high water pressure experienced at these depths. In the absence of light, bacteria are able to utilize the minerals in the water as an energy source to produce organic matter through a process of chemosynthesis, similar to the process of photosynthesis. The bacteria not only provide a direct food source to filter feeders, but also form symbiotic relationships with tube worms, mussels and clams. Residing in the body of their host they receive processed minerals from the host, and in return, they provide the host with organic material produced through chemosynthesis.