The sea occupies 70% of the Earth’s surface. Its average depth is 4,000 meters. The salinity of the sea water is, on average, about 35 grams of salt per liter of water. The temperature of the sea water surface varies between 32 º C in the tropics and -2 º C in polar regions. The movement of water is produced by tides, caused by the attraction exerted on Earth the Moon and the Sun, by waves created by the wind and currents caused by temperature differences in different parts of water.
In the seas and oceans ditinguen two major areas: the neritic and pelagic zone .
Neritic zone . The neritic zone is located on the continental shelf. It is characterized by the continuous movement of water due to the waves, the tides and coastal currents.
In this area lives a variety of species, both of plankton (flagellates, diatoms) as the nekton (seals and fish) and benthic (gastropods, lichens, equidermos). These living things have many adaptations, and competition for space and food has led to a wide variety of organisms.
The plankton species consists qeu small float on the water. The nekton swimmers consists animals, especially fish. The benthos is the set of animals and plants that live on the funds.
Pelagic or ocean . The pelagic zone is the offshore region located downstream of the neritic. This area covers vertically several layers:
– Stratum pelagic , comprising the maritime area illuminated by sunlight and reaches between 100 and 200 meters deep. This layer is rich and varied in living things, both of plankton (protozoa, larvae, algae, crustaceans) and nekton (squid, fish, dolphins). Above this stratum live numerous birds that feed containing fish.
– Stratum bathyal , located below the epipelagic layer, which is between 200 and 2.00 meters deep. Bodies of this layer are adapted to the absence of light and high hydrostatic pressure. There betónicos body (echinoderms, molluscs) and nektonic (fish, squid).
– Stratum abyssal , located between 2,000 and 7,000 meters deep. The fish in this area tend to be small, and many of them are equipped with light organs. There benthic organisms (asteroids, holothurians) and nektonic (fish).
– Stratum ultraabisal , below the previous, corresponding to very deep trenches, including the Mariana Trench (11,000 meters). There benthic organisms (echinoderms, sponges) and nektonic (fish).