71% of the Earth's surface is covered by oceans and seas. The formation of wildlife is mainly determined by the following environmental factors: light, water temperature, oxygen and nutrient content. Sea water absorbs incident light, so the amount of light decreases as we go deeper, and its color also changes. In layers deeper than 200 meters, it is already so dark that photosynthesis is not possible. The concentration of oxygen dissolved in water near the surface is higher in the colder waters. Based on all this we distinguish three habitat types: coastal, open water and deep-sea habitats.

Coastal seas

This is where the richest wildlife evolves due to the high nutrient content of the water (the sediments of the rivers are placed here), the favorable light conditions and the substrate. In the shallow parts, on the seabed, higher order plants also live, while farther in, only algae can be found. The coastal zone, where there are heavy waves, is inhabited mainly by animals with solid shell and that can strongly adhere to the substrate. Herbivores include tiny snails and crabs as well as herbivorous fish. Predators include nettle anthozoons and spiny-skinned starfishes on the seabed. Coral reefs are typical inhabitants of tropical shallow seas. The corals forming the reef are very sensitive to several environmental factors. They only develop properly in low-nutrient, clean water around 25 °C degrees with a salinity of 30 to 35 parts per thousands.

Open seas

A nyílt tengerek élővilága a partoktól távolabbi vizek felső 200 méteres részét népesíti be. A víz hőmérséklete ebben a sávban leginkább a szélességi körtől függ. A fajgazdagság kisebb, mint a partközeli területeké, a tápanyag- és oxigéntartalom csökkenése miatt. Az aljzat hiánya következtében csak lebegő és úszó élőlények fordulnak elő. Többségük planktonevő (bálnák), de számos ragadozó (cápák, cetek) is akad köztük.
The wildlife of the high seas inhabits the upper 200 meters of the waters further from the shores. The water temperature in this zone depends mostly on the latitude. Species richness is lower than at the coastal areas due to reduced nutrient and oxygen content. Due to the lack of a substrate, only floating and swimming creatures occur. Most of them (whales for example) eats plankton, but there are also many predators (sharks, whales) among them.

Deep seas

Deep seas mean constantly dark waters. The darkness is complete here, the temperature is very low, and the oxygen content is moderate. There is no plant life, the food source for the animals living here is mainly organic waste coming from higher waters. Due to the harsh conditions that are almost unimaginable to us, deep-sea communities are poor in species. The animals that live here are filtering animals, carnivores or predators. They move very slowly, many fish have strange, almost shocking shapes.