The ocean can be divided into zones according to depth, and it can be characterized by the amount of sunlight that can penetrate through the water column, the temperature and salinity profiles, as well as the types of habitats & ecosystems that exist there. Organisms in the oceans have adapted unique lifestyles for surviving and thriving in extreme physical conditions.

The ocean can be divided into zones based on the availability of sunlight. These zones, from most to least amount of sunlight, are the: euphotic, photic, disphotic, and aphotic zones. Ocean habitat zones can be determined based on water depth. From shallowest to deepest water, these zones include the neritic, oceanic, bathal, abyssal, and hadal. Scientists have also divided the ocean into zones based on the lifestyle and behavior of organisms. These categories include: plankton, nekton, infauna, and epifauna.

Organisms also live in the tidal and intertidal zones of the ocean. These organisms also have specific requirements and adaptations to be able to survive in the area between high and low tide.