Top-down: Epipelagic, Mesopelagic, Bathypelagic, Abyssopelagic, Hadopelagic.
The open sea is differentiated into the already mentioned zones, due to the fact that different depths of the sea also have different features.
The sunlight is an important issue when scaling the zones as the spectrum of light is decreasing with each meter you go further under water. Another important factor is the pressure, which increases the deeper you go beyond the surface: Every 10 metres, pressure increases by approx. 1 bar.
Ocean layers overview:
The uppermost zone (Epipelagic) stretches from the surface up to around 200 m below sea level. It is the photic zone in which the light is “strong enough” so that photosynthesis can take place. Hence, almost all primary production happens in this zone and you’ll find many species in here as well (e.g. plankton).
Roughly between 200 and 1,000 m below sea level, the Mesopelagic is a transition from the light-flooded zone to darkness. As no red light can penetrate this zone anymore, photosynthesis cannot take place here any longer.
The Bathypelagic reaches from 1,000-4,000 m and is already very dark. As no light penetrates this region at all, you won’t find any plants here either. Without plants, species down here mainly feed on dead material sinking into the zone.
In the Abyssopelagic, the zone between 4,000 m and the ocean basin (approx. 6,000 m), the pressure is very high, temperatures very cold and it’s absolutely dark. Only few species are adapted to those conditions, hence there’s only few life.
The Hadopelagic describes the deepest zone which reaches from the ocean basin into trenches like the Mariana Trench (approx. 11,000 m below sea level). Only very well adapted species can live under such circumstances, hence here’s (almost) no life anymore.