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Scale diagram of the layers of the pelagic zone.

The pelagic zone (from Greek πέλαγος pélagos) is also known as the open-ocean zone. The pelagic zone is further divided into sections creating a number of sub-zones based on their different ecological characteristics. These characteristics are roughly a function of depth.

Epipelagic

From the surface (MSL) down to around 200m (656 ft).

The illuminated surface zone where there is enough light for photosynthesis. Due to this, plants and animals are largely concentrated in this zone. Here one will typically encounter fish such as tuna and numerous amounts of sharks, as well as colorful dolphinfish and jellyfish.

Mesopelagic (the twilight zone)

From 200m down to around 1,000m (3,280 ft).

Although some light penetrates this deep, it is insufficient for photosynthesis. The name stems from Greek μέσον, middle. Animals such as swordfish, squids, wolffish, a few species of cuttlefish, and other semi-deepsea creatures live here.

Bathypelagic

From 1,000m down to around 4,000m (13,123 ft).

By this depth the ocean is almost entirely dark (with only the occasional bioluminescent organism, such as lanternfish). There are no living plants, and most animals survive by consuming the snow of detritus falling from the zones above, or (like the marine hatchetfish) by preying upon others. Giant squid (as well as smaller squids & dumbo octopuses ) live at this depth, and here they are hunted by deep-diving sperm whales. From Greek βαθύς (bathys), deep.

Abyssopelagic

From 4,000m down to above the ocean floor.

No light whatsoever penetrates to this depth, and most creatures are blind and colourless. The name is derived from the Greek άβυσσος (ábyssos), abyss, meaning bottomless (a holdover from the times when the deep ocean was believed to be bottomless).

Hadopelagic

The deep water in ocean trenches.

The name is derived from the Greek Άιδης (Haidēs), Hades, the classical Greek underworld. This zone is mostly unknown and very few species are known to live here (in the open areas). However, many organisms live in hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000m (19,685 ft), whether in a trench or not.

The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some marine biologists elide them into a single zone or consider the latter two to be the same.

• Photic zone (Sunlit zone)
• Aphotic zone (Twilight zone)
• Benthic zone
• Neritic zone

• The Deep Sea pages at Oceanlink
• University of Haifa's pages on deep sea oceanography