The Open Ocean

Many open ocean organisms live out their existence without ever coming into contact with the shore, the seafloor, or the water’s surface. They spend their entire lives surrounded by water on all sides and do not know that anything else even exists. In the case of the deep open ocean, organisms never even see sunlight. As land mammals that breathe air, walk on land, and rely on our sense of sight for almost all functions, it is difficult for people (even experts) to comprehend that most of the organisms on the planet are never exposed to air, land, or sunlight.

The open ocean is an enormous place. In fact, more than 99% of the inhabitable space on earth is in the open ocean. In order to better study and understand this huge ecosystem, scientists divide the it into different zones:

1. The epipelagic zone (or upper open ocean) is the part of the ocean where there is enough sunlight for algae to utilize photosynthesis (the process by which organisms use sunlight to convert carbon dioxide into food). Generally speaking, this zone reaches from the sea surface down to approximately 200 m (650 feet). The epipelagic is home to all sorts of iconic animals, like whales and dolphins, billfishes, tunas, jellyfishes, sharks, and many other groups. Algae that live in the epipelagic zone are responsible for much of the original food production for the entire ocean and create at least 50% of the oxygen in the atmosphere (both through photosynthesis). Organisms that live in the epipelagic zone may come into contact with the sea surface.

2. The mesopelagic zone (or middle open ocean) stretches from the bottom of the epipelagic down to the point where sunlight cannot reach. Generally speaking the deep end of the mesopelagic zone is approximately 1000 m (3300 feet) deep. The mesopelagic zone is much larger than the epipelagic, and the most numerous vertebrates on Earth (small bristlemouth fishes) live in this zone. Many of the species of fishes and invertebrates that live here migrate up into shallower, epipelagic depths to feed, but only under the cover of night.

3. The next deepest zone is called the bathypelagic zone (or lower open ocean). This zone starts at the bottom of the mesopelagic and stretches down to 4000 m (13,000 feet). The bathypelagic is much larger than the mesopelagic and 15 times the size of the epipelagic. It is the largest ecosystem on earth. The upper bound of this zone is defined by a complete lack of sunlight. Organisms in the bathypelagic live in complete darkness, 24 hours per day. The darkness can be interrupted, however, by some light caused by the organisms themselves. This so called bioluminescence can be used to attract prey or to find a mate. Some species have lost their ability to see anything at all.

4. Deeper still is the abyssopelagic zone, which stretches from the bottom of the bathypelagic to the seafloor. This zone is characterized by a relative lack of life. It truly is the abyss.

5. A special zone that only exists in certain places around the world is called the hadopelagic zone. Where deep, wide trenches occur in the otherwise flat seafloor, the open water that fills them is the hadopelagic zone. By this definition, all of the deepest parts of the ocean conclude in the hadopelagic. The deepest known ocean depth is nearly 11,000 m (36,000 feet or almost 7 miles).

Finally, organisms that live on the ocean floor (regardless of depth) are part of the benthos. Benthic ecosystems include coral reefsseagrass beds, and other systems in shallow coastal areas and deep hydrothermal vents, the abyssal plain, and other systems in the deep sea. 

Fun Facts About the Open Ocean

1. Oceans cover more than 70 percent of the Earth’s surface, and half of those waters are at least 1.86 miles (3 km) deep.

2. As far as we know, the ocean is 36,200 feet (11,000 m or almost 7 miles) deep at its deepest point. On average, the ocean is about 12,100 feet (3,688 m) deep.1

3. Animals living in the bathypelagic zone or deeper never see sunlight.1 Some organisms living there, such as vampire squid and humpback anglerfish, produce their own light.2

4. More than 99 percent of Earth’s inhabitable space is in the open ocean.3

5. The ocean is divided into five zones: the epipelagic zone, or upper open ocean (surface to 650 feet deep); the mesopelagic zone, or middle open ocean (650-3,300 feet deep); the bathypelagic zone, or lower open ocean (3,300-13,000 feet deep); the abyssopelagic zone, or abyss (13,000-20,000 feet deep); and the hadopelagic zone, or deep ocean trenches (20,000 feet and deeper).

6. The ocean produces more than 50 percent of the air we breathe.4

7. Humans have only explored 5 percent of the world’s oceans. However, it is believed that humans have impacted every part of the ocean with waste and chemical pollution.5


1 NOAA Ocean Service
2 The MarineBio Conservation Society
4 NOAA Our World Ocean
5 Arizona State University