The Sunlit Zone

When most people think of the ocean, they think of the sunny upper layer called the “Sunlit Zone” or more scientifically, the “Epipelagic” layer or zone. It ranges from the surface to 656 feet (200m) deep.

Here, creatures such as hammerhead sharks, blue whales, loggerhead sea turtles and manta rays live. The ecosystems are as diverse as the tropical coral reefs, the cold kelp forests, or the freezing Antarctic. Because sunlight reaches these depths, algae and coral that photosynthesize (produce food from sunlight) grow here, as well.

The Twilight Zone

The next zone is the “Twilight Zone,” or “Mesopelagic Zone.” It ranges from 656-3,300 feet (200-1,000m). This is a zone where few people have scuba dived because it’s so deep. It is where many of the deep-sea fish you think of live, like the dumbo octopus, lantern fish, and viperfish.

Here, the light is dim, it is very cold, the pressure is high, and there is little food—except each other, or marine snow. Marine snow is the detritus that falls from the sunlit layer above. It can include organic matter such as feces, dead animals, mucus, and bacteria. Marine snow can also include inorganic matter such as sediment and sand. It might sound gross for us to eat, but it’s a buffet for deep sea animals!

The Midnight Zone

The “Midnight Zone” or “Bathypelagic Zone” ranges from 3,300-13,100 feet (1,000-4,000m).

Here there is no light, close to freezing temperatures and even higher water pressure. Animals such as vampire squid, anglerfish, and gulper eels live.

This is where hydrothermal vents are found. Hydrothermal vents are “fissures on the ocean floor that release geothermally heated water.” The animals found there are attracted by the heat, minerals, and the hydrogen sulfide. Some of the animals here, like tubeworms, take in both oxygen and hydrogen sulfide and use it not only to breathe, but produce food. This is known as chemosynthesis.

The Abyssal Zone, or the Abyss

The “Abyssal Zone” or “Abyssopelagic Zone” covers the ocean floor from 13,100-19,700 feet (4,000-6,000m). Here animals live in complete darkness, experience very high pressures, and freezing temperatures. There is also low oxygen compared to the surface.

Animals include sea cucumbers, sea pigs, and sea spiders. About 90% of creatures in the deep-sea use bioluminescence. Most animals living here use bioluminescent light to not only see but communicate with one another, for defense and even to attract prey. Bioluminescence is light that glows-in-the-dark and is usually blue but can range from violet to green.

The Hadal Zone, or the Trenches

The “Hadal Zone” or “Hadalpelagic Zone” is found from 19,700-36,100 feet (6,000-11,000m). They are in the deepest ocean trenches, such as the Mariana Trench. It is in the western Pacific Ocean, and the deepest point is called the Challenger Deep. The deepest part of this trench is 36,037 feet (10,984 m) down. That is 6.8 miles deep. Mount Everest is only 5.5 miles (8.85km) high in comparison.

The temperature is just above freezing at 34-39 degrees Fahrenheit (1-4 degrees Celsius). The pressure is 15,076 psi or 1,086 bar. For comparison, pressure at the surface is 14.7 psi (1 bar), at 300 feet underwater is 130 psi (10 bar), and at 13,000 feet underwater is 5,644 psi (389 bar). The pressure at the deepest point of the Challenger Deep is almost 400x as much as at the surface!

Only the hardiest of creatures live in ocean trenches. Amphipods are crustaceans that live in all layers of the ocean, but in the deep sea, they are much larger. Many species, such as spider crabs, are much larger in the deep sea than they are in shallow seas. This is called deep-sea or abyssal gigantism.

The ocean zones are more diverse than you may have thought and have many different creatures living in each zone that you may never have heard of (until now!). It is estimated that only 5% of the ocean has been explored. 90% of the ocean is considered the deep sea, and it is estimated that 1/3 of fish in the ocean live there.

As overfishing is taking place in the shallower ocean, soon fishing may take place in the deep sea. Plus with possible deep sea mining having effects on all the world’s oceans, ocean explorer Jacques Cousteau said it best, “We only protect what we love, we only love what we understand, and we only understand what we are taught.”