Oceans For Dummies
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The ocean’s water column (a conceptual pillar of water measured from the ocean’s surface to the seafloor) is often divided into five zones—the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadalpelagic zones. The divisions generally correspond to differences in depth, amount of sunlight, temperature, pressure, nutrients, and organisms that live in those zones. In the following sections, we take a deep dive into each of the ocean's five vertical ocean zones.

The ocean’s vertical zones. The ocean’s vertical zones.

Skimming the surface: The epipelagic zone

The epipelagic zone (commonly referred to as the sunlight zone) is the top 200 meters (about 650 feet) of the ocean, where enough sunlight is available for plant life to grow and support a large, diverse population of marine life. Because it forms the ocean’s surface, the epipelagic zone experiences greater variations (compared to the other vertical zones) in temperature and other conditions due to climate, local weather patterns, and proximity to large land masses.

Who lives here? Lots of plankton (tiny plants and animals that float, as shown); nekton (tiny plants and animals that swim); jellyfish; sea turtles (shown); a variety of fish including tuna (shown), marlin, salmon, and sharks; and cetaceans (dolphins and whales).

Plankton Courtesy of Christian Sardet/CNRS/Tara expeditions, from Plankton—Wonders of the Drifting World, Univ Chicago Press 2015 https://upload.wikimedia.org/wikipedia/commons/b/b1/Plankton_species_diversity.jpgLicensed under the Creative Commons Attribution-Share Alike 4.0 International license.


sea turtle Cristina Mittermeier—www.sealegacy.org

Sea turtle.

tuna Keith Ellenbogen—www.keithellenbogen.com


It’s not as though these zones are sectioned off like office cubicles. Plenty of animals roam freely from one zone to another. Animals that need to breathe air, such as sea turtles, dolphins, and whales generally hang out closer to the surface just so they can get their heads (or noses or blowholes) above water regularly. Others prefer the shallower water for other benefits, such as food, light, warmth, and lower water pressure.

Dimming the lights in the mesopelagic zone

Just below the epipelagic zone is the mesopelagic zone (commonly referred to as the twilight zone). This is the region spanning 200 to 1,000 meters (650 to 3,300 feet) below the surface, where some sunlight still penetrates, but not enough for photosynthesis. To feed, most animals in this zone move toward the surface. Others eat whatever detritus (table scraps) and organic matter fall from the epipelagic zone, or they just eat their smaller or weaker twilight zone neighbors.

Some animals in this zone have evolved the ability to produce their own light — a trait referred to as bioluminescence (creating light through biochemical processes). Instead of carrying a flashlight to find their way in the dark, they are the flashlight. Although scientists aren’t quite sure about the purpose of this superpower, they think it might be used to ward off or evade predators, detect or lure prey (ooh, shiny light!), or communicate with members of their own species.

Counter-illumination: Some sea creatures that can bio-luminate may use this skill as camouflage, illuminating their soft underbellies to blend in with light coming from the surface, while the tops of their bodies remain dark to blend in with the darkness below them. This application of bioluminescence, called counter-illumination, protects the creature from predators above and below. When predators from below look up, all they see is light. When predators from above look down, all they see is darkness. Take that, camo pants.

Life starts to get a bit weird in this zone. Here, you’re likely to start bumping into cool fish like the lanternfish, hatchetfish, and barbeled dragonfish all of which can produce their own light. You can also find species of bristlemouths. These fish, generally no larger than your finger, are not only thought to be the most common fish in the ocean, but also the most common vertebrate on Earth — more abundant than humans, chickens, and rats combined. Let that sink in for a second. Some marine mammals and sharks can also be found here, but most will stay in the mesopelagic only for relatively short periods before returning to the surface. Swordfish, ctenophores (see below) and siphonophores (jellyfish relatives), and firefly squid are other interesting animals that can be found in this zone.

swordfish Joe Fish Flynn/Shutterstock


Ctenophores Schmidt Ocean Institute—www.schmidtocean.org


Taking a deeper, darker dive into the bathypelagic zone

Just below the mesopelagic zone is the bathypelagic zone (also called the midnight zone), which extends from 1,000 meters to 4,000 meters (3,300 to 13,000+ feet) below sea-level. No sunlight penetrates this zone, and the temperature is relatively constant at a very chilly 4 degrees Celsius (39 degrees Fahrenheit). Animals in this zone prey on other bathypelagic organisms or grab whatever rains down like manna from above. Some creatures in this zone migrate closer to the surface to feed at certain times of day.

The creatures that live here are too insane to make up, but they’re not the most colorful — just about everything is black or red, which makes everything invisible in water at these depths. (Certain wavelengths are filtered by water faster than others. Because red light has the longest wavelength and is absorbed quickest, once you go deep enough, anything red appears black.)

If you cut yourself diving at around 60 feet deep, your red blood may appear purple and, if you go any deeper, even black. Of course, we’re not recommending that you poke your finger when you’re diving, but if you happen to suffer a small cut underwater at that depth and you’re looking for a cheap thrill . . . .

Calling this zone their home are the weird and wonderful barreleye fish, giant isopods, viperfish, vampire squid, and anglerfish. Occasionally you can find sperm whales here, and if you’re really, really lucky, you can see one battling a giant squid (of course if you do, take a picture because no one has captured that epic battle on film yet). The deepest diving marine mammal, the Cuvier’s beaked whale, can also reach this zone. This elusive and strange-looking animal holds the record for the longest mammalian dive, plunging up to 3,500 meters (11,480 ft) deep (that’ll make your ears pop) in search of deep-water cephalopods and squid.

Many animals in this zone and deeper have adaptations to allow them to eat almost anything, including prey much larger than them. Gulper eels have specialized jaw structures that enable them to open their mouths incredibly wide. Sharks and their relatives, including the Greenland shark (which can live for 400 years), ghost shark, frilled shark and goblin shark can sometimes also be found in this zone, as well as the deepest living octopus, the dumbo octopus shown (although some say it can be found at even greater depths).

gulper eel Woods Hole Oceanographic Institution, P. Caiger—www.whoi.edu

The gulper eel says ahh!

Dumbo Octopus Schmidt Ocean Institute—www.schmidtocean.org

Dumbo Octopus isn’t she (or he) cute?

Delving into the abyss: The abyssopelagic zone

One step down from the bathypelagic is the abyssopelagic zone (also called the abyss), extending from 4,000 to 6,000 meters (13,000+ to nearly 20,000 feet) below the surface. Imagine totally dark, near-freezing temperatures (though stable), and super high pressure. For animals adapted to these harsh conditions, the pressure is no problem. Unlike animals with gas-filled organs (such as lungs and swim bladders) that would be crushed at these depths, deep-sea creatures are pretty much made up of tissue and fluid. While the high pressure may limit species diversification, it isn’t the hardest part about living here. The more challenging factor is the scarcity of food.

Generally, the further down you go, the fewer species you encounter, because these are tough environmental conditions to adapt to. Life here is thought to have changed little over millions of years. Some abyssal species include the common fangtooth, tripod fish shown (they’re hermaphroditic, meaning they have both male and female reproductive organs, which means they can produce young either with another fish or on their own!), hagfish, cusk eels, grenadiers, and viperfish. In some places, you can find deep sea-corals, which don’t need sunlight to survive.

tripod fish Schmidt Ocean Institute—www.schmidtocean.org

The tripod fish.

viperfish Woods Hole Oceanographic Institution, P. Caiger—www.whoi.edu

Viperfish (Chauliodus sloani)—look at those teeth!!!

Deep-sea creatures must be able to tolerate intense pressure (from the weight of the water above), total darkness, and near freezing temperatures. That’s not to say that animals living closer to the surface have an easy life; they face a greater risk from predators and from changes in environmental conditions.

How low can you go? The hadalpelagic zone

The deepest zone in the ocean is the hadalpelagic zone, (also called the trenches) which is anything deeper than 6,000 meters (about 20,000 feet) below the surface, such as in the deep ocean trenches. This realm is named after Hades, the Greek god of the underworld. We don’t know much about this zone, because it’s hard to get to and requires super specialized technology to cope with the immense pressure.

Life becomes very limited in this zone. You can find giant, single-celled xenophyophores, deep sea microbial mats, amphipods, sea cucumbers, jellyfish, and other invertebrates such as tube worms, decapods, bivalves, and sea-anemones. Species of snailfish, cusk eels, and eelpouts can also be found in this region but are limited to relatively shallow areas, and usually closer to the seafloor. And even though it is really hard for people to venture this far down, our trash manages to find a way — namely, our plastic. Yup, even here in the deepest part of the ocean, recent expeditions found a plastic bag at one of the deepest points of the Mariana trench, nearly 11 kilometers (about 7 miles) down. Ugh.

sea cucumber Schmidt Ocean Institute—www.schmidtocean.org

A sea cucumber.

tube worms Source: Woods Hole Oceanographic Institution—www.whoi.edu

Tube worms.

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