1. Zoning In
Sunlit Zone
0-650 ft
Zoning In
Why is Earth called the Blue Planet? It’s because the world’s five oceans (the Pacific, Atlantic, Indian, Southern and Arctic) cover two thirds of its surface.
The oceans support a wide variety of life. But not everything can live everywhere. From top to bottom, there are different zones in the ocean. Each zone is defined by such things as sunlight, temperature, and water pressure.
Twilight Zone 650-3,300 ft
Zoning In
The ocean is divided up into broad zones, according to how far down sunlight penetrates, and water temperature and pressure. In the sunlit zone, there is plenty of light, much water movement, and seasonal changes in temperature. Beneath this is the twilight zone, the maximum depth where light penetrates. Temperatures here decrease rapidly with depth to about 41°F (5°C). Deeper yet is the dark zone, where there is no light and temperatures drop to about 34–36°F (1–2°C). Still in darkness and even deeper is the abyss and then the trenches. There are also zones on the seabed. The shallowest zone lies on the continental shelf. Below this are the continental slope, the abyssal plains, and the seafloor trenches.
Dark Zone
3,300-13,000 ft
Abyss
13,000-20,000 ft
Trenches
over 20,000 ft

2. Surface Sunlight Surface Sunlight
The Great Barrier Reef in Australia is over 1,200 miles long.
Surface Sunlight
Sunlit Zone (0-650 ft)
Coral reefs support an amazing amount of life. However, they are only found in the sunlit zone. That’s because the sunlit zone gets enough light for photosynthesis to occur.
Why is this important? Sunlight allows algae to grow. Living coral need algae so they can produce the hard skeletons that protect their soft bodies.
Surface Sunlight
In the crystal clear, warm waters of the tropics, coral reefs flourish, covering vast areas. Made of the skeletons of stony corals, coral reefs are cemented together by chalky algae. Most stony corals are colonies of many tiny, anemone-like individuals, called polyps. Each polyp makes its own hard limestone cup (skeleton) which protects its soft body. To make their skeletons, the coral polyps need the help of microscopic, single-celled algae that live inside them. The algae need sunlight to grow, which is why coral reefs are found only in sunny, surface waters.
Australia's Great Barrier Reef which is over 1,200 miles (2,000 km) long, is the largest structure in the world made by living organisms. Of the 400 kinds of coral, many spawn on the same night after a full Moon, the water resembling an underwater snowstorm.

3. The Twilight Zone The Twilight Zone Dive below 650 feet and
Large eye helps spot prey in
the dark
The Twilight Zone
Twilight Zone (660-3,300 ft)
Dive below 650 feet and
things really begin to
change. Welcome to the
twilight zone!
Not much light gets to
this zone so the animals
that live here have special
adaptations. This hatchet
fish has light-producing
organs in its mouth to
attract prey.
It also has light organs along its belly
and tail so when the fish is viewed from
underneath, it can better blend in with
the brighter surface water above it.
Light organs under tale
The Twilight Zone
Below 650 feet lies the Twilight Zone. Light is limited in this zone but Hatchet fish, which have bladelike, silvery bodies, manage to thrive at this depth. The light organs along their belly and tail, when viewed from below, help them blend in with the brighter surface water above. These must be the right degree of brightness so as not to outshine the light from above, which would make them easier to see. Hatchet fish live in the Atlantic, Pacific, and Indian Oceans.
Many animals, including some lantern fish and a variety of squid, spend only their days in the twilight zone. At night they journey upward to feed in the food-rich surface water. By doing this, they are less at risk from daytime hunters such as seabirds. Others, such as the lancet fish, spend most of their lives in the twilight zone eating any available food.
Deep-sea hatchet fish
Light organs on belly
Light patches in the mouth help guide prey right into its jaws

4. Darkness Descends Darkness Descends
Dark Zone (3,300-13,000 ft)
The dark zone begins below 3,300 feet. There is no light
in this zone, temperatures are cold, and food is scarce. Yet creatures like this deep-sea anglerfish still survive.
Bacteria that glow
in the dark live in
the structure above
the fish’s mouth. The
anglerfish uses this structure like a fishing pole to lure prey into its huge jaws. After swallowing its prey, the fish’s stomach stretches to hold the whole meal.
Deep-sea anglerfish before meal
Lure has bacteria that can light up
Teeth bend backward to keep prey from escaping
Darkness Descends
No light penetrates into the dark zone. Deep-sea fish make the most of little food by having huge mouths and stretchy stomachs, giving fish a strange appearance. Often they are small or weigh little due to lightweight bones and muscles. Being lightweight helps fish in the dark zone maintain neutral buoyancy (keeping at one level without having to swim), even though most have no gas-filled swim bladders.
This anglerfish (Melanocetus) has been found with a lantern fish twice its own size in its stomach. The prey is guided to its gaping jaws by a glow-in-the-dark lure on the end of a long fin ray. Large teeth, curving backward, make sure that the angler does not let go of its prey once it is within its jaws. The prey is not chewed up, but swallowed whole. Male deep sea anglers are up to 20 times smaller than the females and either feed on much smaller prey, such as shrimp, or do not feed at all. Once a dwarf male finds a female he hangs on by his jaws. In some anglers, the male's whole body fuses with hers so he receives food via her bloodstream.
After meal
Stretchy stomach allows anglerfish to eat huge meals

5. Into the Abyss Into the Abyss Below the dark zone
Abyss (13,000-20,000 ft)
Below the dark zone
is the abyss. Here, the water temperature
is almost freezing
and soft clays and
mud-like ooze cover
the ocean floor.
Can anything exist in such a hostile environment? The deep-sea cucumber does quite well by feeding on the remains of dead animals and plants that have sunk to the ocean floor.
Into the Abyss
This deep-sea cucumber has extra long tube-feet for walking along the soft seabed. Sea cucumbers often leave tracks in the soft seabed behind them as they crawl along. Some kinds of deep-sea cucumbers move by swimming above the bottom.
Deep-sea cucumber
Extra long tube feet for walking along soft ocean floor

6. Hot Water Hideouts Hot Water Hideouts
Plumes of hot, sulfur-rich water rise from vents
Hot Water Hideouts
Trenches (over 20,000 ft)
Deep water trenches and canyons drop below
the ocean floor to form the deepest zone in the ocean. The water pressure in these places can be
an incredible 8 tons per square inch. However,
even in the trenches, scientists have found life.
Gigantic plates that make
up the Earth’s crust move
apart to form cracks, or
vents, in the trenches. Hot,
mineral-rich water gushes
up through these vents and
allows bacteria to grow.
This provides a food
source for strange
animals, like tube
worms, to grow.
Hot Water Hideouts
Animal life abounds in an active vent site, such as this one in the Mid-Atlantic Ridge. If the vent stops producing hot, sulfur-rich water, the community is doomed. Animals from dying vents must colonize a new site, which may be several hundred miles away across the cold, almost barren seafloor.
Tube worms can grow to 10 feet long