Ocean Light Zones (*See below for original article)
Ocean Light Zones
The ocean can be divided from its surface to its depth into
three zones based on the amount of light received. They are the
sunlit, twilight, and midnight zones.
1. Sunlit Zone: This is the top layer, nearest the surface. The
sunlit zone goes down about 600 feet. Here there is enough
light penetrating the water to support photosynthesis. Because
photosynthesis occurs here, more than 90 percent of all marine
life lives in the sunlit zone.
Plankton
Phytoplankton
Plankton occurs in this zone. Plankton are free-floating aquatic
organisms. They are usually microscopic and form the basis of
the food chain in the ocean.
A special type of plankton is called phytoplankton, or “plantlike plankton”. Through the process of photosynthesis, they
have the ability to capture the sun’s energy to make food and
release oxygen.
Phytoplankton have an important adaptation to the sunlit zone.
This is the oil in their cells that allows them to stay afloat in the
surface waters. By staying afloat, they receive enough sunlight
to undergo photosynthesis. Without the oil in their cells, they
would not be able to survive.
Many animals such as dolphins, krill, turtles, and coral inhabit
the sunlit zone. Many of these animals have a special
adaptation called countershading that makes it difficult for
predators to see them. For example, the top of a dolphin’s
body is dark grey while the underside is silvery white. When a
shark sees him from above, the dolphin easily blends in with
the dark waters below. However, if a shark sees him from
below, the silvery white belly easily blends in with the brighter
surface waters and sky above. This hides the dolphin and
protects it from being eaten by the shark!
2. Twilight Zone:
This murky, dim part of the ocean begins at about 600 feet
under the water and extends to the darkest part, which begins
about 3000 feet down. Only a small amount of light can
penetrate the water at these depths. Animals found here include
whales, swordfish, and cuttlefish.
Some animals have adaptations that allow them to survive in
little light. The lantern fish, for example, has extra large eyes
on the top of its head that allow it to gather as much light as
possible. In this way, it can see better and therefore escape
predators or sneak up on its prey.
The cuttlefish has an adaptation called camouflage that allows
it to change its body color. In the upper twilight zone, where
there is more light, the cuttlefish adjusts its body color to match
its surroundings. As it swims to the lower twilight zone where
there is less light, the cuttlefish adjusts its color to the darkness.
In this way, camouflage helps the cuttlefish to escape predators
and also capture prey.
3. Midnight Zone:
Ninety percent of the ocean is in the midnight zone. It is
entirely dark—there is no light. The water pressure is extreme.
The temperature is near freezing.
What can live in the midnight zone? The living things found
here live close to cracks in the Earth's crust. These cracks give
off hydrogen sulfide gas. Special forms of bacteria utilize this
hydrogen sulfide for energy to make food. Then other living
things, such as tube worms, feed on these bacteria.
Living things in the midnight zone, which include the angler
fish, fang tooth fish, and the dumbo octopus, have special
adaptations. Over 90% of these animals have an adaptation
called bioluminescence, which is having special body parts that
produce light. The dumbo octopus, for example, has a
bioluminescent body that attracts its prey. One well-known
bioluminescent fish is the angler fish. It has a glowing “light”
hanging from its head that attracts and tricks prey into being
eaten!
One last adaptation of some midnight zone creatures can be
found in their mouths. Their mouths are especially large and
contain long sharp teeth. How do you think this helps them?
Both the angler and the fang tooth fish have these kinds of
mouths to make it harder for their prey to get away. It’s not
easy to catch food in the dark, so this adaptation clearly helps
them to survive in the midnight zone!
*Adapted from an original article by Missouri Botanical
Gardens http://www.mbgnet.net/salt/oceans/zone.htm