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"Over 60% of our planet is covered by water more than a mile deep. The deep sea is the largest habitat on earth and is largely unexplored. More people have traveled into space than have traveled to the deep ocean realm...."
» Gallery @ HBOI | » Gallery @ ExploreTheAbyss.com
The benthic and abyssal zones are where the cold, dark, deep waters of the ocean are found. These zones are the largest part of the ocean biome covering more than 80% of the vast ocean. The intertidal zone where the water meets land and the pelagic zone, or open ocean, make up a much smaller portion of the total area of the ocean, yet an abundance of ocean life is found in these zones because sunlight penetrates the water. To get an idea of how vast the ocean's depths are, consider that 79% of the entire volume of the earth's biosphere consists waters with depths greater than 1,000 m. Until recently, the deep sea was largely unexplored. But advances in deep sea submersibles and image capturing technologies are increasing the opportunities for marine biologists to observe and uncover the mysteries of the deep ocean realm.
Deep sea research is vital because this area is such an enormous part of the biosphere. Despite its depth and distance, it is still our backyard. There may be life-altering discoveries found at the bottom of the ocean.
IMCS Deep Sea Microbiology Lab videos: Black Smoker | Tube Worms | Pompeii Worms | Zoarcid Fish | Crabs
And The Deep Sea is in trouble too: Sea bed trawling, the greatest threat to deep seas biodiversity | Download the report
The benthic and abyssal zones of the deep sea are found beneath the pelagic zone. Light does not penetrate these waters, yet marine life has adapted to this harsh environment, and a surprising number of species inhabit these waters.
The zone just beneath the pelagic zone is the benthic zone, which reaches depths of about 700-1,000 m. The abyssal zone is found beneath the benthic zone extending down to the deepest depths of the ocean, which are about 33,000 feet. Deep sea thermal vents can be found in this zone. 
There is also the hadalpelagic zone used to define the waters of the deep sea trenches.
Exploration of these zones has presented a challenge to scientists for decades and much remains to be discovered. However, advances in technology are increasingly allowing scientists to learn more about the strange and mysterious life that exists in this harsh environment. Life in the deep sea must withstand total darkness, extreme cold, and great pressure. To learn more about deep-sea marine life, sophisticated data collection devices have been developed to collect observations and even geological and biological samples from the deep. Advances in observational equipment such as fiber optics that use LED light and low light cameras has increased our understanding of the behaviors and characteristics of deep sea creatures in their natural habitat. Remotely operated vehicles (ROVs) have been used underwater since the 1950s. ROVs are basically unmanned submarine robots with umbilical cables used to transmit data between the vehicle and researcher for remote operation in areas where diving is constrained by physical hazards. ROVs are often fitted with video and still cameras as well as with mechanical tools for specimen retrieval and measurements. Manned deep sea submersibles are also used to explore the ocean's depths. Alvin is a deep sea submersible built in 1964 that has been used extensively over the past 4 decades to shed light on the black ocean depths. This sub has been used for more than 4,000 dives reaching a maximum depth of more than 4.5 thousand meters.
Physical Characteristics of the Deep Sea
The physical characteristics of the deep sea are abiotic factors that deep sea life must contend with to survive. Light, pressure, temperature, oxygen and food have all led to the fascinating adaptions of deep sea life used to see, feel, feed, reproduce, move, and avoid being eaten by predators.
Light
The deep ocean waters are as black as night. The deep is also known as the twilight zone. The only light is produced by bioluminescence, a chemical reaction in the creature's body that creates a low level light, so deep sea life must rely on alternatives to sight. Many deep sea fish have adapted large eyes to capture what little light exists. Most often, this light is blue-green, but some creatures have also developed the ability to produce red light to lure curious prey. Lack of light also creates a barrier to reproduction. Bioluminescent light is also used to signal potential mates with a specific light pattern. Deep sea creatures are also often equipped with a powerful sense of smell so that chemicals released into the water can attract potential mates.
Pressure
Considering the volume of water above the deepest parts of the ocean, it's no wonder that pressure is one of the most important environmental factors affecting deep sea life. Pressure increases 1 atmosphere (atm) for each 10 m in depth. The deep sea varies in depth from 700 hundred meters to more than 10,000 m, therefore pressure ranges from 20 atm to more than 1,000 atm. On average, pressure ranges between 200-600 atm. Advances in deep sea technology have enabled scientists to collect species samples under pressure so that they reach the surface for study in good condition. Without this technology, the animals would die shortly after being collected and the absence of pressure would cause their organs to expand and possibly explode. With good samples, we now know that deep sea creatures have adapted to pressure by developing bodies with no excess cavities, such as swim bladders, that would collapse under intense pressure. The flesh and bones of deep sea marine creatures are soft and flabby, which also helps them withstand the pressure.
Temperature
The difference in temperature between the photic, or sunlit, zones nearer to the surface and the deep sea are dramatic. Temperatures vary more in the waters above the benthic zone where thermoclines, or the separation of water layers of differing temperatures, are more common. In most parts of the deep sea, the water temperature is more uniform and constant. With the exception of hydrothermal vent communities where hot water is emitted into the cold waters, the deep sea temperature remains between 2-4°C.
Oxygen
The dark, cold waters of the deep are also oxygen-poor environments. Consequently, deep sea life requires little oxygen. Oxygen is transported to the deep sea from the surface where it sinks to the bottom when surface temperatures decrease. Most of this water comes from arctic regions. Surprisingly, the deep sea is not the most oxygen-poor zone in the ocean. The oxygen minimum zone lies between 500-1,000 m, where there are more species that require oxygen depleting the oxygen in this zone during respiration. In addition, the bacteria that feed on decaying food particles descending through the water column also require oxygen. Oxygen is never depleted in the deepest parts of the ocean because there are fewer animals to deplete the available oxygen.
Food
Deep sea creatures have developed some fascinating feeding mechanisms because of the lack of light and because food is scarce in these zones. Some food comes from the detritus, of decaying plants and animals from the upper zones of the ocean. The corpses of large animals that sink to the bottom provide infrequent feasts for deep sea animals and are consumed rapidly by a variety of species. The deep sea is home to jawless fish such and the lamprey and hagfish, which burrow into the carcass quickly consuming it from the inside out. Deep sea fish also have large and expandable stomachs to hold large quantities of scarce food. They don't expend energy swimming in search of food, rather they remain in one place and ambush their prey using amazing and clever adaptations.
Adaptations of deep-sea organisms
Some of the most fascinating and amazing characteristics of deep sea creatures are the adaptations they have developed to survive the harsh environment. We've briefly covered adaptations such as large eyes, bioluminescence, strong sense of smell, body composition (absence of swim bladder), expandable stomachs, absence of jaws, and bioluminescence. Color is another adaptation developed for camoflauge and protection from predators. Deep sea fish are often transparent, black, silvery and even red in color. The absence of red light at these depths keeps them concealed from both predators and prey. 
Adaptations have also evolved to capture prey. In addition to their large mouths, deep sea fish, such as the deep sea anglerfish, often have extremely long teeth that point inward. This ensures that any prey captured has little chance of escape. Deep sea species such as the gulper eel have huge hinged jaws, which enables them to swallow large prey. Some deep sea species, such as the deep sea anglerfish and the viperfish, are also equipped with a long, thin modified dorsal fin on their heads tipped with a photophore lit with biolumenescence used to lure prey.
Deep sea anglerfish have an interesting reproductive adaptation. Males are tiny in comparison to females and attach themselves to their mate using hooked teeth establishing a parasitic relationship for life. The blood vessels of the male merges with the female's so that he receives nourishment from her. In exchange, the female is provided with a very reliable sperm source.
Hydrothermal Vent Communities
Life in the deep sea is relatively sparse in the deep sea compared to the intertidal zone, with one exciting, and recently discovered exception - hydrothermal vent communities. This surprising discovery occured in 1976 during a deep sea expedition in Alvin near the Galapagos. These dives to depths of about 2,700 km led to the discovery of this remarkable ecosystem where marine life flourished at deep depths in the absence of sunlight.
Hydrothermal vent communities are found at depths ranging from 1,500 to 3,200 m. The water temperature near these communities is much warmer than is normal for this depth, averaging between 8-16°C.(about 2°C). The hot mineral-rich water comes from geyers in the seafloor heated by magma beneath the earth's crust. These vents are most commonly found near mid-ocean ridges where the sea floor is spreading due to the movement of ocean plates. Metal sulfides expelled through the vents settle and form chimneys around which are soon populated by a variety of sea life. Soon after hydrothermal vents were discovered, the communities of giant tube worms that inhabit them were also discovered. In the absence of sunlight, these worms, known as chemoautotrophs, subsist on hydrogen sulfide found in the warm waters surrounding vent communities. The hydrogen sulfide is turned into energy by bacteria that live inside the worms.
References
| Marine Biology, an ecological approach, James W. Nybakken. Chapter 4: Deep Sea Biology Susan M. Libes (1992) An Introduction to Marine Biogeochemistry. John Wiley and Sons, Inc. Dive and Discover : Expeditions to the Seafloor - Woods Hole Oceanographic Institution Deep-Sea News |
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