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.... - The Blue Planet Seas of Life
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 and sampling 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.
The zone beneath the pelagic zones is the benthic zone and is defined as the lowest level of a body of water such as an ocean or a lake, and includes the bottom sediments. Organisms living in this zone are called benthos . They generally live in a close relationship with the bottom of the sea; with many of them permanently attached to it. In oceanic environments, benthic habitats can be further zoned by depth. From the shallowest to the deepest are: the epipelagic (less than 200 meters), the mesopelagic (200–1000 meters), the bathyal (pelagic) (1,000–4,000 meters), the abyssal (pelagic) (4000–6,000 meters) and the deepest, the hadal (pelagic) (below 6,000 meters). 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. Deep sea thermal vents can be found in this zone. The hadal zone is 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 4,500 m.
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 m 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 as the lamprey and hagfish, which burrow into carcasses, quickly consuming them 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.