The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 3,000 to 6,000 metres (9,800 to 19,700 ft), this zone remains in perpetual darkness. It alone makes up over 83% of the ocean and covers 60% of the Earth. The abyssal zone has temperatures around 2 to 3 °C (36 to 37 °F) through the large majority of its mass. Due to there being no light, there are no plants producing oxygen, which primarily comes from ice that had melted long ago from the polar regions. The water along the seafloor of this zone is actually devoid of oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above. This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the above ocean zones and decomposes.
It is the deeper part of the midnight zone which starts in the bathypelagic waters above.
The deep trenches or fissures that plunge down thousands of meters below the ocean floor (for example, the midoceanic trenches such as the Mariana Trench in the Pacific) are almost unexplored. Previously, only the bathyscaphe Trieste, the remote control submarine Kaikō and the Nereus have been able to descend to these depths. However, as of March 25, 2012 one vehicle, the Deepsea Challenger was able to penetrate to a depth of 10,898.4 meters (35,756 ft).
Without plants, a cornerstone of any ecosystem, a unique ecosystem forms. Organisms that live in this zone cannot rely on plants or herbivores to be the bedrock of the ecosystem, instead the species that call the abyssal zone home resort to the only remaining way of acquiring energy. They must feed on each other or the dead organic material, detritus, that falls into the abyssal zone from the levels above it. It is only because of this fallen material that life can exist at this level, to begin with. It replaces plants as the bedrock of the ecosystem while invertebrate decomposers take the place of herbivores as the primary consumer. The biomass of the abyssal zone actually increases when closer to the seafloor as most of the decomposing material and decomposers live there. Since most of the resources or nutrients are present near the seafloor, that area can hold more biomass because it is able to support a more complex food web than the upper areas of the abyssal zone.
The sea floor of the abyssal zone consists of or is layered by different materials depending on the depth of the sea floor. If the seafloor is around 4000m below sea level, the seafloor usually consists of calcareous shells of foraminiferan zooplankton and phytoplankton. At depths greater than 4000m below sea level, the seafloor lacks these shells, as they dissolve once they reach a depth greater than 4000m. This leaves behind a seafloor consisting mostly of brown clay and the remaining silica from dead zooplankton and phytoplankton. In some areas of this zone, organisms are able to sustain themselves off of hydrothermal vents. Some bacterial species use the vents to create and use chemical energy in order to produce their own food. For example, many of these organisms convert hydrogen sulfide to sulfate in order to produce chemical energy. They then take that energy and synthesize the carbon-based compounds they use as food. These organisms are then preyed upon by other organisms, meaning that the bacteria can also take the place of plants as part of the bedrock for this ecosystem.
Organisms that live at this depth have had to evolve to overcome challenges provided by the abyssal zone. Fish and invertebrates had to evolve to withstand the sheer cold and intense pressure found at this level. They also had to not only find ways to hunt and survive in constant darkness but to thrive in an ecosystem that has less oxygen and biomass, energy sources or prey items, than the upper zones. In order to survive in a region with so few resources and low temperatures, many fishes and other organisms developed a much slower metabolism and require much less oxygen than those from the upper zones. Movement for many animals living here is also very slow, which allows them to conserve energy. Their reproduction rates are also very slow in order to decrease competition and conserve energy. The animals living here typically have flexible stomachs and mouths so that when scarce food items are found they can consume as much as possible.
Other challenges faced by life in the abyssal zone are the pressure and darkness caused by the zone’s depth. Many organisms living in this zone have evolved to minimize internal air spaces, such as swim bladders. This adaptation helps to protect them from the extreme pressure, which can reach around 75 MPa (11,000 psi). The absence of light also spawned many different adaptations, such as having large eyes or the ability to produce their own light. Large eyes would allow the detection and use of any light available, no matter how small. Another eye adaptation is that many deep-sea organisms have evolved eyes that are extremely sensitive to blue light. This is because as sunlight shines into the ocean, the water absorbs red light, while blue light, with its short wavelength continues moving down to the waters depths. This means that in the deep ocean, if any light remains then it is most likely blue light so animals wanting to capitalize on that light would need specialized eyes tuned to use it. Many organisms use other specialized organs or methods for sensing their surroundings, some in conjunction with specialized eyes. The ability to make their own light is called bioluminescence. Fishes and organisms living in the abyssal zone have developed this ability in order to not only produce light for vision, but also to lure in prey or a mate and conceal their silhouette. Scientists believe that over 90% of life in the abyssal zone use some form of bioluminescence. Many animals that are bioluminescent will produce blue light since it moves farther underwater than other colored lights, as explained earlier. Due to this lack of light, complex designs and bright colors are not needed. Most fish species have evolved to be transparent, red, or black in order to better blend in with the darkness and not waste energy on developing and maintaining bright or complex designs.
The abyssal zone is surprisingly made up of many different types of organisms, including microorganisms, crustaceans, molluscan (bivalves, snails, and cephalopods), different classes of fishes, and a number of others that might not have even been discovered yet. Most of the fish species in this zone are characterized as demersal or benthopelagic fishes. Demersal fishes are a term that refers to fishes whose habitat is very close to or on the seafloor, typically less than five meters. Most fish species fit into that classification because the seafloor contains most of the abyssal zone’s nutrients so the most complex food web or greatest biomass would be in this region of the zone.
For benthic organisms in the abyssal zone, species would need to have evolved morphological traits that could keep them out of oxygen-depleted water above the sea floor or a way to extract oxygen from the water above, but also, allow the animal access to the sea floor and the nutrients located there. There are also animals that spend their time in the upper portion of the abyssal zone, and even sometimes spending time in the zone directly above, the bathyal zone. While there a number of different fish species representing many different groups and classes, like Actinopterygii or ray-finned fish, there are no known members of the class Chondrichthyes, animals such as sharks, rays, and chimeras, that make the abyssal zone their primary or constant habitat. Whether this is due to the limited recourses, energy availability, or other physiological constraints is unknown. Most Chondrichthyes species will only go as deep as the bathyal zone.
- Anglerfish: Some species of this fish are considered demersal while others swim and live in the upper portions of the abyssal zone. They lack a swim bladder, so that high pressure is not an issue. They use bioluminescence to lure in prey with a specialized lure on their head. Ceratioidei anglerfish have an odd mating process. The male fuses with the much larger female and fertilizes her eggs. Once fused, the male will parasitize off of her for the rest of its life.
- Tripod fish (Bathypterois grallator): Their habitat is along the ocean floor, usually around 4,720 m below sea level. Their pelvic fins and caudal fin have long bony rays protruding from them. They will face the current while standing still on their long rays. Once they sense food nearby, they will use their large pectoral fins to hit the unsuspecting prey towards their mouth. Each member of this species has both male and female reproductive organs so that if a mate cannot be found, they can self fertilize.
- Gulper eel: The gulper eel habitat range typically goes form a depth of 500 to 3,000 meters below sea level. Not only does this animal have a giant mouth, but the mouth is loosely hinged with a pouch built into its lower jaw, making it the perfect mouth for swallowing fish much larger than itself. Like the anglerfish it also lacks a swim bladder. The eel's eyes most likely evolved to detect small traces of light instead of full images.
- Dumbo octopus: This octopus usually lives at a depth between 3,000 to 4,000 meters, no other octopus lives at these depths. They use the fins on top of their head, they look like flapping ears, to hover over the sea floor looking for food. They will use their arms to help change directions or crawl along the sea floor. In order to combat the intense pressure of the abyssal zone, this octopus species lost their ink sac during evolution. They also use their strand-like structured suction cups to help detect predators, food, and other aspects of their environment.
- Cusk eel (Genus Bassozetus): There are no known fish that lives at depths greater than this one. The depth of the cusk eel's habitat can be as great as 8,370 meters below sea level. This animal's ventral fins are specialized forked barbel-like organs that act as sensory organs.
- Abyssal grenadier: This resident of the abyssal zone is known to live at a depth ranging from 800 and 4,000 meters. It has extremely large eyes, but a small mouth. It is thought to be a semelparity species, meaning it only reproduces once and then dies after. This is seen as a way for the organism to conserve energy and have a higher chance of having some healthy strong children. This reproductive strategy could be very useful in low energy environments such as the abyssal zone.
As with all of the rest of the natural world climate change has negative effects. Due to the zone’s depth, increasing global temperatures do not affect it as quickly or drastically as the rest of the world, but the zone is still afflicted by ocean acidification. Along with climate change and ocean acidification, pollutants, such as plastics, are also present in this zone. Plastics are especially bad for the abyssal zone due to the fact that these organisms have evolved to eat or try to eat anything that moves or appears to be detritus, resulting in most organisms consuming plastics instead of nutrients. Both ocean acidification and pollution are decreasing the already small biomass that resides within the abyssal zone. Another problem caused by humans is overfishing. Even though no fishery can fish for organisms anywhere near the abyssal zone, they are still causing harm. The abyssal zone is dependent on the dead organisms from the upper zones sinking to the seafloor in order to sustain an ecosystem since their ecosystem lacks producers, due to the lack of sunlight. If fish and other animals are being removed from the ocean, the frequency and amount of dead material reaching the abyssal zone will also decrease. A future problem for the abyssal zone could be deep sea mining operations. The talks and planning for this industry have not only already begun but is quickly growing. This could be disastrous for this extremely fragile ecosystem since the ecological dangers for mining for deep sea mineral are many. It could increase the amount of pollution in not only the abyssal zone, but in the ocean as a whole, and would physically destroy habitats and the seafloor. While this industry would only be able to occur far into the future, it is still a looming threat to the abyssal zone and the rest of the ocean's inhabitants.
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