Abyssal zone

 

The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. 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.
Without producers, the cornerstone of most ecosystems, a unique ecosystem forms. Rather than relying on producers to form the base of the food pyramid, organisms living in the abyssal zone must feed on the dead organic detritus that falls from oceanic layers above. The biomass of the abyssal zone actually increases near the seafloor as compared to areas above as most of the decomposing material and decomposers rest on the seabed.
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 the products of hydrothermal vents. Some bacterial species use the vents to create and use chemical energy to produce food. For example, many of these organisms convert hydrogen sulfide to sulfate to produce chemical energy. They use that energy to 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.
Other challenges faced by life in the abyssal zone are the pressure and darkness caused by the zones 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 water's 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 not only to 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 produce blue light, since it moves farther underwater than other colors of light, 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 so they better blend in with the darkness and don't waste energy on developing and maintaining bright or complex designs.
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 are 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 only go as deep as the bathyal zone.
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.
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 semelparous 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.