The deep sea is an environment that is unfriendly to humankind. As a result, it represents one of the least explored areas on Earth. It has even been said that the deep sea is less explored than the surface of the moon. The deep sea region begins about 1000 meters beneath the ocean’s surface, located between the thermocline and the seafloor (Gage 1991). Here, light is limited, and after a depth of 100-500 meters, it disappears completely. Temperatures in the deep sea can significantly drop -- nearing 0°C at the very bottom. However, its temperature typically ranges between 2-3°C. The salinity level is relatively stable, with concentrations ranging between 34-35 parts per thousand (ppt). On the other hand, its pressure steadily rises with depth, increasing approximately 10 atmospheric pressure per meter (Gage 1991; George 2013).
Figure 1. https://upload.wikimedia.org/wikiped...HERMOCLINE.png
This figure depicts the thermocline. Temperature of the surface water remains high until it reaches the thermocline where the temperature drops rapidly as the lack of heat from the sun dissipates. Temperature continues to decline, though much less rapidly, at lower depths.
Life at the Depths
While many scientists believed the deep sea was devoid of life due to its harsh conditions, multiple expeditions have demonstrated the opposite. They found that the deep sea possessed intricate and lively ecosystems, despite being thousands of meters below the surface. In the 1870s, the Challenger expedition revealed the existence of a variety of deep water species. More recently, in 1977, scientists aboard DSV Alvin discovered diverse deep sea communities around hydrothermal vents.
Caused by the absence of light, life in the deep sea faces multiple challenges, such as having difficulty producing energy, attracting prey, and finding mates. At these depths, primary producers can no longer rely on the sun for energy, so they use alternative sources, such as falling detritus from above. These falling particles are known as marine snow, consisting of dead or decaying plant, animal material, fecal matter, and other organic and inorganic particulate matter (Silver 2015). The continuous rain of marine snow provides food for many deep-sea creatures, giving them plenty of carbon and nitrogen to sustain the scavengers of the deep and fuel the rest of the ecosystem.
The animals of the deep sea have adapted to survive in their habitats’ extremely harsh conditions: no light, low amounts of food, low amounts of oxygen, high pressure, and cold temperatures. Advantageous physiological changes have developed to enable them to survive in this kind of environment.
Many organisms in the deep sea have adapted a useful physiological trait to aid in the environment’s low light conditions: bioluminescence. Bioluminescence is the production and emission of light by a living organism. The ability to produce their own light allows organisms to lure prey, attract mates, and find paths in the water. Additionally, it can be used for defense, mimicry, and counter-illumination (George 2013). Certain squid species can flash warning lights or secret bioluminescent ink in order to defend themselves and ward off predators. Mimicry can be used to match the light pattern of an otherwise fearsome or undesirable organism in order to fend off predators as well. Counter-illumination employs the use of lights on the underside of an organism to match the light coming from above, allowing an organism to appear dark from above (to match the dark water below it) and light from below (to match the light coming through the water above). A common example of attraction via bioluminescence can be demonstrated by angler fish. Angler fish have an appendage that extends from their head with a small light on the end. As they wave this appendage back and forth, they attract and lure prey towards their mouth (Gage 1991).
Figure 2. upload.wikimedia.org/wikiped...nglerfish).jpg
This image depicts the anglerfish with a light lure. This is a demonstration of the evolutionary adaptation bioluminescence.
Many deep sea fishes are either completely blind or have developed enlarged eyes, which enable them to detect the faintest of lights. To compensate for the loss of vision, they are well-equipped with a lateral line system, tactile organs, and other sensory structures (e.g. fin rays) (Pandev, 2007).
Body color also has become an adaptation for camouflage. Animals’ bodies are often transparent, black, or red. The absence of red light at these depths keeps red-colored animals hidden from predators and prey.
Piezophiles are organisms that thrive in areas with high pressure. In order for them to inhabit such extreme environments, they evolved various mechanisms to counteract the effects of the elevated pressures. They are able to handle such pressure by remaining at a small size and eliminating excess cavities in their bodies that would otherwise collapse.
Many marine organisms have lost appendages and organs to conserve energy output, compensate for the extreme conditions, or because they simply do not use them anymore. The most common organ missing is the swim bladder; this is to escape the high stress of pressure present with extreme depth. In other cases, body parts have become enlarged and exaggerated (sensory organs, eyes, or mouths) in order to adapt to low light or scarce food (Pandey, 2007). An example of exaggerated and reduced body parts is the gulper eel, (Eurypharynx pelecanoides) which has tiny fins but also an enormous mouth, much larger than its body. Its mouth is loosely hinged and can open wide enough to swallow animals bigger than itself. Since food can be sparse in the deep sea, being able to eat whatever comes along, despite its size, is a huge advantage. The gulper eel has also adapted to having highly reduced pectoral fins because of their lack of use.
Figure 3. upload.wikimedia.org/wikiped...urypharynx.jpg
This image depicts the gulper eel with a large mouth and reduced fins.
To Learn More:
What's Hiding at the Most Solitary Place on Earth? The Deep Sea - Kurzgesagt Video
Angler Fish and Gulper Eel
Gage, John D., and Paul A. Tyler. 1991. Deep-sea biology: a natural history of organisms at the deep-sea floor. Cambridge University Press.
George, Robert Y. 2013 "Deep-Sea Organisms."Functional Adaptations of Marine Organisms: 279.
Pandey, Kamleshwar and J.P. Shukla. 2007. Fish and Fisheries. Meerut India: Rastogi. Print.
Silver, Mary. 2015. "Marine Snow: A Brief Historical Sketch."Limnology and Oceanography Bulletin.” 24.1: 5-10.