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12.4 Deep Sea

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  • 12.4 Deep Sea Ecosystems

    12.4.1 Introduction

    12.4.2 Life at Depths

    12.4.3 Physical Adaptations

    12.4.1. Introduction

    The deep sea is an environment completely unfriendly to humankind; 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 1000m beneath the ocean surface laying between the thermocline and seafloor (Gage 1991). Temperature drops significantly nearing 0 C at the bottom, but is typically between 2-3°C for much of the deep ocean. Salinity along the deep sea does not change as significantly, it ranges from 34-35ppt (parts per thousand) concentration. Pressure in the deep sea increases steadily with depth; there is an increase of around 10 atmospheric pressure per meter (Gage 1991, George 2013). Light is one of the most limiting factors of the deep sea and disappears between 100-500m.

    Figure 1.
    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.

    12.4.2 Life at the Depths

    While many scientists believed that the deep see was devoid of life due to its harsh conditions, expeditions have shown there to be intricate and lively ecosystems thousands of meters below the surface. In the 1870s, the Challenger expedition brought to light a great variety of deep water species. In 1977, scientists aboard DSV Alvin discovered diverse deep sea communities around hydrothermal vents.

    Devoid of light, photosynthesis does not occur, predators find it more difficult to attract their prey, and finding a mate becomes a challenge. Primary producers are no longer reliant on the sun for energy, but rather on falling detritus from above, that brings with it nutrients and. These falling particles are known as marine snow. Marine snow is made up of dead or decaying animal parts and plants, fecal matter, and other organic and inorganic 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.

    12.4.3 Physical Adaptations

    The animals in the deep a have to adapt to surviving in the deep sea’s extremely harsh conditions: no light, very little food, low amount of oxygen, high pressure, and freezing temperatures. Advantageous physiological changes have evolved to survive these conditions.

    Some organisms in the deep sea have adapted a very useful physiological trait to aid in the low light conditions: bioluminescence. This is the production and emission of light by a living organism. Aside from attracting prey, mates and finding paths in the water, organisms possess the trait for a variety of other reasons: defense, mimicry, and counter-illumination (George 2013). Certain squids will flash warning lights or even secret bioluminescent ink in order to defend and ward off predators. Mimicry can be used by any organism 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; this allows 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 is in angler fish. They use an appendage like fishing lure attached to their head with a small light on the end, waving it back in forth to attract and lure their prey toward their mouth (Gage 1991).

    Figure 2.
    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 light. To compensate for the loss of vision, they are well equipped with a lateral line system, tactile organs and other sensory structures (ex. Fin rays) (Pandev, 2007).

    Body color also has become an adaptation for camouflage. Animal’s 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 thriving at areas of high pressure. In order for them to inhabit such extreme environments they have evolved various mechanisms to counteract the effects of the elevated pressures they endure. They are able to deal with 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 even organs to conserve energy output, compensate for the extreme conditions, or because they simply did not use them anymore. The most common organ missing is the swim bladder; this is to escape the high stress of pressure that is experienced 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 an enormous mouth much larger than its body, but has tiny fins. Its mouth is loosely hinged and can open wide enough to swallow animals larger 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.
    This image depicts the gulper eel with a large mouth and reduced fins.

    To Learn More:

    Deep Sea Exploration Documentaries 1-3

    Marine Snow

    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.