Life in the Bathypelagic Zone

Here is a paper I wrote in March of 2018 about the outlandish life of the bathypelagic zone, part of the deep ocean layer known as the midnight zone where pressures are extremely high and no light penetrates. I hope you enjoy learning about it and let me know what you think below and comment in the forum with any questions.

Oceanographers divide the open sea into layers, drawing boundaries according to the distance that light penetrates through the ocean. The surface layer of the ocean is known as the epipelagic zone, the sunlit zone, or the euphotic zone. Photosynthesis is prevalent in this zone to utilize the abundant sunlight. It extends from the surface of the ocean down to 200 meters below. Here, little to no light can filter through; the quality of the lighting is eternal dusk. Levels of light are insufficient to support photosynthesis in this zone, but here a new light source shines, known as bioluminescence. This zone is known as the twilight zone, mesopelagic zone, or disphotic zone. It extends from 200 meters below sea level to 1000 meters below sea level. The next layer receives no sunlight whatsoever; it is called the aphotic zone or the midnight zone. This layer is commonly divided into three sub-layers: the bathypelagic zone, abyssopelagic zone, and hadalpelagic zone. Sometimes the bathypelagic zone by itself is called he aphotic zone or the midnight zone. Since no sunlight passes past 1000 meters below sea level, the next layers must be determined on an alternative basis: The  bathypelagic zone beginning at the continental slope and extending past it, extending from about -1000 meters -4000 meters; Below is the Abyssopelagic zone which is the zone beginning where the continental slope levels off, extending from approximately -4000 meters to -6000 meters; The lowest zone is the hadalpelagic zone which is the volume inside oceanic trenches, extending from around -6000 meters to a maximum depth of -10994 meters (2). (1).

            Since light penetrates to varying depths in different areas according to the transparency of the water, the boundaries determined according to light penetration cannot be absolute or precise. For example, in some tropical waters, light can penetrate as far as 600 meters (3), but in other places . The same degree of uncertainty applies to the aphotic zones as the continental slope is not entirely uniform and oceanic trenches vary in depth (2).

            The particular focus of this paper shall be the bathypelagic zone. It is unique in several ways: there is no sunlight, there is very high pressure (100-400 atm.), it is relatively cold, it has a high mineral and nutrient density, and the conditions are constant, due to lack of wind, sunlight, and because water in the deep sea comes from dense, polar water which sinks to the bottom and slowly flows across the ocean floor and thus the deep sea water is a constant temperature. These extreme conditions have selected for some rather extreme adaptations, the lack of sunlight having the greatest adaptive repercussions; since there is no sunlight, there is no photosynthesis which means that almost no primary production occurs. All of the food in the bathypelagic zone comes in the form of organic particles drifting down from the layers above. There is only enough to support a very low population density; even though the bathypelagic zone accounts for ninety percent of the oceans’ volume,  it has a very low population and biodiversity relative to the layers above. And these things continue to decline as a function of depth. (4).

            What few organisms are supported by this organic snow are not over-abundantly supplied with sustenance; they were forced to develop means of conserving energy and, in a world of dark darkness, of luring the prey to them. Creatures, such as jellyfish and angler-fish of the deep can often be found floating motionless. (2). Due to cold temperatures, they have very low metabolic rates which helps further to conserve energy. Since they don’t move very often, they don’t waste energy in forming a streamlined body; they tend to be bulky and lumpy. Many are merely living lures; traps baited with light. (4).

            The light is produced by a phenomenon called bioluminescence, caused by reaction between a molecule called luciferin and oxygen. Some animals that produce luciferin also produce a catalyst to speed up the reaction called luciferase. An organism can control the intensity and color of the reaction called bioluminescence, as well as when they light up. Some organisms borrow bioluminescence from glowing bacteria; they provide a favorable environment and the bacteria glow for them. (5).

            This is a tool used all throughout the animal kingdom from insects to plankton to deep sea fishes and invertebrate. Even humans bioluminesce, although the intensity is one thousand times fainter than would be visible and does not involve luciferin or serve any purpose (6).. Nor is it limited to the deep sea; the phenomenon can be observed all throughout the water column, only it is very common in the aphotic zone; about ninety percent of deep sea species have the capability to produce bioluminosity. Bioluminescence can serve multiple purposes from mate attraction to luring prey to startling predators. (5).

            Living in an area devoid of visibility, the organisms inhabiting the bathypelagic zone have developed alternative sensory techniques, enhanced old ones, and dropped others. Some organisms, like angler-fish, have long tentacles that act like feline whiskers to increase the distance at which they can detect predators and prey. (7). Another adaptation of the angler-fish is its extreme sexual dimorphism and sexual parasitism. The male is minute in comparison to the female and lacks a fishing rod and bioluminescence. He locates a female mainly utilizing his enlarged nasal orifices, but also perhaps by the female’s bioluminescence once he reaches the appropriate proximity. When he finds her, he bites into her underside and fuses with her body, sharing her blood in exchange for sperm. This is often how bioluminescence is used, to distinguish between the sexes. (8).

            But some organisms lack functional eyes because they serve no purpose in a world of darkness, save for detecting bioluminescence. And if an organism has no eyes, it can’t be lured in the bell of a jellyfish or the maw of an angler-fish. (9).

            This leads to an interesting question: why are organisms attracted to the light in the first place? At night, when humans introduce an artificial light, little zooplankton become illuminated. This makes them a target for small fish that hunt by sight to whom they were invisible prior to illumination. When the little fish begin feeding, they too become illuminated and attract larger predators, and so on. Fishermen take advantage of this phenomenon. (10). A similar chain of events may occur in the bathypelagic zone as well.

            Another important characteristic of the bathypelagic zone is the extreme pressure. This does not have an enormous effect on the creatures because they were born there and spend most of their lives their, so the pressures inside and outside their bodies are equalized leaving no net effect. But, at least one adaptation has arisen due to high pressure and it involves an organ common to many fish species called a swim bladder. It is a gas-filled chamber inside some fish that allows for passive flotation. Regulation of the amount of gas stored can help the fish rise or sink. The swim bladder is absent from the physiology of bathypelagic organisms, or it is filled with fluid, not gas. This is because gases are compressible while fluids are not. This also explains the complete lack of air spaces in deep sea organisms. (11).

            Oxygen is something that might be expected to exist only in very small amounts in the bathypelagic zone; Because no photosynthesis occurs, no oxygen is replenished and it is constantly consumed. But, the cold polar waters are actually saturated with oxygen. (4). However, above the bathypelagic zone exists, from about -300 to -400 meters, a so called oxygen minimum zone. Here, adaptations to this lower oxygen environment may include the utilization of more efficient oxygen processing enzymes (12) and increased surface area. (13).

            Many deep sea organisms at some point travel nearer to the surface, for various reasons. An innumerable quantity of small organisms move up at night and descend to safety during the day, when they would be visible beyond the bathypelagic zone. At night, they like to take advantage of the increased availability of food in shallower, warmer layers. (10). This exposes them to varying pressure and temperature, which alters certain bilayer membranes of functional importance. Adaptations are required to handle this. (14). Some organisms, like the angler-fish, rise near the surface to breed, and require the same environmental versatility to survive, although many of them don’t survive anyway (7).

            Those described above are just a few of the numerous adaptations necessary for the survival of life in the bathypelagic zone.


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  2. Stenstrom, Jonas. (2018). “Pelagic Biome.” Untamed Science. Date-accessed: 4/4/2018.
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  7. Langin, Katie. (2018). “Exclusive: ‘I’ve never seen anything like it.’ Video of mating deep-sea anglerfish stuns biologists.” Science.
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  13. Levin, Lisa A. (2002). “Deep Ocean Life Where Oxygen is Scarce.” American Scientist.
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