(5) Pelagic Zone

Pelagic Zone

Any water in sea that is neither close to the bottom nor near the shore are called the Pelagic Zone.

Epipelagic (Sunlight) (660 ft)

The is the zone at the surface of the sea where enough light can penetrate the sea for photosynthesis. Nearly all primary production in the ocean occurs here. Consequently, plants and animals are largely concentrated in this zone. E.g plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.

Mesopelagic (Twilight)(3,300 ft)

 Although some light penetrates this second layer it is impossible for photosynthesis to occur. At about 500 m the water also becomes depleted of oxygen. Organisms survive this environment by having more efficient gills or by minimizing movement. 
e.g 

swordfish, squid, wolfish 

Bathypelagic (midnight)(13,000 ft)

At this depth, the ocean is made of near darkness apart from occasional bio luminescent organisms, such as lantern fish. No living plant life exists here. The only inhabitants are giant squid, smaller squids and the dumbo octopus. 

Abyssopelagic (lower midnight) (13,000 ft) down to above the ocean floor

Very few creatures live in the freezing temperature, high pressures and complete darkness of this depth. Among the species found in this zone are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider.

PS: Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.

Pelagic ecosystem

The pelagic ecosystem is created by phytoplankton which occupy the beginning of the food chain. Phytoplankton made their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone.

Pelagic birds
Pelagic birds, also called oceanic birds, live on the open sea, rather than around waters adjacent to land or around inland waters. Pelagic birds feed on planktonic crustaceans, squid and forage fish.

Pelagic fish

Pelagic fish live in the water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake.  These fish are often migratory forage fish, which feed on plankton, and the larger fish that follow and feed on the forage fish. 

Abyssal (8)

 

The Abyssal Zone, also known as the Abyssopelagic Zone is the part of the ocean which is typically characterized by uniform darkness, low temperature (around 3 degrees celsius), and unique fauna. It lies between the Bathyal Zone and Hadal Zone. It roughly starts at a depth of 4,000 m (13,000 ft) and ends at 6,000 m (19,500), which is where the Hadal Zone,  begins. The latter is primarily made up of deep trenches, like the Mariana Trench of the Challenger Deep fame.

In the absence of photosynthesis, the oxygen content of this zone depends on the oxygen that gets dissolved into the water in polar areas and makes it to the other parts of the ocean.

Its nutrient salt concentration is uniform―at times, higher than what it is in shallow water; courtesy, the dead biological matter that settles in this zone.

Animals of the Abyssopelagic Zone

• ·   The ability to withstand tremendous amount of pressure is a necessity. 
• The animals inhabiting this zone seem to excel in this context.·        
• As there is no sunlight to support the process of photosynthesis, the Abyssal Zone is devoid of plants. 

Crustaceans living at these depths have adapted by becoming transparent and have evolved without eyes because they serve no purpose at these depths.

Tiny microbes( cormophytes) act as producers in this zone by resorting to the process of chemosynthesis and oxidizing hydrogen sulfide coming from vents in the ocean floor.

Tubeworms are best suited for the conditions prevailing in the Abyssal Zone. They don't have mouth or digestive system. So they absorb inorganic compounds through their skin and depend on bacteria in their body for food.

Examples of abyssal zone animals


 giant squid (Architeuthis dux),

 colossal squid (Mesonychoteuthis hamiltoni)

black swallower (Chiasmodon niger) 

humpback anglerfish (Melanocetus johnsonii)

 deep-sea glass squid (Teuthowenia pellucida)

 ogrefish (Anoplogaster cornuta),
 
hagfish (Eptatretus cirrhatus),

 hydrothermal vent crab (Bythograea thermydron),

 giant tubeworm (Tevnia jerichonana), 

Fish in this zone:

·         various physical and behavioral adaptations to survive.

·        

·         The humpback anglerfish (Melanocetus johnsonii), for instance, has a large mouth with long teeth, which helps it grab its prey the moment it is in close range.

 Other physical adaptations include gray or black color, (camouflage) in the deep sea environment,

·         Bioluminescence( ability to produce light by physiological processes) which helps the species lure their prey.

·         ambush predators(wait for their prey to come to them), instead of expending energy chasing them.

·       This zone is characterized by the presence of hydrothermal vents, also known as black smokers that release toxic chemicals. Interestingly, the area around these vents is known for its rich biodiversity. The organisms found here resort to chemosynthesis for energy.

Other animals of the abyssopelagic zone include basket stars, seapigs, and seaspiders. Basket stars are echinoderms with an arm span larger than 25 cm living on the ocean floor or on another animal, feeding on detritus floating down from above. The seapig is another echinoderm that looks like an ugly sea cucumber and can eat mud like an earthworm. Some echinoderms, like the swimming cucumber, use wing-like structures to swim as if they were flying through the water. The sea spider is a crustacean and actually resembles a spider walking on the ocean floor. The sea spider has a hose-shaped mouth and eats anemones and echinoderms. Its leg span reaches 30 cm or larger