Marine Biology (2+1)

Pelagic realm

The pelagic realm is further subdivided vertically into five zones viz. epi pelagic, mesopelagic , bathypelagic , abyssopelagic  and hadalpelagic zones.

i. Epipelagic zone  It covers fromthe surface of the ocean and extends upto 200m depth (all light rays are seen here initially) It is also called as the photic zone or euphotic zone.

ii. Mesopelagic zone -It extends from 200 to1000 meters depth.  It is also called as  disphotic zone as; only blue light is seen here. It is also referred to as the  "twilight zone"; its lower boundary in the tropics is the 10º C isotherm.

iii. Bathypelagic zone - This zone extends from 1000 upto 4000 meters deep (aphotic zone; no light reaches this depth, there is  total darkness);  It lies between the boundaries of water with 10 and 4º C isotherm layers.

iv. Abyssopelagic zone – It lies below 2000 and extends upto 6000 meters depth (aphotic zone).

v. Hadalpelagic zone - has a depth of 6000-10000 meters (aphotic zone).

The epipelagic photic  euphotic zone  is the ideal place for about 90% of all ocean life to live because of warm temperature and sunlight  .This is the only zone to support plant life because it has the light needed for photosynthesis. As this region supports  diverse  plant life,  variety of animals such as zooplankton, crustaceans, moll                                                                                                       usks, sharks, sting rays, mackerels, tuna, seals, sea lions, sea turtles, etc., are abundant here.  

Therefore this region has to plant life though some sunlight penetrates through mesopelagic zone, it is not enough for photosynthesis. However this zone has octopus, squid, and hatchet fish.  There animals tolerate cold temperatures, increased water pressure and darkness . Some fish have extra big eyes to help them see, while others produce their own light called bioluminescence using special organs in their bodies called photophores. Most fish do not don’t chase their food but  either wait for it or stalk it. Some have sharp fangs or big mouths to help them catch their food.

 The bathypelagic and hadalpelagic zones, do not have as many fish as the earlier zones. The coloration of the fish is black or red, and have  bioluminescence (used to lure prey). The shape of the  fishes living have is globular (round is shape and no streamlining). Most the weak swimmers live here and the fishes are mostly small but some are large. The eyes of the fishes  are almost small or absent.

This zone is also called midnight zone or dark zone. This zone has a very intense water pressure which can be as great as two tons per square inch. Just like the mesopelagic zone, there are no plants and fewer animals which include vampire squid, giant squid, amphipod, slime stars, snake dragon fish, anglerfish, oarfish and gulper eel. The sperm whale  dives to these depths  search of food. Only about 1 % of all ocean species live in this zone, and some do not have eyes.

 The Hadal zone is covers the deepest parts of the ocean. This zone is  totally dark and  cold, with intense pressure. Creatures found here  have adapted themselves  to the darkness by reducing the use of eyesight. Fishes occurring heredo have eyes,  and they are usually enormous, which indicates enough flashes of bioluminescent light to keep their eyes from totally deteriorating.