The ocean is one of the most interesting areas one could ever explore. It holds the remains of the Titanic, the wonders of the Mariana Trench, and the unknown of the Bermuda Triangle. While those are all great treasures and wonders of the sea; we are here to simply help you understand what we all call the Pelagic Zone.
What is the Pelagic Zone?
The pelagic zone is an area in open water that is neither near the shore nor near the bottom. The area by the shore and at the bottom of the water body have their own zones and associated properties.
Open Ocean and the Pelagic Zone are often used interchangeable. This is because while the Pelagic Zone encompasses open water, it also contains the neritic part just before the shore. So, while using ‘Open Ocean’ is not incorrect, it is not an all-encompassing term replacement for the ‘Pelagic Zone’.
The Pelagic Zone is the largest habitat in the world with a whopping volume of 330 million cubic miles! This column of open water, the ‘Pelagic Zone’, can be further divided into multiple regions based off depth.
Division of the Pelagic Zone
Many variables go into dividing such a large section of most bodies of water: depth, pressure increase, temperature drop, salinity levels, amount of dissolved oxygen, and the amount of decreased light. The Pelagic Zone is broken up into 5 zones:
- Epipelagic (surface – 660 ft) – As this zone begins at the surface of the water, this zone is considered the ‘illuminated zone’, where sunshine can still be seen, absorbed, and used for photosynthesis. Since almost all primary production occurs in this layer there is a large concentration of plants and animals that live in this zone. Organisms like plankton, jellyfish, tuna, floating seaweed, sharks and dolphins favor this zone.
- Mesopelagic (661 ft – 3,300 ft) – Often referred to as the twilight zone, due to the limitations in light and the lower levels of oxygen. Some creatures that live in the mesopelagic zone come up to the epipelagic zone, when it is dark, to feed. The most abundant of organisms that live in this zone are heterotrophic bacteria. Many of these organisms are bioluminescent. Examples of larger animals that stay in this layer are deep-water sharks & whales, squid, swordfish, cuttlefish and wolffish.
- Bathypelagic (3,301 ft – 13,000 ft) – This is the zone where the ocean becomes pure darkness. The occasional bioluminescent organism, like the anglerfish, may light up but the remaining layer remains pitch black. Plants are unable to thrive at this depth and darkness. Animals like the hatchet fish, giant squid, dumbo octopi, and other smaller squids either feed on the ‘marine snow’ (dissolved organic material) falling from the above layers or prey on other animals living in this zone. Fun Fact: some Sperm Whales that live in the mesopelagic zone may dive deep to this zone to feed on the elusive giant squid.
- Abyssopelagic (13,001 ft – to just above the seabed) – This is the zone just before the ocean floor. The name ‘Abyssopelagic’ comes from ancient Greek meaning “bottomless”. Very few organisms can survive in such cold temperatures, complete darkness, and high pressure of the abyss. Despite this some animals like squid, echinoderms (sea urchin, starfish, sand dollars, etc.), the swimming cucumber, the sea pig and even sea spiders! Due to the utter darkness, many of these species are transparent and eyeless.
- Hadopelagic (20,000 ft+) – Hadal environments are located int eh deepest parts of the ocean. These generally consist of deep-sea trenches. There are 46 known hadal habitats throughout the world, most found in the Pacific Ocean. Organisms like sea cucumber, bristle worms, bivalves, sea anemones, some crustaceans and even some fish can survive in these trenches. Fun Fact: The Mariana Trench is the deepest known point in the ocean, at over 36,000 ft.
Pelagic Marine Life
The pelagic marine life comprises three main categories: phytoplankton, zooplankton, and nekton.
When talking about organisms that can be found in the pelagic zone, one of the most abundant is phytoplankton. Phytoplankton are microscopic algae living in the epipelagic zone and they represent food for marine mammals or larger types of fish. Phytoplankton won’t be found in the mesopelagic zone, as the level of light decreases, and they can no longer access the light to complete photosynthesis.
Zooplankton are small, aquatic microorganisms. Most zooplankton consume phytoplankton which in turn end up falling pray to larger oceanic animals. Krill is one of the most widely known zooplankton. Other zooplankton include small crustaceans, insect larvae, rotifers, and aquatic mites.
Nektons are the pelagic animals and organisms that swim independent of any water or wind movement. There are 3 types of nektons found in the pelagic zone:
The chordate nekton includes species that have bones and/or cartilage. Animals such as sharks, eels, whales, dolphins, and sea turtles are among this group of nektons.
The Molluscan nekton will be soft bodies invertebrates, some of which can be protected by a calcium-based shell. This includes mostly cephalopods, octopods and squids.
The arthropod nekton class is primarily made up of decapods. This includes animals like shrimp, lobsters, and crabs.
Popular Pelagic Fish Species
The list of the most popular pelagic fish can be broken down based on the zones in which they can be found.
Fish Found in the Epipelagic Zone
Broadly, epipelagic fish can be divided into 2 groups, forage fish and predator fish.
Popular forage fish used as bait include herring, sardines, anchovies, shad, and sprats. Other small forage fish in this zone may include halfbeaks, smelt, and caplin.
Predator fish in the epipelagic zone includes both coastal and offshore fish like billfish, mackerel, salmon, tuna, bonita, and even blue sharks.
Fish Found in the Mesopelagic Zone
Moving on to the mesopelagic zone, the list of the most popular pelagic fish changes a bit. Animals like swordfish, bigeye tuna, squid, cuttlefish, the bioluminescent lanternfish, marine hatchetfish, or stoplight loosejaw are just a few of the fish found in this zone.
Fish Found in the Bathypelagic Zone
As we get deeper fish verse smaller organisms become smaller and smaller. The most popular fish found in the bathypelagic zone includes the larger whales, humpback anglerfish, squid, octopi, and the black swallower.
Fish Found in the Abyssopelagic Zone
There are actually hardly any fish found in the abyssopelagic zone. Fish normally skip this zone and either live above in the Bathypelagic zone or on the ocean floor, known as the Benthic Zone. However, some squid, jellyfish, and sea cucumbers do thrive in this environment.
What is the pelagic zone, in the end? It is a vast column of water that runs from the surface to just above the ocean floor. The pelagic zone remains an area that is fascinating not only for scientists but for fishermen too. It’s a challenging environment for species, as there are different factors that can affect which zone they thrive in.