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Depth and layers

The pelagic zone is subdivided into five vertical regions. From the top down, these are:

Epipelagic (sunlight)

See also: Photic zone and Epipelagic fish

From the surface (MSL) down to around 200 m (660 ft)

The illuminated zone at the surface of the sea with sufficient light for photosynthesis. Nearly all primary production in the ocean occurs here, and marine life is concentrated in this zone, including plankton, floating seaweed, jellyfish, tuna, many sharks and dolphins.

Mesopelagic (twilight)

See also: Mesopelagic zone and Pelagic fish § Mesopelagic fish

From 200 m (660 ft) down to around 1,000 m (3,300 ft)

The most abundant organisms thriving into the mesopelagic zone are heterotrophic bacteria.⁵ Creatures living in this zone include swordfish, squid, wolffish and some species of cuttlefish. Many organisms living here are bioluminescent.⁶ Some mesopelagic creatures rise to the epipelagic zone at night to feed.⁷

Bathypelagic (midnight)

See also: Bathyal zone and Pelagic fish § Bathypelagic fish

From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from Ancient Greek βαθύς 'deep'. The ocean is pitch black at this depth apart from occasional bioluminescent organisms, such as anglerfish. No plants live here. Most creatures survive on detritus known as "marine snow" falling from the zones above or, like the marine hatchetfish, by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are giant squid, smaller squid, viperfish, gulper eel, and dumbo octopus.

Abyssopelagic (abyssal zone)

Main article: Abyssal zone

From around 4,000 m (13,000 ft) down to above the ocean floor

The name is derived from Ancient Greek ἄβυσσος 'bottomless' – a holdover from times when the deep ocean was believed to be bottomless. Among the very few creatures living in the cold temperatures, high pressures and complete darkness here are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider. Many species at these depths are transparent and eyeless.⁸

Hadopelagic (hadal zone)

Main article: Hadal zone

The name is derived from the realm of Hades, the Greek underworld. This is the deepest part of the ocean at more than 6,000 m (20,000 ft) or 6,500 m (21,300 ft), depending on authority. Such depths are generally located in trenches.

Pelagic ecosystem

The pelagic ecosystem is based on phytoplankton. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.⁹

Pelagic invertebrates

Some examples of pelagic invertebrates include krill, copepods, jellyfish, decapod larvae, hyperiid amphipods, rotifers and cladocerans.

Thorson's rule states that benthic marine invertebrates at low latitudes tend to produce large numbers of eggs developing to widely dispersing pelagic larvae, whereas at high latitudes such organisms tend to produce fewer and larger lecithotrophic (yolk-feeding) eggs and larger offspring.¹⁰¹¹

Pelagic fish

Main article: 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. They can be contrasted with demersal fish, which do live on or near the bottom, and coral reef fish.¹²

Pelagic fish are often migratory forage fish, which feed on plankton, and the larger predatory fish that follow and feed on the forage fish. Examples of migratory forage fish are herring, anchovies, capelin, and menhaden. Examples of larger pelagic fish which prey on the forage fish are billfish, tuna, and oceanic sharks.

Pelagic reptiles

Hydrophis platurus, the yellow-bellied sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The yellow-bellied sea snake is the world's most widely distributed snake species.

Many species of sea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.

Pelagic birds

See also: Seabird

Pelagic birds, also called oceanic birds or seabirds, live on open seas and oceans rather than inland or around more restricted waters such as rivers and lakes. Pelagic birds feed on planktonic crustaceans, squid and forage fish. Examples are the Atlantic puffin, macaroni penguins, sooty terns, shearwaters, and Procellariiformes such as the albatross, Procellariidae and petrels.

Further reading

• Ryan, Paddy "Deep-sea creatures" Te Ara – the Encyclopedia of New Zealand, updated 21 September 2007
• "Pelagic-zone (oceanography)" Encyclopædia Britannica Online. 21 March 2009.
• Grantham HS, Game ET, Lombard AT, et al. (2011) "Accommodating Dynamic Oceanographic Processes and Pelagic Biodiversity in Marine Conservation Planning" PLOS One 6(2): e16552. doi:10.1371/journal.pone.0016552.
• Wrobel, David; Mills, Claudia (2003) [1998]. Pacific Coast Pelagic Invertebrates: A Guide to the Common Gelatinous Animals. Sea Challengers and Monterey Bay Aquarium. ISBN 0-930118-23-5.

References

1. "pelagic (adj.)". Online Etymology Dictionary. Retrieved 17 February 2020. https://www.etymonline.com/word/pelagic ↩

2. Costello, Mark John; Cheung, Alan; De Hauwere, Nathalie (2010). "Surface Area and the Seabed Area, Volume, Depth, Slope, and Topographic Variation for the World's Seas, Oceans, and Countries". Environmental Science & Technology. 44 (23): 8821–8. Bibcode:2010EnST...44.8821C. doi:10.1021/es1012752. PMID 21033734. /wiki/Bibcode_(identifier) ↩

3. Charette, Matthew; Smith, Walter (2010). "The Volume of Earth's Ocean". Oceanography. 23 (2): 112–4. doi:10.5670/oceanog.2010.51. hdl:1912/3862. https://doi.org/10.5670%2Foceanog.2010.51 ↩

4. Ocean's Depth and Volume Revealed Archived 23 August 2011 at the Wayback Machine OurAmazingPlanet, 19 May 2010. http://www.ouramazingplanet.com/oceans-depth-and-volume-revealed-0206/# ↩

5. Mazuecos, E.; Arístegui, J.; Vázquez-Domínguez, E.; Ortega-Retuerta, E.; Gasol, J.M.; Reche, I. (2012). "Temperature control of microbial respiration and growth efficiency in the mesopelagic zone of the South Atlantic and Indian Oceans". Deep Sea Research Part I: Oceanographic Research Papers. 95: 131–138. doi:10.3354/ame01583. hdl:10261/95626. https://doi.org/10.3354%2Fame01583 ↩

6. The Open Ocean - MarineBio.org https://web.archive.org/web/20100410222157/http://www.marinebio.com/Oceans/open-ocean.asp ↩

7. The Open Ocean - MarineBio.org https://web.archive.org/web/20100410222157/http://www.marinebio.com/Oceans/open-ocean.asp ↩

8. The Open Ocean - MarineBio.org https://web.archive.org/web/20100410222157/http://www.marinebio.com/Oceans/open-ocean.asp ↩

9. Walker P and Wood E (2005) The Open Ocean (volume in a series called Life in the sea), Infobase Publishing, ISBN 978-0-8160-5705-4. https://books.google.com/books?id=O0GpAZsmrVAC&q=intitle:The+intitle:Open+intitle:Ocean ↩

10. Thorson, G (1957). "Bottom communities (sublittoral or shallow shelf)". In Hedgpeth, J.W. (ed.). Treatise on Marine Ecology and Palaeoecology. Geological Society of America. pp. 461–534. ↩

11. Mileikovsky, S. A. (1971). "Types of larval development in marine bottom invertebrates, their distribution and ecological significance: a re-evaluation". Marine Biology. 10 (3): 193–213. doi:10.1007/BF00352809. S2CID 84623588. /wiki/Doi_(identifier) ↩

12. Lal, Brij V.; Fortune, Kate (January 2000). The Pacific Islands: An Encyclopedia. University of Hawaii Press. p. 8. ISBN 978-0-8248-2265-1. 978-0-8248-2265-1 ↩