The deep sea may be regarded as that part of the ocean below the upper limit of the continental slopes (see illustration). Its waters fill the deep ocean basins, cover about two-thirds of the Earth's surface, have an average depth of about 12,000 ft (4000 m), and provide living space for communities of animals that are quite different from those inhabiting the land-fringing waters which overlie the continental shelves (neritic zone). See also Ecological communities.
The deep-sea fauna consists of pelagic animals (swimming and floating forms between the surface and deep-sea floor) and below these the benthos, or bottom dwellers, which live on or near the ocean bottom. Pelagic animals can be divided into the usually smaller forms that tend to drift with the currents (zooplankton) and the larger and more active nekton, such as squids, fishes, and cetaceans. Pelagic, deep-sea animals are frequently termed bathypelagic in contrast to the epipelagic organisms of the surface waters (see illustration). See also Zooplankton.
Classification of marine environments. Right side of diagram illustrates the proposal to divide the bathypelagic zone into mesopelagic, bathypelagic, and abyssopelagic zones. Division of benthic region into bathyal, abyssal, and hadal zones also is shown. 1 m = 3.3 ft.
All animal life in the sea, pelagic and benthic, depends on the growth of microscopic plants (phytoplankton). From the surface down to a maximum depth of about 300 ft (100 m) there is sufficient light for photosynthesis and vigorous phytoplanktonic growth. This layer is known as the photic zone. See also Phytoplankton.
Bathypelagic fauna
The typical bathypelagic animals begin to appear below depths of about 600 ft (200 m). The bathypelagic fauna is most diverse in the tropical and temperate parts of the ocean. Numerous species are found in all three temperature zones, but many appear to have a more limited distribution. Each species also has a definite vertical occurrence. Findings suggest that there are three main vertical zones, each with a characteristic community. Here the term bathypelagic is used for the fauna between about 3000 and 6000 ft (1000 and 2000 m), that above (between 600 and 3000 ft or 200 and 1000 m) being called mesopelagic and that below 6000 ft (2000 m) abyssopelagic (see illustration). The typical forms of the mesopelagic fauna (stomiatoids and lantern fishes) live in the twilight zone of the deep sea (between the 68 and 50°F or 20 and 10°C isotherms), while the bathypelagic species (ceratioid angler fishes and Vampyroteuthis) occur in the dark, cooler parts below the 50°F (10°C) isotherm.
Perhaps the most conspicuous feature of pelagic deep-sea life is the widespread occurrence of luminescent species bearing definite light organs (photophores). Many of the squids and fishes have definite patterns of such lights, as do some of the larger crustaceans (hoplophorid and sergestid prawns and euphausiids). Investigations have shown that flashes from luminescent organisms could be detected down to depths of 12,300 ft (3750 m). See also Bioluminescence; Photophore gland.
Benthic fauna
There are two main ecological groups of bottom-living animals in the ocean: organisms that attach to the bottom and those that freely move over the bottom. The benthic fauna is most diverse in the temperate and tropical ocean, although the arctic and antarctic areas have their characteristic species. As in the pelagic fauna, certain species occur in all three oceanic zones, while others appear to have a more restricted occurrence. While a number of species—particularly among the polychaete worms, gastropod mollusks, and the brittle stars (Ophiuroidea)—range from littoral to abyssal regions, most forms tend to live within smaller ranges of depth. Data suggest that there are typical communities of animals over the continental slopes (see illustration) extending down to about 9000 ft (3000 m; bathyal zone); others occur below this in the abyssal zone. See also Marine ecology.
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
