Deep-sea fauna

 The deep sea may be regarded as that part of the ocean below the upper limit of the continental slopes (Fig. 1). 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

 

 Fig. 1  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.

 

 

The systematic exploration of the deep sea began with the voyage of the HMS Challenger (1872–1876). Since that time there have been numerous large-scale, deep-sea expeditions. In 1948 the Swedish Deep Sea Expedition in the Atlantic developed new techniques for trawling and the winch used by later expeditions. See also: Marine biological sampling

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 (Fig. 1).

Bathypelagic fauna

 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. In the deep sea, plants can exist only as saprophytes. The productivity of the plants, however, is reflected down to the deepest parts of the sea through complex food chains. These consist of zooplankton that graze on phytoplankton, carnivorous species that feed on zooplankton, and large predators that eat the other animals. The typical bathypelagic animals (Fig. 2) begin to appear below depths of about 600 ft (200 m). See also: Food web; Phytoplankton; Zooplankton

 

 Fig. 2  Pelagic animals of the deep sea. (a) Copepod (Haloptilus acutifrons). (b) Prawn (Acanthephyra multispina). (c) Salp (Salpa, or Thalia, democratica). (d) Squid (Abraliopsis morisii). (e) Lantern fish (Myctophym punctatum). (f) Anglerfish (Lophodolus acanthognathus).

 

 

 

 Zooplankton

 The planktonic or drifting forms of animal life in the ocean include the Protozoa, larval stages of deep-sea fishes, and even larger organisms with limited powers of movement. 

1. Protozoa. Included in this group are various families of Foraminiferida and Radiolaria, the skeletons of which form an important part of the deep-sea sediments. See also: Protozoa

2. Coelenterata. Scyphomedusae such as Atolla and Periphylla are not uncommon. Other jellyfishes include various Trachymedusae (Crossota and Colobonema) and Narcomedusae. Siphono­phorida, particularly the diphyids, are found down to depths of at least 9000 ft (3000 m) but are more common in the upper several hundred feet. See also: Cnidaria

3. Nemertea. This group of worms has bathypelagic species belonging to some 10 families.

4. Crustacea. In numbers of species and individuals, the small Copepoda are the dominant group of crustaceans in the ocean. There are numerous bathypelagic species. Certain of the Ostracoda (Gigantocypris) are purely bathypelagic, as are some of the Amphipoda (the gammarid genera Cyphocaris and Hyperiopsis and most species of the hyperiid families Scinidae and Lanceolidae). See also: Amphipoda; Copepoda; Ostracoda

The larger and more active pelagic crustaceans (Euphausiacea, various Mysidacea, and prawns) are usually classed as plankton but might well be called “micronekton,” a group intermediate between thrusting nekton and feebler-swimming plankton. Of the euphausiid shrimps, Benthenphausia and various species of Thysanopoda, Nematoscelis, and Stylocheiron have centers of abundance in the bathypelagic zone. Deep-water genera of mysids include Gnathophausia, Lophogaster, and Eucopia, while the prawn families Hoplophoridae and Sergestidae have numerous bathypelagic representatives. See also: Crustacea

5. Chaetognatha. Certain species of arrowworms, such as Eukrohnia fowleri and Sagitta macrocephals, are predominantly bathypelagic. See also: Chaetognatha

6. Echinodermata: Holothuroidea. The genera Pelagothauria, Enypiastes, and Galatheathuria are bathypelagic, the first two being medusalike. See also: Echinodermata

7.Protochordata: Thaliacea. While they are more abundant in the surface layers, the salps, doliolids, and pyrosomes have been fished down to 9000 ft (3000 m).

 

Deep-sea nekton

This group consists largely of squids, octopods, and fishes. The sperm whale also enters the deep sea, where it finds some of the squid upon which it feeds. 

1. Mollusca: Cephalopoda. Several families of squids form an important part of the deep-sea nekton together with a few octopods, such as Cirrothauma, Amphitretus, Vitreledonella, and Vampyroteuthis. See also: Cephalopoda

2. Fishes. Apart from a few squaloid sharks, the bathypelagic fish fauna consists of teleosts. The most diverse groups are the stomiatoids (Elupeiformes), with about 300 species; Myctophidae (lantern fishes; Salmoniformes), about 250 species; and the ceratioid angler fishes (Lophiiformes), about 90 species. The few species forming the orders Anguilliformes (gulper eels) and Cetomimiformes (whale fishes) are entirely bathypelagic, as are certain of the eels (Cyemidae, Nemichthyidae) and Beryciformes (for example, Melamphaidae). See also: Teleostei

 

Distribution

 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 (Fig. 1). 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.

Lastly, numerous species of mesopelagic animals, such as euphausiids, prawns, squids, and fishes (particularly lantern fishes), undertake extensive diurnal, vertical migrations, moving upward into the productive surface layers to feed at night. Toward sunrise they begin to descend to their daytime levels. See also: Scattering layer

 

Bioluminescence

 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 (Fig. 3) in the ocean: organisms that attach to the bottom and those that freely move over the bottom.

 

 Fig. 3  Benthic animals of the deep sea. (a) Sea-pen (Umbellula) showing bulbous holdfast. (b) Sea cucumber (Elpidia glacialis). (c) Head and forepart of the trunk of a pogonophoran (Birsteinia witjasi). (d) Isopod crustacean (Macrostylus hadalis). (e) Bathypteroid fish (Benthosaurus).

 

 Attached benthic organisms

 This group consists of species that attach themselves to the sediments, rocks, or to other organisms. The more typical forms include the following: Hexactinel­lida (Porifera) (glass sponges), with about 375 species; certain hydroids, gorgonians, pennatulids (sea fans), antipatharians (black corals), actiniarians (sea anemones), and madrepore corals (Lophohelia and Amphihelia); Cirripedia (barnacles), such as Scalpellum and Verruca spp.; numerous species of stalked crinoids living in the deep sea together with a number of unstalked forms; and Pogonophora (beard-bearers) and certain ascidians (Culeolus sp.).

 

Benthic crawlers and swimmers

This group comprises the freely moving animals, those that swim or crawl over the bottom or burrow into the sediments, the upper layer of which has a rich bacterial flora. (1) Annelida: a few species of Polychaeta (bristle worms). (2) Gephyrea: certain species of echiuroid and sipunculoid worms. (3) Crustacea. In numbers of species and individuals the most important group of benthic deep-sea crustaceans is the Peracarida, represented by various species of cumaceans (Bathycuma, Macrocylindrus), isopods (Ischnomesus and Eurycope), amphipods, and tanaids (Apseudes, Neotanais). Of the Eucarida, the most prominent groups are the penaeid prawns and the Eryonidae. There are also a number of crabs (Platymaia, Geryon, Ethusa, and Scyramathia) and hermit crabs (numerous species of Axiidae). (4) Pycnogonida (sea spiders): numerous species of the families Colossendeidae and Nymphonidae. (5) Mollusca. Certain of the Octopodidae (octopuses) and the cirromorph octopods live on the deep-sea floor, as do various gastropods, scaphopods, and lamellibranchs. (6) Echinodermata. These form an important part of the benthic fauna, particularly the sea cucumbers (Holothuroidea) of the orders Elasipodida and Molpadida. Among the sea urchins (Echinoidea), the order Cidaroida and the suborder Meridosternata mainly consist of deep-sea species. There are also various brittle stars (Ophiuroidea) and starfishes (Asteroidea). (7) Fishes. The species of one group of cartilaginous fishes (Holocephali) live over the continental slope. The main groups of benthic, deep-sea teleosts are the Bathypteroidae (Salmoniformes); Halosauridae and Notacanthidae (Notacanthiformes); Macrouridae (rattails) and Morinae (deep-sea cods) (Gadiformes); and Brotulidae, Liparidae, and Zoarcidae (Perciformes). See also: Echiura; Polychaeta; Pycnogonida; Sipuncula

 

Distribution

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 (Fig. 1) extending down to about 9000 ft (3000 m; bathyal zone); others occur below this in the abyssal zone. Danish and Russian exploration also suggested that the deep-sea trenches (with depths over 21,000 ft or 7000 m) form another ecological zone (hadal zone) having certain characteristic species—those capable of living under pressures of 700–1000 atm (70–100 megapascals; barophilic species). This work also showed that life could exist at the very bottom of the ocean (down to depths of more than 30,000 ft or 10,000 m) and that species of certain groups, such as sea anemones, echiuroid and polychaete worms, bivalves, isopod and amphipod crustaceans, sea cucumbers, and Pogonophora, occurred at depths beyond 27,000 ft (9000 m).

Lastly, there is a decrease in the numbers of species and individuals with depth. Russian biologists found that at depths of 24,000–30,000 ft (8000–10,000 m) the weight of animals per square meter of sea floor was about one-fifth to one-fifteenth the weight at depths of 3000–12,000 ft (1000–4000 m). As the deep-sea benthic fauna is dependent on organic matter originating in the upper, plant-bearing waters and as the amount reaching the bottom must decrease with depth, the above findings are comprehensible. It is also interesting that there are very few carnivorous animals, such as crabs, brittle stars, and starfishes, below a depth of 21,000 ft (7000 m). It is the particle-catchers, such as the Pogonophora, and ooze-eaters, such as sea cucumbers and echiuroid worms, that make up most of the hadal fauna. See also: Marine ecology; Seawater fertility

Bibliography

  •  C. P. Idyll, Abyss: The Deep Sea and the Creatures That Live in It, 3d rev. ed., 1976
  • Alifazeli = egeology.blogfa.com
  • N. B. Marshall, Aspects of Deep Sea Biology, 1977
  • Alifazeli = egeology.blogfa.com
  • N. B. Marshall, Developments in Deep-Sea Biology, 1980
  • Alifazeli = egeology.blogfa.com