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16.2 the WORLD of PERPETUAL DARKNESS the Lack of Food Final PDF to printer 376 Part Three Structure and Function of Marine Ecosystems O2 abyssopelagic zone lies from 4,000 to 6,000 m (13,000 to 20,000 ft). The hadopelagic, or hadal pelagic, zone consists of the waters of the ocean trenches, from below 6,000 m to Amount of dissolved oxygen just above the sea floor, as deep as 11,000 m (36,000 ft). Low High Each of the depth zones supports a distinct community of animals, but they also have much in common. Here we hotosynthesi CO P s Organic 2 stress the similarities, rather than the differences, among the + matter H O + Epipelagic depth zones of the deep sea. 2 n O Respiratio 2 The conditions of life in the deep pelagic environment Decomposition change very little. Not only is it always dark, it is always 200 m cold: The temperature remains nearly constant, typically at 1 to 2 °C (35 °F). Salinity and other chemical properties of the water are also remarkably uniform. Oxygen minimum zone CO Organic 2 matter The deep sea also includes the ocean bottom beyond + H O Respiration + the continental shelf. Bottom-living organisms are covered 2 O Decomposition 2 Mesopelagic separately (see “The Deep-Ocean Floor,” below). The deep sea includes the bathypelagic, from 1,000 to 4,000 m; the abyssopelagic, 4,000 to 6,000 m; and the hadopelagic, 6,000 m to 1,000 m the bottom of trenches. The physical environment in these zones is quite constant. The deep sea also includes the deep-sea floor. In the darkness of the deep sea there is no need for countershading. Many animals, especially zooplankton, are CO Organic 2 a drab gray or off-white. Deep-sea fishes are generally black + matter H O Respiration + or a reddish brown, which in the deep sea has the same effect 2 O Decomposition 2 Deep sea as being black. A few deep-sea fishes are bright red, as are many deep-sea shrimps. As in mesopelagic animals, bioluminescence is very common in animals that live in the upper part of the deep sea. Deep-sea animals do not use bioluminescence for counterillumination, however, since there is no sunlight 2,000 m to create a silhouette. They have fewer photophores than midwater species, and the photophores are usually on the head and sides of the body rather than on the ventral FIGURE 16.18 Surface waters are rich in oxygen, because oxygen both enters from the surface. In the deep sea the primary uses of biolumines- atmosphere and is released by photosynthesis. In the mesopelagic zone neither the atmosphere cence are probably prey attraction, communication, and nor photosynthesis can contribute oxygen to the water, but there is extensive bacterial decom- courtship. Bioluminescence becomes less common in position of organic matter sinking from shallow water. This uses up oxygen and results in an oxygen minimum zone. Below the oxygen minimum zone, most of the organic matter has already the deeper parts of the deep sea, for reasons that are not decayed on its way down, and oxygen remains dissolved in the water. Additional oxygen is understood. brought in by the deep thermohaline circulation (see Figs. 3.24 and 3.25). The large eyes of midwater animals are not needed in the deep sea, where not even dim sunlight penetrates. The there is. They also tend to be relatively inactive, which lowers deep sea is not completely dark, however, because of bio- their oxygen consumption. Many also have complex biochemical luminescence. Many deep-sea animals have functional eyes, espe- adaptations, like hemoglobin that functions well at low oxygen cially in the upper parts of the deep sea, but the eyes are generally concentrations. small (Figs. 16.19 and 16.20). Animals from the deepest regions tend to have even smaller eyes or be blind altogether. Deep-sea fishes that are blind are not bioluminescent, one indication that the 16.2 THE WORLD main function of vision in the deep sea is to see bioluminescence. OF PERPETUAL DARKNESS Below the mesopelagic lies the little-known world of the deep The Lack of Food sea, where sunlight never penetrates. This alien environment is Deep-sea organisms may not have to adapt to variations in the vast, indeed. It is the largest habitat on Earth and contains about physical environment, but they face a continual shortage of food. 75% of our planet’s liquid water. The deep sea can be divided Very little, only about 5%, of the food produced in the photic zone into several pelagic depth zones. The bathypelagic zone includes makes it past all the hungry mouths in the waters above to reach depths between 1,000 and 4,000 m (3,300 and 13,000 ft), and the the deep sea. Deep-sea animals do not make vertical migrations cas23068_ch16_365-386.indd 376 6/29/15 8:07 PM Final PDF to printer CHAPTER 16 The Ocean Depths 377 Lure on dorsal Swim bladder have flabby, watery muscles, weak skel- spine or chin reduced or absent etons, no scales, and poorly developed Flabby muscles respiratory, circulatory, and nervous sys- tems. Nearly all lack functional swim blad- Small eyes ders. These fishes hang in the water column, expending as little energy as possible, until Large mouth a meal comes along. Most deep-sea fishes and teeth Lack of have huge mouths and often fearsome teeth streamlining (Fig. 16.21), can consume prey much larger than themselves. This trend reaches its peak in the swallowers (Saccopharynx) and gulp- ers (Eurypharynx; Fig. 16.22), which look Black or reddish-brown color like swimming mouths. To go along with their large mouths, many species have Relatively small size stomachs that can expand to accommodate FIGURE 16.19 Some typical characteristics of deep-sea pelagic fishes. Compare these with the adaptations the prey once it has been engulfed. shown in Figures 15.18 and 16.9. Deep-sea pelagic fishes are typically small and black, with small eyes, large mouths, expandable Mesopelagic (Gonostoma denudatum) Deep-sea (Gonostoma bathyphilum) stomachs, flabby muscles, weak bones, and poorly developed swim bladders. Bristlemouths Brain Brain and anglerfishes are the most common. Like their mesopelagic counterparts, bathypelagic anglerfishes have a first spine on their dorsal fin that is modified into a “pole” that dangles a bioluminescent bait to attract prey. In some species the bait even resembles a shrimp or a worm. In most spe- Gill Gill filaments cies only the females have a pole and bait. filaments Many other deep-sea fishes also attract prey with bioluminescent lures. FIGURE 16.20 Comparison of typical adaptations in mesopelagic and deep-sea pelagic fishes. Shown are Sex in the Deep Sea closely related bristlemouths from the mesopelagic (G. denudatum) and the deep sea (G. bathyphilum). The deep- sea fish has smaller eyes, less muscle, and fewer light organs. It also has less-developed nervous and circulatory Food is not the only thing that is scarce in systems, as indicated by the smaller brain and gill filaments. the deep sea. In such a vast, sparsely pop- ulated world, finding a mate can be difficult—even harder than to the rich surface waters, probably because the surface is too far finding food. After all, most deep-sea animals are adapted to away and the change in pressure too great. With food critically eat just about anything they can get, but a mate has to be both scarce, deep-sea animals are few and far between. the right species and the opposite sex! At least one deep-sea Deep-sea fishes, the most common of which are bristlemouths squid (Octopoteuthis) doesn’t worry about that. Male squids and anglerfishes (Fig. 16.22), are relatively small, generally 50 cm mate by implanting a sperm packet on the female. Male Octo- (20 in) or less, but on average they are larger than mesopelagic poteuthis take no chances, and implant sperm packets indis- fishes. It is somewhat surprising that deep-sea pelagic fishes tend criminately on both males and females, presumably getting it to be larger than mesopelagic ones, since there is even less food right half the time. available in the deep sea than in the mesopelagic. It is thought that deep-sea fishes put their energy into growth, reproducing slowly and late in life, while mesopelagic fishes spend less energy on Hemoglobin A blood protein that transports oxygen in many animals; growth and more on reproduction. In addition, vertically migrat- in vertebrates it is contained in erythrocytes (red blood cells). ing mesopelagic fishes expend a lot of energy in their migrations, 8.3, Biology of Fishes reducing the energy available for growth. Trenches Deep depressions in the sea floor that are formed when The energy-saving adaptations to food shortage seen in mid- two plates collide and one sinks below the other. water organisms are accentuated in the deep sea. Deep-sea fishes 2.2, The Origin and Structure of Ocean Basins; Figures 2.12 and 2.13 are even more sluggish and sedentary than midwater fishes. They cas23068_ch16_365-386.indd 377 6/29/15 8:07 PM Final PDF to printer 378 Part Three Structure and Function of Marine Ecosystems Many deep-sea fishes have solved the problem by becom- ing hermaphrodites. After all, it would accomplish nothing if two members of the same species finally got together but were both the same sex. If every individual can produce both eggs and sperm, the ability to breed is guaranteed. Deep-sea animals probably actively attract mates. Biolu- minescence, for example, could send a signal that draws other members of the same species.
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