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Home , Benthos, Bioluminescence, Hydrothermal vent, Mesopelagic zone, Saccopharynx, Saccopharynx ampullaceus

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376 Part Three Structure and Function of Marine

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 of the trenches, from below 6,000 m to Amount of dissolved just above the floor, as deep as 11,000 m (36,000 ft). Low High Each of the depth zones supports a distinct of , 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 . 2 n O Respiratio 2 The conditions of in the deep pelagic environment 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 are also remarkably uniform. CO Organic 2 matter The deep sea also includes the ocean bottom beyond + H O Respiration + the . Bottom-living 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 . Many animals, especially , are CO Organic 2 a drab gray or off-white. Deep-sea 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, 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 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 . In the 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 (see Figs. 3.24 and 3.25). The large 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 . Many deep-sea animals have functional eyes, espe- adaptations, like 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 on Earth and contains about physical environment, but they face a continual shortage of food. 75% of our planet’s water. The deep sea can be divided Very little, only about 5%, of the food produced in the 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

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CHAPTER 16 The Ocean Depths 377

Lure on dorsal 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 , 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 () 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 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 has smaller eyes, less muscle, and fewer 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 (Octopoteuthis) doesn’t worry about that. Male 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 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

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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. The lure of female anglerfishes differs among species, so it could have a role in attracting mates as well as prey. Chemical attraction is also important. Male ang- lerfishes have a very powerful sense of smell, which they use to locate females. The females apparently release a special chemical that the male can detect and follow. Such chemicals are called . Some anglerfishes (Cryptopsaras, Ceratias) have evolved an extreme solution to the problem of finding mates. When a male locates a female, who is much larger, he bites into her side, where he remains attached for the rest of his life (Fig. 16.22). In some species the male’s modified jaws fuse with the female’s tissue. Their cir- culatory systems join, and the female ends up nourishing the male. This arrangement, sometimes called male , ensures that the male is always available to fertilize the female’s eggs. Neither hermaphroditism nor male parasitism seem to be common in deep-sea . The mechanisms that bring FIGURE 16.21 The fearsome jaws and teeth of a deep-sea dragonfish, the males and females together, if any, are unknown. There is evi- scaleless black dragonfish (Melanostomias biseriatus). The long barbel below the chin dence that some groups aggregate to breed, perhaps attracted by has a lure with bioluminescent tissue. bioluminescent signals.

Finding mates, a problem for deep-sea animals, is eased by the use of bioluminescent and chemical signals and by the development of hermaphroditism and male parasitism. Deep-sea (Gigantactis macronema) 35 cm Living Under Pressure Under the weight of the overlying water Deep-sea devilfish column, the pressure in the deep sea is (Caulophryne pelagica) Deep-sea bristlemouth 15 cm (Gonostoma bathyphilum) tremendous. This is one reason so little is 20 cm known about the deep sea. The submers- ibles and equipment needed to study the deep sea must withstand the pressure with- out being crushed and are very expensive. Only a very few can ven- ture into the deepest trenches, where the Attached male Swallower pressure can exceed 1,000 atmospheres () Female deep-sea anglerfish 160 cm (14,700 psi). It is just as difficult to bring with attached male animals up from the deep sea as it is to (Cryptopsaras couesi) 45 cm go down to them. Unable to endure the enormous change in pressure, they usu- ally die when brought to the surface. A few scientists have succeeded in retriev- Gulper ing organisms from the deep sea in special (Eurypharynx pelecanoides) pressurized chambers. Much has been 100 cm learned from such work, but it is frustrat- FIGURE 16.22 Some deep-sea fishes and their approximate maximum length. ingly difficult.

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CHAPTER 16 The Ocean Depths 379

It is clear that pressure has important one such craft succeeded for the first time effects on deep-sea organisms. The absence in reaching the deepest part of the ocean. of a functional swim bladder in most deep- The Japanese-built Kaiko descended 10,991 sea fishes, for example, is probably due to m (36,061 ft) to the floor of the Challenger the high energetic cost of filling the bladder Deep in the . This broke with under extreme pressure. Along with the long-standing record of the submers- the availability of food, pressure seems to be ible , which in 1960 descended to a major factor causing zonation in deep-sea 10,919 m (35,810 ft), also in the Mariana pelagic organisms, that is, dividing the deep Trench. Completely autonomous submers- sea into bathypelagic, abyssopelagic, and ibles, which are pre-programmed to operate hadopelagic zones. FIGURE 16.23 The ROV (remotely operated independently of direct human control, are In shallow-water organisms the vehicle) Hercules. Controlled from the surface, Jason is also making contributions. used to take samples, install equipment, and take pho- that control , the vast tographs and videos to depths of 4,000 m (13,000 ft). set of chemical reactions that sustains life, are strongly affected by pressure, and would cease to function at Feeding in the Deep-Sea the pressures of the deep sea. Deep-sea organisms, however, have much more pressure- resistant enzymes. Some also have high con- Food shortage is critical on the floor of the deep sea. Very little centrations of a chemical that helps to stabilize the enzymes. Such of the surface production makes it all the way to the bottom. molecular adaptations enable deep-sea organisms to survive under Benthic organisms, however, have a major advantage over the pressures that would kill any surface-dweller. deep-sea pelagic ones above. In the water column, food par- Nonetheless, pressure probably limits the depth range of ticles that are not immediately located and eaten sink away and most organisms, and the number of species declines going are lost. Once food reaches the bottom, by contrast, it stays put deeper and deeper. The deepest known fish was recorded at a until it is found. Thus, although pelagic animals can get first depth of 8,370 m (27,460 ft). Invertebrates and , how- crack at food sinking out of the photic zone, benthic animals ever, have been found at the deepest depths. have more time to find and eat it. Food particles that reach the bottom tend to be those that sink fairly rapidly, minimizing the chance that they will be eaten on the way down. Fecal pellets, Hydrostatic pressure is great in the deep sea and partially controls the for example, are an important source of organic matter for the depth distribution of deep-sea organisms. Deep-sea organisms have molecular adaptations that allow their enzymes to function at high deep-sea benthos. pressure. Still, the rain of organic matter to the sea floor is actually more like a drizzle. Very little food is available to the benthic community. Furthermore, much of the material that reaches the sea floor, like the chitinous remains of zooplankton, is not immediately digestible. On the sea floor, however, bacte- 16.3 THE DEEP-OCEAN FLOOR ria decompose the chitin and become food for other organisms. The floor of the deep sea shares many characteristics with the pelagic Most of the deep-sea floor is covered in fine, muddy sedi- waters immediately above: the absence of sunlight, constant low ment. The meiofauna, tiny animals that live among the sediment temperature, and great hydrostatic pressure. Nevertheless, the bio- particles (see Box 13.2, “Life in Mud and Sand”), are the most logical communities of the deep-sea floor are very different from abundant organisms on the deep-sea floor, typically outnumber- pelagic communities because of one key factor: the presence of ing larger organisms by a factor of 10. The meiofauna graze on the bottom. and absorb dissolved organic matter (DOM) from the Marine biologists have learned a bit more about the benthos, water between the sediment grains. The energy in the bacteria and or bottom- inhabiting organisms, of the deep sea than about deep- DOM is thus made available to larger animals, or macrofauna, water pelagic communities, but much is still unknown. Of the that graze on the meiofauna. 270 million km2 (105 million mi2) of deep-sea floor, only about 2 2 500 m (5,400 ft ), the floor area of a large house, have been quan- Hermaphrodites Individuals with both male and female gonads. titatively sampled. What we do know has been learned using a 7.5, Molluscs: The Successful Soft Body variety of techniques. Devices called epibenthic sleds are dragged along the bottom, scooping up organisms, and corers bring a piece Enzymes Proteins that speed up and control chemical reactions in organisms. of the bottom to the surface. Deep-sea cameras photograph fast- Chitin Highly resistant carbohydrate found in the skeleton of swimming animals like fishes that are hard to catch in nets. Sci- and other structures. entists have even developed a miniature transmitter that can be 4.1, The Ingredients of Life hidden in bait and used to track the movements of a fish that swal- lows it. Deep submersibles like Alvin (see Fig. 1.10) have also Suspension Feeders Animals, including filter feeders, that eat particles suspended in the water column. been very useful, even more so than in the water column, as have Deposit Feeders Animals that eat organic matter that settles to the bottom. remotely operated vehicles, or ROVs (Fig. 16.23), which are used 7.1, ; Figure 7.4 to collect samples, take photos, and perform experiments. In 1995

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380 Part Three Structure and Function of Marine Ecosystems

Tr ipod fish (Bathypterois viridensis) Deep-sea spiny 20 cm (Notacanthus bonapartei) 20 cm Grenadier (Lionurus carapinus) 45 cm Eelpout (Lycenchelys jordani) 35 cm

Hagfish (Eptatretus stouti) Brotulid 50 cm (Holcomycteronus profundissimus) 30 cm

FIGURE 16.24 Some typical deep-sea bottom fishes and their approximate maximum length.

Suspension feeders are rare among the larger organisms in blind, these fishes sit on the bottom on their elongated fins (Fig. 16.24), the deep-sea benthos. Instead, deposit feeders dominate. Many facing into the current and snapping up passing . of these are infauna, burrowing in the sediments. Others, the The slow rain of food to the bottom is interrupted by an occa- epifauna, rest on the sediment surface. sional “storm.” Large pieces of food that sink rapidly, like the dead worms are usually the most abundant macrofauna bodies of large fishes or , are an important source of food on the deep-sea floor, followed by crustaceans and bivalve mol- to the benthos. Mobile deep-sea animals rapidly congregate around luscs, but there is considerable variation from place to place. Sea such “baitfalls” (Fig. 16.25). Among the most common of these are cucumbers are sometimes dominant, for example. Sea cucumbers crustaceans, especially amphipods, which arrive soon after the bait from the deep sea often have strange, highly modified body forms. touches down. Many deep-sea benthic amphipods are generalists that Some have leg-like and walk across the bottom in feed on and perhaps prey on live organisms if nothing else search of organic-rich sediment. “Herds” of some species have is available. Some, however, seem to specialize as . They been observed from submersibles. Other species can swim above apparently have a well- developed sense of smell, which probably the sea floor by undulating their bodies or squirting jets of water. helps them find new baitfalls. When caught in traps, these amphi- Some parts of the deep sea are dominated by brittle stars, and sea pods often have nothing in their guts except the bait, which indicates stars can also be abundant. that they have not fed for some time before. This, and the fact that they have an expandable gut, could mean that the amphipods are adapted The deep-sea benthos is dominated by deposit feeders. The dominant to capitalize on large but infrequent meals. groups are the meiofauna, polychaete While most deep-sea benthic animals are worms, crustaceans, bivalve molluscs, sea small, the deep-sea members of some groups cucumbers, brittle stars, and sea stars. are giants compared to their shallow-water relatives (Fig. 16.26). Most amphipods, isopods, and sea spiders are less than 1 cm There are predators in the deep-sea (0.4 in) in size, but deep-sea isopods can benthos, but they seem to be fairly rare. reach 36 cm (14 in) and amphipods 34 cm The main predators on deposit-feeding ani- (13 in) in length, and deep-sea sea spiders mals are probably sea stars, brittle stars, and can reach 80 cm (30 in) across. This phe- . Members of the , like fishes FIGURE 16.25 A variety of fishes quickly locate nomenon, known as deep-sea gigantism, is and squids, are also important predators. large pieces of food on the deep-sea floor. These cusk thought to be an adaptation for fast, efficient Sea spiders, or pycnogonids, prey on other (Genypterus blacodes), spiny dogfish sharks (Cen- locomotion so that the animals can cover a invertebrates by sucking out their soft parts. trophorus squamosus), cutthroat eels (Diastobranchus capensis), and snubnose eels (Simenchelys parasiticus) larger area in search of baitfalls. The giant Tripod fishes are another interesting have come to a large piece of fish placed at 1,500 m size also provides for more energy storage group of deep-sea benthic predators. Nearly (5,000 ft) on the sea floor near . between meals.

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CHAPTER 16 The Ocean Depths 381

search of the occasional bonanza. In several groups the parts of the brain devoted to olfaction are highly developed compared to close relatives from the mesopelagic, where visual areas dominate. Thus, deep-sea bottom fishes, like invertebrates, probably rely a lot on their sense of smell. At depths above 2,000 m, sharks can also show up at deep-sea bait falls, quickly putting an end to the feed.

The Nature of Life in the Deep-Sea Benthos There is a growing realization that life in the deep sea proceeds at a very different pace than at the surface. Most deep-sea animals seem to grow very slowly, probably because of the lack of food. On the other hand, they live for a long time. Deep-sea have been estimated to be 50, 60, or even 100 years old. Some fishes live even longer (see Box 16.2, “ in the Deep Sea”). Perhaps the FIGURE 16.26 The deep-sea isopod (Bathynomus giganteus) is related to low temperature and high pressure slow down the processes of life common garden pillbugs, but can grow to 36 cm (14 in) long. in the deep sea. It may also be that deep-sea animals need to live a long time Various fishes also find freshly placed bait quickly. The most to store up enough energy to reproduce. The larvae of deep-sea common are , or rattails, brotulas and cusk eels, deep-sea forms do not spend time in the food-rich photic zone. The chances spiny eels, and hagfishes (Fig. 16.24, also see Fig. 8.2). These bot- of making it all the way to the surface and then back to the deep-sea tom scavengers tend to be larger on average than deep-sea pelagic floor are simply too small. Instead, deep-sea animals tend to pro- fishes, relatively muscular, and active, unlike bathypelagic fishes duce large eggs, with enough yolk to see the larva through its early (Fig. 16.27). They are adapted for cruising along the bottom in stages without eating. It takes a lot more energy to produce a large

Mesopelagic Mesopelagic Deep-sea Epipelagic (vertical migrators) (non-migrators) Deep Pelagic bottom

Appearance

Size Wide size range Small Small Relatively small, larger Relatively large from tiny to huge than mesopelagic

Shape Streamlined Relatively elongated Relatively elongated No streamlining Very elongated and/or laterally and/or laterally often globular in compressed compressed shape

Musculature Strong muscles, fast Moderately strong Weak, flabby muscles Weak, flabby muscles Strong muscles swimming muscles

Eye Large eyes Very large, sensitive Very large, sensitive Eyes small, sometimes Small eyes characteristics eyes eyes, sometimes absent tubular eyes

Coloration Typical counter- Black or black with Black or black with Black or reddish brown, Dark brown or black shading: dark back silver sides and silver sides and occasionally bright red, and white or silver belly; counter- belly; counter- often lack coloration belly illumination illumination at greatest depths

Bioluminescence Bioluminescence Bioluminescence Bioluminescence Bioluminescence Only a few groups relatively common, often common, often common, often bioluminescent uncommon used for counter- used for counter- used to attract illumination illumination prey

FIGURE 16.27 Comparison of the typical characteristics of fishes from different depth zones in the pelagic realm.

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382 Part Three Structure and Function of Marine Ecosystems

Box 16.2 Biodiversity in the Deep Sea

ith its immense pressure, , which like the rest near-freezing waters, 400 of the deep sea have hardly been absence of sunlight, and Deep-sea benthos studied, also have very high diversity.

W ed chronic food shortage, the deep Samples from only 24 seamounts in sea seems like one of Earth’s least 300 the southwest Pacific near hospitable environments. Indeed, produced more than 850 species, in the nineteenth century the pio- and the species accumulation curve neering oceanographer William was still rising, indicating that many 200 Forbes (see “The Challenger Expedi- more species would have been found Shallow-water benthos tion,” in 1.1, The Science of Marine had more samples been taken. What Biology), observing that fewer and is more, about a third of the spe- 100

fewer organisms were caught in Number of species collect cies collected were new to science, nets as depth increased, hypoth- and probably restricted to small esized that no life at all existed chains or even individual below about 600 m (2,000 ft). seamounts. Seamounts seem to be This “azoic hypothesis” was quickly 0 20,000 40,000 60,000 80,000 100,000 deep-sea islands, each holding a col- proven wrong when other scien- Number of individuals collected lection of organisms found nowhere tists caught animals from greater else. depths. Nevertheless, animal abun- Species accumulation curves from the deep-sea and shallow-water benthos off With some 30,000 seamounts in the of New England. dance does decrease with depth— the deep sea, nearly all unexplored, animals are scarce in the deep sea. there must be vast numbers of sea- It was once thought that in addition to to level off near the total number of species that mount species waiting to be discovered, but many low there were relatively few ani- live in the environment. In highly diverse , may go extinct before that happens. Seamounts mal species in the deep sea. Just the opposite ones with many species, the number of species are home to the ( Hoplostethus is true: The deep sea could be the most biologi- increases rapidly as more individuals are collected atlanticus) and several other commercially valu- cally diverse environment on Earth. Biologists are and takes longer to level off. able fishes. These fishes live for more than a unable to collect and identify every species pres- When this was done on the deep-sea floor, hundred years, but take decades to reach repro- ent in most environments, so they never know the species curve went through the roof, - ductive age and reproduce very slowly, so they the exact number of species there. They can ing the researchers to estimate that certainly are vulnerable to (see “Optimal Yields estimate species numbers, however, from “spe- more than 1 million, and possibly more than and Overfishing,” in 17.1, The Living Resources cies accumulation curves.” This involves analyzing 10 million, species live on the deep-sea floor, of the Sea). Numerous newly discovered popu- how often they find new species as they collect making it as or more diverse than rain forests lations on seamounts in the , for more and more individual organisms. The first and reefs. This was little short of aston- example, were completely fished out less than collected will always be a species new ishing because up until then most biologists three years after their exploitation began. Even to the study. The second organism could be had thought that no more than a few hundred worse, the bottom trawls used to catch these from the same species, leaving the total number thousand species lived in the entire ocean! fish rip animals from the bottom, reducing the of species collected at one, or it might be a new The estimate of 10 million species on abundance of benthic organisms by 80% or more. species, bringing the species total to two. If the deep-sea floor has been controversial, in part With many seamounts located on the “high ,” number of species in the habitat—one measure because it is extrapolated from samples of only that is, outside the jurisdiction of any country, of its biodiversity—is small, then as more and a tiny fraction of the sea floor. Whatever the protecting the rich biodiversity of seamounts more individuals are collected the number of new true number, it is clear that the deep-sea floor will require an international effort to control the species increases relatively slowly and soon starts is among the planet’s most diverse habitats. damage inflicted by seamount .

egg than a small one, so deep-sea animals produce only a few indication of how much there is to learn. As the crew of Alvin was eggs. In at least some animals, reproduction could be tied to feed- preparing for a dive the was swamped by a wave, its ing. In some species of amphipods, individuals caught in baited supporting cable snapped, and Alvin, hatch open, dropped 1,540 m traps are all sexually immature. Biologists have speculated that (5,000 ft) to the bottom. The crew escaped but left their lunch on they do not reproduce until they manage to find a good meal. board. It was to become the most famous lunch in . When Alvin was recovered 10 months later, scientists discov- ered that the long-lost lunch was in amazingly good condition. Microbes in the Deep Sea Though soggy, the sandwiches looked almost fresh; the bologna Bacteria, , and play many important roles in was still pink inside. The rest of the lunch—apples and a thermos of deep-sea benthic ecosystems that we are only beginning to under- soup—also looked good enough to eat. Once brought to the surface stand. In 1968 a famous unplanned “experiment” gave an early the food soon spoiled, even though it was refrigerated.

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CHAPTER 16 The Ocean Depths 383

Box 16.3 Alvin Reborn

he research submersible Alvin (see for Alvin had to be scrapped. Instead, Alvin improved was Alvin’s launch system and stor- Fig. 1.10) has played a central role in was transformed through her largest upgrade age in its support vessel, the research vessel Tsome of the greatest discoveries in deep- ever, which was accomplished in two phases. Atlantis. Alvin made her last dive before her sea biology, but she wasn’t getting any younger. The first phase replaced the heart of Alvin, the rebuild late in 2010, and the reborn Alvin was Launched in 1964, and with more than 4,600 dives titanium sphere that houses the crew. The launched in 2014. under her belt, Alvin has been a starring tool for new sphere is capable of depths to 6,500 m Even the new Alvin won’t be able to go deep-sea science for nearly five decades. Over the (21,300 ft). Alvin will eventually have access to the very deepest trenches. Some ROVs, years Alvin has often had facelifts and upgrades, to 98% of the ocean floor. The new sphere as well as “landers,” equipment packages and not much of the original sub remains. Even has five instead of the original three win- that are simply dropped to the bottom and so, moving into the 21st century Alvin seemed dows. The new windows are larger to further retrieved, can do so now. In addition, private to have reached the end of her working life, and increase visibility. The interior is being rede- groups have developed glider-like submers- her depth limit of 4,500 m (14,800 ft) meant signed to be more comfortable—or, perhaps ibles that will be able to “fly” to the deepest that much of the deep ocean was out of reach. more accurately, less uncomfortable—during parts of the ocean to help reveal the secrets In 2004, a plan was developed to retire Alvin and the long descents into the abyss. Other of the least-explored places on Earth. build her replacement. improvements in Phase 1 included better Escalating costs and tight research fund- and high-definition camera systems For more information, explore the links provided on ing meant that the plan for a new replacement as well as an improved control system. Also the Marine Biology Online Learning Center.

Why was the food preserved in the deep sea? Though the deep sediment, causing their cells to burst and release large amounts sea is cold, so is a refrigerator. Are bacteria absent from the deep sea? of (DOC). Other microbes take up the Does the pressure somehow inhibit bacterial decay? Is there some DOC as an energy source, so the viruses promote rapid carbon other explanation? These questions sparked a flurry of research. recycling, as well as recycling of nutrients like nitrogen and It is now known that bacteria do live in huge numbers in the phosphorus. It thus appears that viruses play a major role in the deep sea, as in every other environment on Earth. Pressure and deep-sea . cold slows bacterial growth, however, and most shallow-water Deep-sea sediments also contain large numbers of bacteria cannot grow at the pressure and temperature of the deep chemo synthetic , which are probably an important sea. Therefore, the bacteria that were already in the lunch probably food source for deposit feeders. They could be involved in the died when Alvin sank. formation of polymetallic nodules and other mineral deposits (see Even if the surface bacteria died, there are still plenty of bac- “Ocean Mining,” in 17.2, Non-Living Resources from the Sea teria already living in the deep sea. Deep-sea bacteria don’t just Floor). Chemosynthetic bacteria are even gradually digesting the tolerate the prevailing cold and high pressure, many of them grow wreck of the Titanic! best or even only under these conditions. Even though these cold- loving (psychrophilic) and pressure-loving (barophilic) bacteria grow best under deep-sea conditions, they still grow slower than 16.4 HOT SPRINGS, COLD SEEPS, shallow-water bacteria and can take up to 1,000 times longer to decompose organic matter. It has also been suggested that deep- AND DEAD BODIES sea bacteria are adapted to use organic matter in low concentra- The year 1977 marked one of the most exciting discoveries in tions, and are overloaded by such unusually rich fare as a bologna the history of biology, and it wasn’t even made by biologists! A sandwich. Furthermore, the lunchbox could have prevented many group of marine geologists and chemists was using Alvin to look bacteria, like those that live on or in amphipods and other deep-sea for hydrothermal vents on a section of mid-ocean ridge near the animals, from getting to the food in the first place. Vast numbers of microbes live in deep-sea sediments. Until the last few years it was thought that bacteria dominate, but Chemosynthetic Prokaryotes Autotrophic bacteria and archaea that new surveys show that archaea are more abundant in deep-sea use energy contained in inorganic chemicals rather than sunlight to make sediments. The earlier studies used techniques that are not well- organic matter. suited for detecting archaea. Other studies indicate that viruses 5.2, Prokaryotes; Table 5.1 are a key driver of the microbial in deep-sea sedi- Hydrothermal Vents Undersea hot springs associated with mid-ocean ridges. ments, where they are extremely abundant. Researchers estimate 2.3, The Geologic Provinces of the Ocean that viruses kill some 80% of the bacteria and archaea in the

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384 Part Three Structure and Function of Marine Ecosystems

Galápagos Islands in the eastern Pacific. When the scientists found the vents, they also found something completely unex- pected: a rich, flourishing community unlike anything ever imagined (Fig. 16.28). In the days that followed, more vents were found, each teeming with animals. There were gigantic worms up to 1 m (3.3 ft) long, 30-cm (12-in) clams, dense clusters of mus- sels, shrimps, crabs, fishes, and a variety of other unexpected life. The vents were like oases of life on the barren deep-sea floor. Expeditions were soon mounted to hydrothermal areas around the world. With every dive, it seems, come new discoveries. Over 400 new animal species—and counting—have been found in hydrothermal- FIGURE 16.28 Rich, colorful animal communities live at many hydrothermal vents. This community at vent communities, which vary considerably a depth of over 2,500 m (8,200 ft) on the includes giant tube worms (Riftia), zoarcid fish, or eelpouts, and crabs (Bythograea). Image courtesy of Richard A. Lutz. from place to place. Vestimentiferan tube- worms, clams, crabs, and shrimps typically dominate the eastern Pacific vent communi- ties that were first discovered. Unusual and bar- nacles are most common at vents in the western Pacific, while vents on the Mid-Atlantic Ridge are typically dominated by a shrimp (Rimicaris). With no sunlight to support photosynthesis, the primary Plume producers in hydrothermal-vent communities are chemo- synthetic archaea and bacteria. Around the mid-ocean ridge, trickles down through cracks and fissures in the earth’s crust and is heated to very high temperatures. Rich Heart in minerals, it emerges at hydrothermal vents to form “black smokers,” “chimneys,” and other mineral deposits. Feeding body The hot water also contains large amounts of hydrogen Dorsal blood sulfide (H2S), which is toxic to most organisms but is an Tube vessel energy-rich molecule. Chemosynthetic prokaryotes that use the energy in and sulfide minerals to Capillary Ventral blood make inorganic matter are the base of the . Some vessel Body cavity of these microbes are that can live at tempera- () tures over 120 °C (250 °F), the highest temperature at which Trophosome life is known to occur (see “Archaea,” in 5.2, Prokaryotes). (“feeding body”) New studies are expected to shatter this record.

Capillary Deep-sea hydrothermal vents harbor rich communities. Sulfide, Organic The that supports these communities CO2 matter comes from microbial , not photosynthesis. Body cavity (coelom) Bacteria Water near the vents contains so many microbes that Tube they cloud the water. Some vent animals feed by filtering them from the water, but this isn’t the principal mode of feeding. One of the dominant tubeworms in the eastern Pacific vent communities (Riftia) does not filter-feed. In fact, it doesn’t even have a mouth or digestive tract! Instead, these worms have a highly specialized organ FIGURE 16.29 The anatomy of the giant hydrothermal-vent tubeworm (). called a trophosome (“feeding body”; Fig. 16.29), packed The plume at the end acts like a gill, except that it exchanges hydrogen sulfide (H2S) as well as (CO2) and oxygen (O2). The carbon dioxide and sulfide are carried in the blood to with symbiotic bacteria. The bacteria perform chemo- the feeding body, where symbiotic bacteria use them to make organic matter by chemosynthesis. synthesis inside the worm’s body and pass much of the

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CHAPTER 16 The Ocean Depths 385

organic matter they produce on to the worm. The worm, in turn, Chemosynthetic communities aren’t confined to hydrother- supplies the bacteria with raw materials. The bright-red plume acts mal areas. Cold seeps are places, mostly along continental mar- like a gill, exchanging not only carbon dioxide and oxygen, but gins or in sediment-rich basins like the Gulf of , where also hydrogen sulfide. The tubeworm’s blood has special hemo- hydrogen sulfide and (CH4) produced by the decay of globin that chemically binds the hydrogen sulfide, protecting the organic matter seep out from the sea floor. Primary production by worm from its toxic effects, and transports it to the bacteria in the chemosynthetic prokaryotes that can use one or the other of these feeding body. Most other vent animals chemically process hydro- energy-rich molecules supports communities that are similar in gen sulfide to avoid being poisoned. many ways to those at hydrothermal vents, though the individual Numerous other vent animals, including (Bathymo- species are mostly different. diolus) and large clams (Calyptogena), contain symbiotic bacteria, Communities based on chemosynthesis have even been found though they can filter-feed as well. Non-symbiotic microbes are on deep-sea “graves.” As previously discussed, occasional bait- also an important food source. For example, the shrimp (Rimica- falls like dead whales are an important source of food for deep-sea ris) that dominates vent communities on the Mid-Atlantic Ridge scavengers. When the scavengers are through, the decomposing scrapes off and eats bits of microbe-covered mineral from the remains produce hydrogen sulfide and methane, supporting a chimneys that form at vents (see Fig. 2.26). The microbes are community similar to those at vents and seeps. digested and the mineral eliminated. Unlike most deep-sea organisms, those at hot springs, cold Rimicaris do not have recognizable eyes, but they do have seeps, and dead bodies enjoy an energy-rich environment and grow two light-sensitive patches on their upper surface. The patches can fast and large. On the other hand, their specialized habitats are tiny detect much fainter light than humans can see. Before the discov- oases separated by vast distances. These oases are also unreliable. ery that these shrimp could “see,” no one suspected there was any Hot springs and cold seeps can be wiped out by volcanic eruptions, light at all at deep-sea vents. A special low-light camera similar undersea landslides, and other disturbances. As long as the flow of to those used to study distant stars revealed a faint glow, invis- energy-rich molecules continues the communities recover fairly ible to the human , around the vents. The source of the glow is quickly as new larvae recolonize the vent (see Box 3.2, “Larval uncertain, although some of it can be explained by the heat of the Transport Near Hydrothermal Vents”) but sometimes the flow is emerging water. Biologists speculate that the shrimp use this dim blocked or dries up. In 2002 scientists returned to the Rose Garden, light both to locate active vents and to avoid getting cooked by a near the Galápagos Islands first discovered in coming too close to the scalding water. 1979 and revisited several times since, to find no trace of the rich Boiling-hot black smokers are not the only hydrothermal hab- vent community—only fresh lava flows. To avoid extinction, vent itat in the deep sea. “White smokers” occur a little further away and seep species must be able to disperse to new oases, and indeed from the active and are cooler, so that black sulfide miner- they do. Only 200 m (660 ft) from the Rose Garden site scientists als precipitate out before the water emerges from the sea floor. discovered a new vent, dubbed “Rosebud,” already colonized by White minerals remaining in the water form the “smoke.” “Snow- juvenile tubeworms, clams, and mussels. The communities that blower vents” form where chemosynthetic bacteria living in rocks recolonize new vents or areas wiped out by an eruption, however, beneath the sea floor produce filaments of that coalesce can be different from those before the eruption, with species that into snowflake-like particles when they emerge from the vent into were common becoming rare and vice versa, and vents can be colo- cold water. Cooler vents at greater distance from the mid-ocean nized by species from hundreds of kilometers away. ridge or associated with and trenches (see “The The carcasses of large organisms quickly rot away, so the spe- Mid-Ocean Ridge and Hydrothermal Vents,” in 2.3, The Geo- cies they support must also “island hop.” The remains of whales, logical Provinces of the Ocean) support snails, sponges, deep-sea seals, sharks, and other large organisms are particularly tiny habi- , and other organisms, but not in the abundance seen at mid- tats in the expanse of the deep sea, but compared to hot springs ocean ridge vents. Thick mats of chemosynthetic archaea and bac- and cold seeps there are a lot of them. Or at least there were. By teria also grow in some of these areas, using carbonate minerals hunting the giants of the sea to the brink of extinction, we may rather than sulfide minerals as an energy source. “Blue smokers” have already removed these stepping stones and doomed an entire have also been discovered. ecosystem that we have only just discovered.

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