Download Chapter 13: PELAGIC COMMUNITIES

Chapter 13: PELAGIC COMMUNITIES
Answer to End of Chapter Study Questions
1. What are plankton? How are plankton collected? How are zooplankton different from
phytoplankton?
Plankton drift or swim weakly, going where the ocean goes, unable to move consistently
against waves or current flow. The plankton contain many different plant-like species and
virtually every major group of animals. The term is not a collective natural category like
mollusks or algae, which would imply an ancestral relationship between the organisms; instead
it describes a basic ecological connection. Members of the plankton community, informally
referred to as plankters, can and do interact with one another—there is grazing, predation,
parasitism, and competition among members of this dynamic group.
Plankton are usually collected in fine-mesh nets customarily made of nylon or Dacron
cloth woven in a fine interlocking pattern to assure consistent spacing between threads. The
net is hauled slowly for a known distance behind a ship, or cast to a set depth, then reeled in.
Trapped organisms are flushed to the net's pointed end and carefully removed for analysis.
Quantitative analysis of plankton requires the organisms and an estimate of the sampled
volume of water. Very small plankton can slip through a plankton net. Their capture and study
requires concentration by centrifuge, or entrapment by a plankton filter through which water is
drawn. The filter is later disassembled and the plankton studied in place.
Autotrophic plankton is generally called phytoplankton, a term derived from the Greek
word meaning plant. A huge, nearly invisible mass of phytoplankton drifts within the sunlight
surface layer of the world ocean. Phytoplankton is critical to all life on Earth because of its
great contribution to food webs and its generation of large amounts of atmospheric oxygen
through photosynthesis. Heterotrophic plankton—the planktonic organisms that eat these
primary producers—is collectively called zooplankton. Zooplankters are the most numerous
primary consumers of the ocean. They graze on the diatoms, dinoflagellates, and other
phytoplankton at the bottom of the trophic pyramid in a way analogous to cows grazing on
grass. The variety of zooplankton is surprising; nearly every major animal group is
represented.
2. Describe the most important phytoplankters. Which are most efficient in converting solar
energy to energy in chemical bonds? By what means is this conversion achieved?
The dominant photosynthetic organisms in the plankton—and in the world—are the
diatoms. More than 5,600 species of diatoms are known to exist. The larger species are barely
visible to the unaided eye. Most are round, but some are elongated or branched or triangular.
Typical diatoms are shown in Figure 13.5 – a truly beautiful photomicrograph. Diatoms are
encased in a "shell" (actually a frustule) that consists of silica (SiO2), giving this beautiful
covering the optical, physical, and chemical characteristics of glass—an ideal protective
window for a photosynthesizer. Inside the frustule lies the most nearly perfect photosynthetic
machine yet to evolve on the planet. Fully 55 percent of the energy of sunlight absorbed by a
diatom can be converted into the energy of carbohydrate chemical bonds, one of the most
efficient energy conversion rates known. Excess oxygen not needed in the cell's respiration is
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released through the perforations in the frustule into the water. Some oxygen is absorbed by
marine animals, some is incorporated into bottom sediments, and some diffuses into the
atmosphere. Most of the oxygen we breathe has moved recently through the many glistening
pores of diatoms.
Dinoflagellates are the second most important phytoplankters. Like diatoms, they are
single-celled autotrophs. A few species live within the tissues of other organisms, but the great
majority of dinoflagellates live free in the water. Most have two whip-like projections called
flagella in channels grooved in their protective outer covering of cellulose (see Figure 13.6a).
One flagellum drives the organism forward while the other causes it to rotate in the water.
Their flagella allow dinoflagellates to adjust orientation and vertical position to make the best
photosynthetic use of available light. Dinoflagellates are responsible for red tides. A red tide
can be dangerous because some dinoflagellate species synthesize potent toxins as
byproducts of metabolism.
Coccolithophores. Most other types of phytoplankton are extraordinarily small, and so
are called nanoplankton. The coccolithophores, for example, are tiny single cells covered with
discs of calcium carbonate (coccoliths) fixed to the outside of their cell walls (see Figure
13.7b). Coccolithophores live near the ocean surface in brightly lighted areas. Coccoliths can
build seabed deposits of ooze. The famous White Cliffs of Dover in England consist largely of
fossil coccolith ooze deposits uplifted by geological forces.
Each of these autotrophs fixes carbon into glucose by photosynthesis.
3. Where in the ocean is plankton productivity the greatest? Why?
Because of coastal upwelling and land runoff, nutrient levels are highest near the
continents. Plankton is most abundant there, and productivity highest. The water above some
continental shelves sustains productivity in excess of 1 g C/m2/day!
Where in the open ocean is plankton productivity the highest? With the tropics generally
out of the running for reasons of nutrient deficiency and the north polar ocean suffering from
slow nutrient turnover and low illumination, the overall productivity prize is left to the temperate
and southern subpolar zones. Thanks to the dependable light and the moderate nutrient
supply, annual production in the nearshore temperate and southern subpolar ocean areas is
the greatest of any open ocean area. Typical productivity in the temperate zone is about 120 g
C/m2/yr. In ideal conditions, southern subpolar productivity can approach 250 g C/m 2/yr!
4. Describe five nektonic organisms that are not fishes.
Nektonic organisms are active swimmers (in contrast to the largely passive, drifting
nature of plankton). Nekton include squid, nautiluses, sea turtles, whales, sea lions, seals,
penguins, the occasional polar bear, and even occasional human surfers and divers.
5. What are the major categories of fishes? What problems have fishes overcome to be
successful in the pelagic world?
Familiar fishes are divided into two major groups—the cartilaginous fishes and the bony
fishes—based on the material forming their skeletons. All members of the Class
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Chondrichthyes—the group that includes sharks, skates, rays, and chimaeras—have a
skeleton made of a tough, elastic tissue called cartilage. Though there is some calcification in
the cartilaginous skeleton, true bone is entirely absent from this group. These fish have jaws
with teeth, paired fins, and often active lifestyles. Sharks and rays tend to be larger than bony
fishes, and except for some whales, sharks are the largest living vertebrates. The bony fishes,
members of Class Osteichthyes, owe much of their great success to the hard, strong,
lightweight skeleton that supports them. These most numerous of fish—and most numerous,
most diverse, and successful of all vertebrates—are found in almost every marine habitat from
tidepools to the abyssal depths. Their numbers include the air breathing lungfishes and lobefinned coelacanths, whose ancient relatives broke from the path of fish evolution to establish
the dynasties of land vertebrates.
Seawater may seem to be an ideal habitat, but living in it does present difficulties.
These most successful vertebrates have structures and behaviors to cope. Among them are
adaptations of movement, shape, and propulsion. Active fish usually have streamlined shapes
that make their propulsive efforts more effective. A fish's resistance to movement, or drag, is
determined by frontal area, body contour, and surface texture. A fish's forward thrust comes
from the combined effort of body and fins. Muscles within slender flexible fish (such as eels)
cause the body to undulate in S-shaped waves that pass down the body from head to tail in a
snake-like motion. Advanced fishes have a relatively inflexible body, which undulates rapidly
through a shorter distance, and a hinged scythe-like tail to couple muscular energy to the
water. Maintenance of level is crucial to any swimming animal. The density of fish tissue is
typically greater than that of the surrounding water, so fishes will sink unless their weight is
offset by propulsive forces or by buoyant gas- or fat-filled bladders. Cartilaginous fishes have
no swim bladders and must swim continuously to maintain their position in the water column.
Bony fishes that appear to hover motionless in the water usually have well-developed swim
bladders just below their spinal columns. The volume of gas in these structures provides
enough buoyancy to offset the animal's weight. Gas exchange is also important: How can fish
breathe underwater? Gas exchange, the process of bringing oxygen into the body and
eliminating carbon dioxide (CO2), is essential to all animals. Fish take in water containing
dissolved oxygen at the mouth, pump it past fine gill membranes, and exhaust it through rearfacing slots. The higher concentration of free oxygen dissolved in the water causes oxygen to
diffuse through the gill membranes into the animal; the higher concentration of CO2 dissolved
in the blood causes CO2 to diffuse through the gill membranes to the outside. The gill
membranes themselves are arranged in thin filaments and plates efficiently packaged into a
very small space. Feeding and defense is also critical to success. Competitive pressure among
the large number of fish species has caused a wonderful variety of feeding and defense tactics
to evolve. Sight is very important to most fishes, enabling them to see their prey or avoid being
eaten. Even some deep-water fishes that live below the photic zone have excellent eyesight
for seeing luminous cues from potential mates or meals. Hearing is also well developed, as is
the ability to detect low-frequency vibrations with the lateral-line system. More subtle means of
offense or defense depend on trickery—looking like something you're not, or changing color to
blend with the background. These kinds of cryptic coloration or camouflage may be active or
passive. Schooling behavior is also useful—about a quarter of all bony fish species exhibit
schooling behavior at some time during their life cycle. I can personally attest to the
effectiveness of schooling as a means of defense. On a few diving trips, I've noticed a large
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moving mass just beyond the limit of clear visibility. Is it a fish school, or is it a single large
animal? Many predators might not stick around long enough to find out!
6. How does a marine bird differ from a terrestrial bird—a pigeon, for example?
Truly marine birds are typically fairly large animals—there are no sparrow-sized pelagic
seabirds. If they fly, they have very long, thin, pointed, cupped wings. (Penguins, which "fly"
through water, have a small wing hydrodynamically adapted to a different fluid than air.) The
legs of marine birds are usually farther back on the body than the legs of a terrestrial bird, so
they stand up straighter than, say, a chicken or pigeon. The eggs of seabirds tend to be more
sharply conical, presumably so they'll roll in a smaller diameter circle if they fall out of the nest,
not a bad adaptation for an animal whose nests are often built in confined areas on the edges
of cliffs. Marine birds often have salt glands in their heads, tissue able to extract the excess
salt from the blood that results from drinking seawater. Their sense of smell is often highly
developed. Marine birds have exceedingly lightweight skeletons and other weight-saving
adaptations that enable them to spend very long times aloft at sea with relatively little energy
output. Many, like the albatrosses, also have highly developed homing systems that enable
them to return to the nesting site after months or years at sea.
7. What characteristics are shared by all marine mammals?
All marine mammals share four common features: (1) They have a streamlined body
shape with limbs adapted for swimming that makes an aquatic lifestyle possible. Thin, stiff
flippers and tail flukes situated at the rear of the animal drive it forward, and similarly shaped
forelimbs act as rudders for directional control. Drag is reduced by a slippery skin or hair
covering. (2) They generate internal body heat from a high metabolic rate, and conserve this
heat with layers of insulating fat and, in some cases, fur. Their large size gives them a
favorable surface-to-volume ratio—with less surface area per unit of volume they lose less
heat through the skin. This is why there are no marine mammals smaller than a sea otter; a
small mammal would lose body heat too rapidly. (3) Their respiratory systems are modified to
collect and retain large quantities of oxygen. The air duct "plumbing" of marine mammals is
typically much different from that of land mammals, and the lungs can be more thoroughly
emptied before drawing a fresh breath. (4) They share a number of osmotic adaptations that
free them from any requirement for fresh water. Unlike other marine vertebrates, the marine
mammals do not have salt-excreting glands or tissues. They swallow little water during feeding
(or at any other time), and their skin is impervious to water. This minimal seawater intake,
coupled with their kidneys' abilities to excrete a concentrated and highly saline urine, permit
them to meet their water needs with the metabolic water derived from the oxidation of food.
8. Compare and contrast the groups of living marine mammals (use the technical classification
terms).
The three living groups of marine mammals are the porpoises, dolphins, and whales of
Order Cetacea; the seals, sea lions, walruses, and sea otters of Order Carnivora; and the
manatees and dugongs of Order Sirenia.
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The 90+ living species of cetaceans are thought to have evolved from an early line of
ungulates—hoofed land mammals related to today's horses and sheep—whose descendants
spent more and more time in productive shallow waters searching for food. Modern whales
range in size from 1.8 meters (6 feet) to 33 meters (110 feet) in length and weigh up to
100,000 kilograms (110 tons). Modern cetaceans are further divided into two suborders.
Suborder Odontoceti, the toothed whales, are active predators and possess teeth to subdue
their prey. Suborder Mysticeti, the whalebone or baleen whales, have no teeth. Filter feeders
rather than active predators, these whales subsist primarily on krill, a relatively large shrimplike crustacean zooplankter obtained in productive polar or sub-polar waters. Figure 13.21
shows representative whales in each division.
The Order Carnivora includes of land predators ranging from dogs and cats to bears
and weasels, but the members of the carnivoran suborder Pinnipedia—the seals, sea lions,
and walruses—are almost exclusively marine. Unlike the cetaceans, the gregarious pinnipeds
leave the ocean for varying periods of time to mate and raise their young. The Order Carnivora
also includes sea otters and polar bears. Because they spend large amounts of time on sea
ice, polar bears may be considered marine mammals.
The bulky, lethargic, small-brained dugongs and manatees, collectively called sirenians,
are the only herbivorous marine mammals. Like the cetaceans, they appear to have evolved
from the same ancestors as modern ungulates. They make their living grazing on sea grasses,
marine algae, and estuarine plants in coastal temperate and tropical waters of North America,
Asia, and Africa.
9. How are odontocete (toothed) whales different from mysticete (baleen) whales? Which are
the better known and studied? Why?
Whales of the suborder Odontoceti, the toothed whales, are active predators and
possess teeth to subdue their prey. Toothed whales have a high brain-weight-to-body-weight
ratio, and though much of their "extra" brain tissue is involved in formulating and receiving the
sounds on which they depend for feeding and socializing, many researchers believe them to
be quite intelligent. Smaller whales in this group include the killer whale and the familiar
dolphins and porpoises of oceanarium shows. The largest toothed whale is the 18 meter (60
foot) sperm whale, which can dive to at least 1,140 meters (3,740 feet) in search of the large
squids that provide much of its diet. They search for prey using echolocation, the biological
equivalent of sonar; they generate sharp clicks and other sounds that bounce off prey species
and return to be recognized. Odontocete whales are now thought to use sound offensively as
well.
Whales of the suborder Mysticeti, the whalebone or baleen whales, have no teeth.
Filter feeders rather than active predators, these whales subsist primarily on krill, a relatively
large shrimp-like crustacean zooplankter obtained in productive polar or sub-polar waters.
They do not dive deep, but commonly feed a few meters below the surface. Their mouths
contain interleaving triangular plates of bristly horn-like baleen (see Figure 13.26) used to filter
the zooplankton from great mouthfuls of water. The plankton is concentrated as water is
expelled, swept from the baleen plates by the whale's tongue, compressed to wring out as
much seawater as possible, and swallowed through a throat not much larger in diameter than
a grapefruit. A great blue whale, largest of all animals, requires about 3 metric tons (13,200
pounds) of krill each day during the feeding season. The short, efficient food chain from
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phytoplankton to zooplankton to whale provides the vast quantity of food required for their
survival.
10. Without looking ahead to Chapter 15, can you list some of the influences human activities
have had on pelagic communities?
The list is considerable. First, think about direct capture. As you read about fishes in this
chapter, you may have wondered how many are caught for human use. Nearly 140 million
metric tons of fishes were caught in 2007, most of them pelagic organisms. For each human
killed annually by sharks, humans now kill more than 1 million sharks. Most of the recent
increase in shark capture is related to the now-discredited idea that consuming shark cartilage
will somehow prevent or cure human cancers.
Next, consider the chemical and mechanical effects of oil spills, plastic floating on the
ocean (which if sometimes confused for food by pelagic organisms), and sewage outfalls.
Then think about indirect interference. Just one example: Stratospheric ozone layer
depletion (about which more will be said in Chapter 15) allows more ultraviolet radiation to
reach the surface of the Earth. Plankton in the uppermost 2 meters of ocean have been
affected. Recent research indicates an alarming decrease of phytoplankton productivity in the
coastal waters around Antarctica.
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