6.06.2016

Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea.

Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows), or both may be observed outside the cell envelope, and a spore may sometimes be seen within the cell.
A typical prokaryotic cell.
Figure.1: A typical prokaryotic cell.

Cell Membrane

Capsule Ribosomes Cytoplasm Enclosing the cytoplasm of a prokaryotic cell is the cell membrane (also known as the plasma, cytoplasmic, or cellular membrane). This membrane is similar in structure and function to the eukaryotic cell membrane. Chemically, the cell membrane consists of proteins and phospholipids. Being selectively permeable, the membrane controls which substances may enter or leave the cell. It is flexible and so thin that it cannot be seen with a compound light microscope. However, it is frequently observed in transmission electron micrographs of bacteria.

Many enzymes are attached to the cell membrane, and various metabolic reactions take place there. Some scientists believe that inward foldings of the cell membranes—called mesosomes—are where cellular respiration takes place in bacteria. This process is similar to that which occurs in the mitochondria of eukaryotic cells, in which nutrients are broken down to produce energy in the form of ATP molecules. On the other hand, some scientists think that mesosomes are nothing more than artifacts created during the processing of bacterial cells for electron microscopy.

In cyanobacteria and other photosynthetic bacteria (bacteria that convert light energy into chemical energy), infoldings of the cell membrane contain chlorophyll and other pigments that serve to trap light energy for photosynthesis. However, prokaryotic cells do not have complex internal membrane systems similar to the ER and Golgi complex of eukaryotic cells. Prokaryotic cells do not contain any membrane-bound organelles or vesicles.

Chromosome 

The prokaryotic chromosome usually consists of a single, long, supercoiled, circular DNA molecule, which serves as the control center of the bacterial cell. It is capable of duplicating itself, guiding cell division, and directing cellular activities. A prokaryotic cell contains neither nucleoplasm nor a nuclear membrane. The chromosome is suspended or embedded in the cytoplasm. The DNA-occupied space within a bacterial cell is sometimes referred to as the bacterial nucleoid.

The thin and tightly folded chromosome of E. coli is about 1.5 mm (1,500 μm) long and only 2 nm wide. Because a typical E. coli cell is about 2 to 3 μm long, its chromosome is approximately 500 to 750 times longer than the cell itself—quite a packaging feat! Bacterial chromosomes contain between 575 and 55,000 genes, depending on the species. Each gene codes for one or more gene products (enzymes, other proteins, and rRNA and tRNA molecules). In comparison, the chromosomes within a human cell contain between 20,000 and 25,000 genes.

Small, circular molecules of double-stranded DNA that are not part of the chromosome (referred to as extra-chromosomal DNA or plasmids) may also be present in the cytoplasm of prokaryotic cells (Fig. 2). A plasmid may contain anywhere from fewer than 10 genes to several hundred genes. A bacterial cell may not contain any plasmids, or it may contain one plasmid, multiple copies of the same plasmid, or more than one type of plasmid (i.e., plasmids containing different genes). Plasmids have also been found in yeast cells.
A typical bacterial genome. The hypothetical bacterial cell illustrated here possesses a chromosome containing 3,000 genes and a plasmid containing 5 to 100 genes. (Redrawn from Harvey RA et al. Lippincott’s Illustrated Reviews. Microbiology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)
Figure.2: A typical bacterial genome. The hypothetical bacterial cell illustrated here possesses a chromosome containing 3,000 genes and a plasmid containing 5 to 100 genes. (Redrawn from Harvey RA et al. Lippincott’s Illustrated Reviews. Microbiology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)

Cytoplasm 

The semiliquid cytoplasm of prokaryotic cells consists of water, enzymes, dissolved oxygen (in some bacteria), waste products, essential nutrients, proteins, carbohydrates, and lipids a complex mixture of all the materials required by the cell for its metabolic functions. There is some evidence to suggest that bacterial cytoplasm contains a cytoskeletal structure similar to that of eukaryotic cells.

Beware of similar sounding words - A plasmid is a small, circular molecule of double-stranded DNA. It is referred to as extrachromosomal DNA because it is not part of the chromosome. Plasmids are found in most bacteria. A plastid is a cytoplasmic organelle, found only in certain eukaryotic cells (e.g., algae and plants). Plastids are the sites of photosynthesis.

Cytoplasmic Particles Within the bacterial cytoplasm, many tiny particles have been observed. Most of these are ribosomes, often occurring in clusters called polyribosomes or polysomes (poly meaning many). Prokaryotic ribosomes are smaller than eukaryotic ribosomes, but their function is the same—they are the sites of protein synthesis. A 70S prokaryotic ribosome is composed of a 30S subunit and a 50S subunit. It has been estimated that there are about 15,000 ribosomes in the cytoplasm of an E. coli cell.

Cytoplasmic granules occur in certain species of bacteria. These may be stained by using a suitable stain, and then identified microscopically. The granules may consist of starch, lipids, sulfur, iron, or other stored substances.

Bacterial Cell Wall 

The rigid exterior cell wall that defines the shape of  bacterial cells is chemically complex. Thus, the structure of bacterial cell walls is quite different from the relatively simple structure of eukaryotic cell walls, although they serve the same functions—providing rigidity, strength, and protection. The main constituent of most bacterial cell walls is a complex macromolecular polymer known as peptidoglycan (also known as murein), consisting of many polysaccharide chains linked together by small peptide (protein) chains (Fig. 3). Peptidoglycan is only found in bacteria. The thickness of the cell wall and its exact composition vary with the species of bacteria. The cell walls of certain bacteria, called Gram-positive bacteria, have a thick layer of peptidoglycan combined with teichoic acid and lipoteichoic acid molecules (Fig. 4). The cell walls of Gram-negative bacteria (also explained in Chapter 4) have a much thinner layer of peptidoglycan, but this layer is covered with a complex layer of lipid macromolecules, usually referred to as the outer membrane, as shown in Figure 4. Although most bacteria have cell walls, bacteria in the genus Mycoplasma do not. Archaea (described in Chapter 4) have cell walls, but their cell walls do not contain peptidoglycan.
Structure of peptidoglycan. The peptidoglycan (murein) layer in a bacterial cell is a crystal lattice. Polysaccharide chains consisting of two alternating amino sugars are attached to a short peptide chain. Some of the peptide chains of one polysaccharide chain are cross-linked to peptide chains of another polysaccharide chain, thus producing a three-dimensional lattice structure. (Redrawn from Engleberg NC, et al. Schaechter’s Mechanisms of Microbial Disease. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)
Figure.3: Structure of peptidoglycan. The peptidoglycan (murein) layer in a bacterial cell is a crystal lattice. Polysaccharide chains consisting of two alternating amino sugars are attached to a short peptide chain. Some of the peptide chains of one polysaccharide chain are cross-linked to peptide chains of another polysaccharide chain, thus producing a three-dimensional lattice structure. (Redrawn from Engleberg NC, et al. Schaechter’s Mechanisms of Microbial Disease. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)

Some bacteria lose their ability to produce cell walls, transforming into tiny variants of the same species, referred to as L-form or cell wall–deficient (CWD) bacteria. Over 50 different  species of bacteria are capable of transforming into CWD bacteria, some of which might be responsible for chronic diseases such as chronic fatigue syndrome, Lyme disease, rheumatoid arthritis, and sarcoidosis. Clinicians are often unaware that CWD bacteria are present in their patients because they will not grow under standard laboratory conditions; they must be cultured in a different medium and at a different temperature than typical bacteria.
Differences between Gram-negative and Gram-positive cell walls. The relatively thin Gram-negative cell wall contains a thin layer of peptidoglycan, an outer membrane, and lipopolysaccharide (LPS). The thicker Gram-positive cell wall contains a thick layer of peptidoglycan and teichoic and lipoteichoic acids. (From Engleberg NC, et al. Schaechter’s Mechanisms of Microbial Disease. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)
Figure.4: Differences between Gram-negative and Gram-positive cell walls. The relatively thin Gram-negative cell wall contains a thin layer of peptidoglycan, an outer membrane, and lipopolysaccharide (LPS). The thicker Gram-positive cell wall contains a thick layer of peptidoglycan and teichoic and lipoteichoic acids. (From Engleberg NC, et al. Schaechter’s Mechanisms of Microbial Disease. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013.)

Glycocalyx (Slime Layers and Capsules) 

Some bacteria have a thick layer of material (known as glycocalyx) located outside their cell wall. Glycocalyx is a slimy, gelatinous material produced by the cell membrane and secreted outside of the cell wall. There are two types of glycocalyx. One type, called a slime layer, is not highly organized and is not firmly attached to the cell wall. It easily detaches from the cell wall and drifts away. Bacteria in the genus Pseudomonas produce a slime layer, which sometimes plays a role in diseases caused by Pseudomonas species. Slime layers enable certain bacteria to glide or slide along solid surfaces, and seem to protect bacteria from antibiotics and desiccation.

The other type of glycocalyx, called a capsule, is highly organized and firmly attached to the cell wall. Capsules usually consist of polysaccharides, which may be combined with lipids and proteins, depending on the bacterial species. Knowledge of the chemical composition of capsules is useful in differentiating among different types of bacteria within a particular species; for example, different strains of the bacterium H. influenzae, a cause of meningitis and ear infections in children, are identified by their capsular types. A vaccine, called Hib vaccine, is available for protection against disease caused by H.influenzae capsular type b. Other examples of encapsulated bacteria are Klebsiella pneumoniae, Neisseria meningitidis, and Streptococcus pneumoniae.

Capsules can be detected using a capsule staining procedure, which is a type of negative stain. The bacterial cell and background become stained, but the capsule remains unstained (Fig. 5). Thus, the capsule appears as an unstained halo around the bacterial cell. Antigen–antibody tests may be used to identify specific strains of bacteria possessing unique capsular molecules (antigens).
Capsule staining. A. Drawing illustrating the results of the capsule staining technique. B. Photomicrograph of encapsulated bacteria that have been stained using the capsule staining technique. The capsule staining is an example of a negative staining technique. Note that the bacterial cells and the background stain, but the capsules do not. The capsules are seen as unstained “halos” around the bacterial cells. ([B] From Winn WC Jr, et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.)
Figure.5: Capsule staining. A. Drawing illustrating the results of the capsule staining technique. B. Photomicrograph of encapsulated bacteria that have been stained using the capsule staining technique. The capsule staining is an example of a negative staining technique. Note that the bacterial cells and the background stain, but the capsules do not. The capsules are seen as unstained “halos” around the bacterial cells. ([B] From Winn WC Jr, et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.)

Encapsulated bacteria usually produce colonies on nutrient agar that are smooth, mucoid, and glistening; they are referred to as S-colonies. Nonencapsulated bacteria tend to grow as dry, rough colonies, called R-colonies. Capsules serve an antiphagocytic function, protecting the encapsulated bacteria from being phagocytized (ingested) by phagocytic white blood cells. Thus, encapsulated bacteria are able to survive longer in the human body than nonencapsulated bacteria.

Flagella 

Flagella (sing., flagellum) are thread-like, protein appendages that enable bacteria to move. Flagellated bacteria are said to be motile, whereas nonflagellated bacteria are usually nonmotile. Bacterial flagella are about 10 to 20 nm thick; too thin to be seen with the compound light microscope.
Flagellar arrangement. The four basic types of flagellar arrangement on bacteria: peritrichous, flagella all over the surface; lophotrichous, a tuft of flagella at one end; amphitrichous, one or more flagella at each end; monotrichous, one flagellum.
Figure.6: Flagellar arrangement. The four basic types of flagellar arrangement on bacteria: peritrichous, flagella all over the surface; lophotrichous, a tuft of flagella at one end; amphitrichous, one or more flagella at each end; monotrichous, one flagellum.

The number and arrangement of flagella possessed by a certain species of bacterium are characteristic of that species and can, thus, be used for classification and identification purposes (Fig. 6). Bacteria possessing flagella over their entire surface (perimeter) are called peritrichous bacteria. Bacteria with a tuft of flagella at one end are described as being lophotrichous bacteria, whereas those having one or more flagella at each end are said to be amphitrichous bacteria. Bacteria possessing a single polar flagellum are described as monotrichous bacteria. In the laboratory, the number of flagella that a cell possesses and their locations on the cell can be determined using what is known as a flagella stain. The stain adheres to the flagella, making them thick enough to be seen under the microscope (Fig. 7).
Salmonella cells, showing peritrichous flagella. Salmonella is a bacterial genus. The cells were stained using a flagella stain. (Courtesy of the CDC.)
Figure.7: Salmonella cells, showing peritrichous flagella. Salmonella is a bacterial genus. The cells were stained using a flagella stain. (Courtesy of the CDC.)

Bacterial flagella consist of three, four, or more threads of protein (called flagellin) twisted like a rope. Thus, the structures of bacterial flagella and eukaryotic flagella are quite different. You will recall that eukaryotic flagella (and cilia) contain a complex arrangement of internal microtubules, which run the length of the membranebound flagellum. Bacterial flagella do not contain microtubules, and their flagella are not membrane-bound. Bacterial flagella arise from a basal body in the cell membrane and project outward through the cell wall and capsule (if present), as was shown in Figure 1.

Some spirochetes (spiral-shaped bacteria) have two flagella-like fibrils called axial filaments, one attached to each end of the bacterium. These axial filaments extend toward each other, wrap around the organism between the layers of the cell wall, and overlap in the midsection of the cell. As a result of its axial filaments, spirochetes can move in a spiral, helical, or inchworm manner.

Pili (Fimbriae) 

Pili (sing., pilus) or fimbriae (sing., fimbria) are hair-like structures, most often observed on Gram-negative bacteria. They are composed of polymerized protein molecules called pilin. Pili are much thinner than flagella, have a rigid structure, and are not associated with motility. These tiny appendages arise from the cytoplasm and extend through the plasma membrane, cell wall, and capsule (if present). There are two types of pili: one type merely enables bacteria to adhere or attach to surfaces; the other type (called a sex pilus) facilitates transfer of genetic material from one bacterial cell to another following attachment of the cells to each other.

The pili that merely enable bacteria to anchor themselves to surfaces (e.g., tissues within the human body) are usually quite numerous (Fig. 8). In some species of bacteria, piliated strains (those possessing pili) are able to cause diseases such as urethritis and cystitis, whereas nonpiliated strains (those not possessing pili) of the same organisms are unable to cause these diseases.
Proteus vulgaris cell, possessing numerous short, straight pili and several longer, curved flagella; the cell is undergoing binary fission. P. vulgaris is a bacterial species. (From Volk WA, et al. Essentials of Medical Microbiology. 5th ed. Philadelphia, PA: Lippincott-Raven; 1996.)
Figure.8: Proteus vulgaris cell, possessing numerous short, straight pili and several longer, curved flagella; the cell is undergoing binary fission. P. vulgaris is a bacterial species. (From Volk WA, et al. Essentials of Medical Microbiology. 5th ed. Philadelphia, PA: Lippincott-Raven; 1996.)

A bacterial cell possessing a sex pilus (called a donor cell)—and the cell only possesses one sex pilus—is able to attach to another bacterial cell (called a recipient cell) by means of the sex pilus. Genetic material (usually in the form of a plasmid) is then transferred from the donor cell to the recipient cell—a process known as conjugation.

Spores (Endospores) 

A few genera of bacteria (e.g., Bacillus and Clostridium) are capable of forming thick-walled spores as a means of  survival when their moisture or nutrient supply is low. Bacterial spores are referred to as endospores, and the process by which they are formed is called sporulation. During sporulation, a copy of the chromosome and some of the surrounding cytoplasm becomes enclosed in several thick protein coats. Spores are resistant to heat, cold, drying, and most chemicals. Spores have been shown to survive for many years in soil or dust, and some are quite resistant to disinfectants and boiling. When the dried spore lands on a moist, nutrient-rich surface, it germinates, and a new vegetative bacterial cell (a cell capable of growing and dividing) emerges. Germination of a spore may be compared with germination of a seed. However, in bacteria, spore formation is related to the survival of the bacterial cell, not to reproduction. Usually, only one spore is produced in a bacterial cell and it germinates into only one vegetative bacterium. In the laboratory, endospores can be stained using what is known as a spore stain. Once a particular bacterium’s endospores are stained, the laboratory technologist can determine whether the organism is producing terminal or subterminal spores. A terminal spore is produced at the very end of the bacterial cell, whereas a subterminal spore is produced elsewhere in the cell (Fig. 9). Where a spore is being produced within the cell and whether or not it causes a swelling of the cell serve as clues to the identity of the organism.
Terminal and subterminal spores. A. Gramstained Clostridium tetani bacteria, revealing terminal spores (arrows). C. tetani causes the disease known as tetanus. (Courtesy of Dr. Holdeman and the CDC.)
Figure.9A: Terminal and subterminal spores. A. Gramstained Clostridium tetani bacteria, revealing terminal spores (arrows). C. tetani causes the disease known as tetanus. (Courtesy of Dr. Holdeman and the CDC.)
Continued. B. Gramstained Clostridium difficile bacteria, revealing subterminal spores (the light areas within the cells). C. difficile causes a diarrheal disease. (Courtesy of Dr. Gilda Jones and the CDC.)
Figure.9B: Continued. B. Gramstained Clostridium difficile bacteria, revealing subterminal spores (the light areas within the cells). C. difficile causes a diarrheal disease. (Courtesy of Dr. Gilda Jones and the CDC.)

The Discovery of Endospores 

While performing spontaneous generation experiments in 1876 and 1877, a British physicist named John Tyndall concluded that certain bacteria exist in two forms: a form that is readily killed by simple boiling (i.e., a heat-labile form), and a form that is not killed by simple boiling (i.e., a heat-stable form). He developed a fractional sterilization technique, known as tyndallization, which successfully killed both the heatlabile and heat-stable forms. Tyndallization involves boiling, followed by incubating, and then reboiling; these steps are repeated several times. The bacteria that emerge from the spores during the incubation steps are subsequently killed during the boiling steps. In 1877, Ferdinand Cohn, a German botanist, described the microscopic appearance of the two forms of the “hay bacillus,” which Cohn named Bacillus subtilis. He referred to small refractile bodies within the bacterial cells as “spores” and observed the conversion of spores into actively growing cells. Cohn also concluded that when they were in the spore phase, the bacteria were heat resistant. Today, bacterial spores are known as endospores, whereas active, metabolizing, growing bacterial cells are referred to as vegetative cells. The experiments of Tyndall and Cohn supported Louis Pasteur’s conclusions regarding spontaneous generation and dealt the final death blow to that theory.
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10.14.2013

Marine Mammals: Dugongs

Marine Mammals: Dugongs
Shy and retiring, dugongs are marine mammals that spend their days feeding in the shallow waters of reefs and along coastlines in the Indian Ocean, the Indonesian archipelago, and the southwestern Pacific around the Philippines. Even though they resemble a cross between a seal and a walrus, dugongs are more closely related to elephants. The slow-moving mammals are easily identified by their triangular, whalelike tails, broad trunklike snouts, and long bodies, which reach 9 feet (2.7 m). Dugongs have a thick layer of blubber under their skin, a feature that gives them a round-shouldered look. Their mouths look like vertical slits on their upper jaws, and their flippers are small and paddle shaped.

Dugongs have unusually slow metabolic rates for mammals but function well in their warm water environments where they float and feed, expending very little energy. With very few predators and plenty of food, migration and other energetic types of behavior are not necessary. Most of their time is spent grazing on sea grass blades and digging up the grass roots, their favorite parts of the plants. Roots of sea grasses are rich in carbohydrates, but to reach these treats, the animals must dig around on the bottom of the reef, behavior that has earned them the nickname “sea pigs.” Equipped with very few teeth, a dugongs bites with a mobile disk at the end of its snout. The disk works like a rake, pulling in food and sending it back to the grinding plates in the mouth. The males also have tusks, enlarged incisors that project from below the upper lip.

Sexually mature between eight and 18 years, female dugongs give birth to one calf every three or four years. After a gestation period of 13 months, their cream-colored calves are born in shallow water. Mothers help the calves, measuring only 39.4 inches (100 cm), to the surface for their first breaths. A calf nurses for two years, always remaining close enough to its mother to touch her. During an average life span of 55 years, a female produces only five or six offspring.

Dugongs are so shy that not much is known about their social interactions. Attempts to observe their behavior disturb them and often kindle curiosity about the observers. Dugongs are spotted singly or in small groups of six to eight animals. Within a group there seems to be no leader or organized social structure.
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10.12.2013

The minke whale (Balaenoptera acutorostrata)

The minke whale (Balaenoptera acutorostrata)
The minke whale (Balaenoptera acutorostrata) is the second smallest of the baleen whales, measuring about 32.8 feet (10 m) in length. A distinctive triangular head, narrow and pointed snout, and sickle-shaped dorsal fin make this whale easy to identify. Generally, minke whales are black, gray, or brown on their dorsal surface and a light color on their ventral surface. These active, agile whales have good maneuverability and speed, able to travel at 10.4 miles per hour (17 kph) for short periods of time.

As baleen whales, minkes are carnivores whose mouths are equipped with smooth baleen plates for filtering small organisms from gulps of water. Minkes can be found worldwide and are known to live in deep oceans, along coasts, and in coral reefs. Instead of seasonal migration, they only travel to follow their food. Sometimes minkes chase schools of small fish such as sardines and herring, swimming beneath and scooping them up in their open mouths. As in humpbacks, the throats of minkes are pleated so that they can expand their bite size.

Females enter their reproductive periods on 14-month cycles. Gestation lasts for 10 months, and calves are born in mid-winter. There is usually only one calf, but twins and triplets do occur. A newborn calf is only about 8.53 feet (2.6 m) long but grows quickly on its mother’s milk, which supports the baby whale for four or five months. Young whales reach sexual maturity at seven years of age and live to be about 50 years old.

Minke whales are most often solitary animals, although they are also seen in small groups. When food is plentiful, several hundred animals may congregate in the feeding grounds where they communicate with grunts, clicks, and breaching. The sounds they produce are very low-frequency waves that can travel long distances under water. Worldwide populations of minke whales are larger than most other groups of whales, consisting of about a million animals. Because the whales are small, they have escaped predation by humans and maintained almost-normal population sizes.
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Humpback Whales

Humpback Whales
The humpback whale (Megaptera novaeangliae) is much bigger than the spinner dolphin, measuring 40 to 50 feet (12.9 to 15.2 m) long and weighing up to 55 tons. The common name humpback describes the motion this whale makes as it jumps out of the water. A typical humpback whale is black on the dorsal side, with a white ventral surface and distinctive 15-foot flippers on the sides of its body.

The head of a humpback whale is large in proportion to the rest of its torpedo-shaped body. This figure below shows that the mouth line runs high along the entire length of head, and the eyes are set above the ends of the mouth. The small ear slits are located behind and below the eyes.

On the top of its head, a humpback has a raised area in front of its two blowholes that functions like a splashguard to keep water from flowing back into the holes where it breathes. Rounded knobs called tubercles are also located on the head, often on the upper and lower jaws. Each tubercle contains a hairlike structure, a vibrissa, that is about 0.5 inch (1.3 cm) long. The vibrissa’s function is not clearly understood, but it is believed to be important in detecting vibrations in the water. On the ventral side of the head, running from the tip of the lower jaw to the naval, there is an area of grooves known as ventral pleats, which are creased tissues that unfold when the whale opens its mouth, allowing the animal to expand the size of its bite to three times its normal width. The throat pleats can be seen when the whales breach, or jump in the water, as in the lower color insert on countershading (One type of protective, two-tone coloration in animals in which surfaces that are exposed to light are dark colored and those that are shaded are light colored.).

Baleen whales have two blowholes and large mouths filled with baleen plates. (Courtesy Sanctuary Collection, NOAA)
Baleen whales have two blowholes and
large mouths filled with baleen plates. (Courtesy
Sanctuary Collection, NOAA)
Whales are divided into two groups based on their feeding adaptations: the baleen whales and the toothed whales. Humpbacks are baleen whales, so named for the large plates in their mouths that act as food-catching sieves. Baleen is made of a flexible tissue that is chemically and physically similar to a fingernail. Plates are rooted in and grow from bases in the roof of the mouth. On each side of the upper jaw, there are 480 baleen plates. Each plate overlaps the adjacent one, forming dense mats that filter plankton from the water. The tongue wipes food off the plates and sweeps it into the whale’s throat.

Several of the cetaceans migrate, and their paths and periods of migration vary by species. Migratory baleens divide their year between the rich feeding grounds of the cold seas and the warm oceans where they breed and calve.

Humpbacks live in small groups similar to the pods of spinner dolphins. From June to September, they feed in the waters around Alaska and in other cold regions where food is plentiful, leaving the area in early fall for the long trip to the tropics.

Led by the sexually mature members of the group, the entire group makes the trip of about 3,500 miles (5,600 km) to warm coral reef waters, cruising at speeds of 2.3 miles per hour (2.0 kph), where females give birth.

Male humpback whales are extremely vocal and sing complex songs that can go on for hours. Within a population of whales, all of the males begin the breeding season singing the same song, but as the season progresses, each male creates his own version. By the end of a breeding season, individual songs have evolved so that every male’s vocalizations are distinct. The exact functions of these songs are not known but
are most likely associated with mating behaviors such as attracting females or warning off rival males.

Feeding occurs within the top 164 feet (50 m) of water. Humpbacks consume tons of plankton and krill, small, insectlike animals that live in the upper layers of water. To eat, a whale engulfs enormous gulps of water, then filters out food by sieving the water through the meshlike screen of baleen plates. Humpbacks have several feeding techniques, including one called bubble-netting. In this strategy, a whale dives beneath a school of prey and slowly begins to spiral upward around them, blowing bubbles as it goes. These bubbles herd the prey in the center of the circle. The whale then dives beneath the prey and swims up through the bubble net with its mouth open, gulping prey as it ascends.
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10.11.2013

Spinner Dolphins

Spinner dolphins (Stenella longirostris)
Spinner dolphins (Stenella longirostris) are easily spotted as they swim in the clear waters of coral reefs. Named for their ability to spin during acrobatic jumps, these dolphins are slender, with long thin beaks, sloping foreheads, and a stripe that runs from the eyes to the flippers. An adult measures 4.25 to 6.89 feet (1.3 to 2.1 m) long, and weighs between 100 and 165 pounds (45 and 75 kg). There are several varieties of spinners in different geological locations, and they vary slightly in shape and color.

Spinner dolphins feed at night on fish and squid in the deeper waters of the reef, although they will also eat organisms that live on the reef floor. Their mouths are equipped with 45 to 65 pairs of sharp teeth in each jaw. After feeding, spinner dolphins can often be found resting in protected areas of shallow water.

As social animals, spinners depend on interaction with others for hunting, defense, and reproduction and form small, long-lasting social groups called pods. In this species, pods do not have a highly organized social structure with a leader or dominant animal; instead, spinner pods are loose associations of a few key individuals, as well as dolphins who come and go.

A pod of spinner dolphins may also spend time with other sea animals, such as pilot whales, spotted dolphins, or tuna. Like most dolphins, spinners have good eyesight; however, they primarily rely on their sense of hearing and two kinds of voices, the sonic voice and the sonar voice, to let them know about their environment. The sonic or audible voice includes a vocabulary of clicks and whistles that are performed above or below the water. Along with these sounds, these mammals incorporate several mechanically produced sounds like jawsnapping, flipper slapping, and crash dives. Sonic voice and mechanical sounds are associated with communication between animals. The sonar or echolocation voice is used to navigate. Spinners send out high frequency sounds that are reflected back to the senders as echoes. The dolphins listen for the echoes and use them to locate objects.

A female spinner calves (has offspring, called a calf) once every two or three years. After a gestation period of about 10 1/2 months, a single newborn, 29.5 to 33.5 inches (75 to 85 cm) long, is born. The calf is immediately pushed to the surface by its mother, so it can take in its first breath. A nursing mother lies on her side at the surface, enabling her offspring to feed and breathe. The mother’s fat-rich milk supports the young dolphin for about seven months.

Spinner dolphins may use their pectoral fins to reach out and stroke each other, acts that strengthen the social bonds between them. Pairs of dolphins often swim along face to face, touching their flippers. Closely bonded animals, such as mother and calf, may swim in perfect synchrony as if mirror images of each other. The dolphins are also playful animals that make “toys” from materials in the environment and pass them back and forth to each other.
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7.27.2013

Marine Mammals

Marine Mammals
Mammals are the most obvious group of animals on land, but they are relatively rare in marine environments. There are just a few types of mammals whose bodies have become specialized for marine life. Among these are the cetaceans, a group that includes whales and dolphins, porpoises, and dugongs.

Marine Mammal Anatomy

Mammals are warmblooded vertebrates that have hair and breathe air. All females of this group have milk-producing mammary glands with which to feed their young. Mammals also have a diaphragm that pulls air into the lungs and a four-chambered heart for efficient circulation of blood. The teeth of mammals are specialized by size and shape for particular uses.

Marine mammals are subdivided into four categories: cetaceans, animals that spend their entire lives in the ocean; sirenians, herbivorous ocean mammals; pinnipeds, web-footed mammals; and marine otters. Animals in all four categories have the same characteristics as terrestrial mammals, as well as some special adaptations that enable them to survive in their watery environment.

The cetaceans, which include whales, dolphins, and porpoises, have streamlined bodies, horizontal tail flukes, and paddle-like flippers that enable them to move quickly through the water. Layers of blubber (subcutaneous fat) insulate their bodies and act as storage places for large quantities of energy. Their noses (blowholes) are located on the tops of their heads so air can be inhaled as soon as the organism surfaces above the water.

Manatees and dugongs are the only sirenians. These docile, slow-moving herbivores lack a dorsal fin or hind limbs but are equipped with front limbs that move at the elbow, as well as with a flattened tail. Their powerful tails propel them through the water, while the front limbs act as paddles for steering.

The pinnipeds—seals, sea lions, and walruses—are carnivores that have webbed feet. Although very awkward on land, the pinnipeds are agile and aggressive hunters in the water. This group of marine mammals is protected from the cold by hair and blubber. During deepwater dives, their bodies are able to restrict blood flow to vital organs and slow their heart rates to only a few beats a minute, strategies that reduce oxygen consumption. All pinnipeds come onto land or ice at breeding time.

The sea otters spend their entire lives at sea and only come ashore during storms. They are much smaller than the other marine mammals. Even though otters are very agile swimmers and divers, they are clumsy on shore. Their back feet, which are flipperlike and fully webbed, are larger than their front feet. Internally, their bodies are adapted to deal with the salt in seawater with enlarged kidneys that can eliminate the excess salt.

Body Temperature

Animals that are described as warm blooded, or endothermic, maintain a constant internal temperature, even when exposed to extreme temperatures in their environment. In mammals, this internal temperature is about 97°F (36°C), while in birds, it is warmer, around 108°F (42°C).

Warm-blooded animals have developed several physiological and behavioral modifications that help regulate body temperature. Since their bodies generate heat by converting food into energy, they must take in enough food to fuel a constant body temperature. Once heat is produced, endotherms conserve it with insulating adaptations such as hair, feathers, or layers of fat. In extreme cold, they also shiver, a mechanism that generates additional heat.

Heart rate and rate of respiration in warm-blooded animals does not depend on the temperature of the surroundings. For this reason, they can be as active on a cold winter night as they are during a summer day. This is a real advantage that enables warm-blooded animals to actively look for food year round.

The internal temperature of coldblooded, or ectothermic, animals is the same as the temperature of their surroundings. In other words, when it is hot outside, they are hot, and when it is cold outside, they are cold. In very hot environments the blood temperature of some cold-blooded animals can rise far above the blood temperature of warmblooded organisms. Furthermore, their respiration rate is dependent on the temperature of their surroundings. To warm up and speed their metabolism, cold-blooded animals often bask in the sun. Therefore, cold-blooded animals such as fish, amphibians, and reptiles, tend to be much more active in warm environments than in cold conditions.
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4.27.2013

Marine Bird Anatomy

Marine Bird Anatomy
Birds are warm-blooded vertebrates that have feathers to insulate and protect their bodies. In most species of birds, feathers are also important adaptations for flying. As a general rule, birds devote a lot of time and energy to keeping their feathers waterproof in a process called preening. During preening, birds rub their feet, feathers, and beaks with oil produced by the preen gland near their tail.

The strong, lightweight bones of birds are especially adapted for flying. Many of the bones are fused, resulting in the rigid type of skeleton needed for flight. Although birds are not very good at tasting or smelling, their senses of hearing and sight are exceptional. They maintain a constant, relatively high body temperature and a rapid rate of metabolism. To efficiently pump blood around their bodies, they have a four-chambered heart.

Like marine reptiles, marine birds have glands that remove excess salt from their bodies. Although the structure and purpose of the salt gland is the same in all marine birds, its location varies by species. In most marine birds, salt accumulates in a gland near the nostrils and then oozes out of the bird’s body through the nasal openings.

The term seabird is not scientific but is used to describe a wide range of birds whose lifestyles are associated with the ocean. Some seabirds never get further out into the ocean than the surf water. Many seabirds are equipped with adaptations of their bills, legs, and feet. Short, tweezerlike bills can probe for animals that are near the surface of the sand or mud, while long, slender bills reach animals that burrow deeply. For wading on wet soil, many seabirds have lobed feet, while those who walk through mud or shallow water have long legs and feet with wide toes.

Other marine birds are proficient swimmers and divers who have special adaptations for spending time in water. These include wide bodies that have good underwater stability, thick layers of body fat for buoyancy, and dense plumage for warmth. In swimmers, the legs are usually located near the posterior end of the body to allow for easy maneuvers, and the feet have webs or lobes between the toes.

All marine birds must come to the shore to breed and lay their eggs. Breeding grounds vary from rocky ledges to sandy beaches. More than 90 percent of marine birds are colonial and require the social stimulation of other birds to complete the breeding process. Incubation of the eggs varies from one species to the next, but as a general rule the length of incubation correlates to the size of the egg: Large eggs take longer to hatch than small ones do.
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Seabirds

Birds, vertebrates of the class Aves, are the second largest group of vertebrates on Earth, following fish. Unlike marine reptiles, seabirds do not actually live in the water; however, they depend on the sea for their food, and their bodies are highly specialized for aquatic life. Many of the seabirds who feed on animals in the coral reefs also spend some of their time in other marine areas. All of them go to shore during parts of their lives, and some migrate from one ocean area to another.
 
Seabirds are among the longest-lived birds in the world, many with life spans of 30 years or more. For these animals, reproducing is a serious investment of time and energy. Compared to terrestrial avians, seabirds produce fewer offspring and the young are slower to mature, taking an average of seven years. Many choose their mates for life, and the males and females work together to incubate the eggs.

There are many species of birds that nest near coral reefs and interact with the reef food webs. Some of the largest seabird families represented on the reef include frigate birds (family Fregatidae), tropic birds (Phaethontidae), petrels (Procellariidae), boobies (Sulidae), terns and noddies (Sternidae), and albatrosses (Diomedeidae).

Magnificent frigate birds (Fregata magnificens) are striking black birds with deeply forked tails and wingspans of about 7.5 feet (24.6 m). They are easily recognized by their gular sacs, red membranous pouches that the males inflate during courtship. Unlike other seabirds, magnificent frigate birds do not produce a lot of preening oil. These animals rarely float or paddle in the ocean, even to gather food, so their wings require little waterproofing. Instead, most of their meals are stolen from other birds using a highly effective technique of harassment, pestering the victim so much that the irritated bird regurgitates its meal. When the stomach contents are finally disgorged, the magnificent frigate bird deftly catches the mass, often before it hits the water, and whisks it away. If there are no other birds feeding in the area, magnificent frigate birds, who are capable fishers, revert back to the predator mode and catch their own fish or squid from the water.

Magnificent frigate birds (Fregata magnificens)
Magnificent frigate birds (Fregata magnificens)

Reefs also support white-tailed tropic birds (Phaethon lepturus). The adults of this species have wingspans of about 37 inches (94 cm) and can be identified by the long central streamers of tail feathers, black markings on the wings, and yellow bills. Generally feeding at twilight, the white-tailed tropic bird flies high over the ocean, then gracefully dives a distance of 50–70 feet (15–20 m) in pursuit of fish and squid. To lessen the impact of the dive, the birds have shockabsorbent air-filled pouches on their chests. In nesting season, the female lays one pink-and-brown egg on the bare ground or among rocks.

white-tailed tropic birds (Phaethon lepturus)
White-tailed tropic birds (Phaethon lepturus)


A reef bird that is capable of both diving and skimming for food is Audubon’s shearwater (Puffinus iherminieri). Its dark head and brown upper body are set off by a white belly and throat. To feed, Audubon’s shearwater flies close to the water, alternately flapping and gliding, picking up small crustaceans and fish larvae that swim near the surface. If it spots a fish or squid in deeper water, the bird dives after it. Like most shearwaters, it lays a single egg inside a hole in seaside cliffs.

Puffinus iherminieri
Puffinus iherminieri

The red-footed booby is the smallest member of the booby family, having a wingspan of 36 to 40 inches (91.4 to 01.6 cm). Its coloring is unusual among birds because individuals can vary from white to brown. The adults have torpedoshaped bodies, long pointed wings, distinctive bright red feet, and conical blue bills. Red-footed bobbies feed by diving for fish and squid. During mating season, a pair builds a nest in the tops of trees and usually lays two eggs; however, they only hatch one of the eggs, perhaps because competition for food is keen among marine bird populations.

Sooty terns, members of the group of birds known as “sea swallows,” have forked tails, long, pointed wings, and slender bills that curve downward. Adults display distinctive blackand-white plumage, but juveniles have sooty-colored feathers on their heads and chests. Favorite foods of sooty terns include small fish and crustaceans. Each spring, thousands of the birds migrate to tropical islands to form nesting colonies that cover acres of ground. Within these sprawling assemblies, new parents work together to create nurseries where all of the young birds are kept and protected.
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4.20.2013

Marine Reptile Anatomy

Reptiles are not usually associated with marine environments. In fact, of the 6,000 known species of reptiles, only about 1 percent inhabits the sea. Members of this select group include lizards, crocodiles, turtles, and snakes. Each of these organisms shares many of the same anatomical structures that are found in all reptiles: They are cold-blooded, air-breathing, scaled animals that reproduce by internal fertilization. Yet, to live in salt water, this subgroup has evolved some special adaptations not seen in terrestrial reptiles.

Marine Reptile Anatomy

In turtles, the shell is the most unique feature. The lightweight, streamline shape of the shell forms a protective enclosure for the vital organs. The ribs and backbone of the turtle are securely attached to the inside of the shell. The upper part of the shell, the carapace, is covered with horny plates that connect to the shell’s bottom, the plastron. Extending out from the protective shell are the marine turtle’s legs, which have been modified into paddle-like flippers capable of propelling it at speeds of up to 35 miles per hour (56 kph) through the water. These same legs are cumbersome on land, making the animals slow and their movements awkward.

Most air-breathing vertebrates cannot drink salty water because it causes dehydration and kidney damage. Seawater contains sodium chloride and other salts in concentrations three times greater than blood and body fluids. Many marine reptiles drink seawater, so their bodies rely on special saltsecreting glands to handle the excess salt.

To reduce the load of salt in body fluids, these glands produce and excrete fluid that is twice as salty as seawater. The glands work very quickly, processing and getting rid of salt about 10 times faster than kidneys.
Salt glands are located on the head, often near the eyes.

There are more than 50 species of sea snakes that thrive in marine environments. Sea snakes possess adaptations such as nasal valves and close-fitting scales around the mouth that keep water out during diving. Flattened tails that look like small paddles easily propel these reptiles through the water. The lungs in sea snakes are elongated, muscular air sacs that are able to store oxygen. In addition, sea snakes can take in oxygen through the skin. Their adaptations to the marine environment enable sea snakes to stay submerged from 30 minutes up to two hours; however, this ability comes at a cost. Because marine snakes routinely swim to the surface to breathe, they use more energy and have higher metabolic rates than land snakes. To balance their high energy consumption, they require more food than their terrestrial counterparts.

Finally, crocodiles usually occupy freshwater, but there are some species that live in brackish water (in between salt water and freshwater) and salt water. These animals have salivary glands that have been modified to excrete salt. Their tails are flattened for side-to-side swimming and their toes possess well-developed webs. Saltwater crocodiles are equipped with valves at the back of the throat that enable them to open their mouths and feed underwater without flooding their lungs.
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4.16.2013

Marine Reptiles

Marine Reptiles
Among the reptiles, only a small number of species have become adapted for life in the sea. Of these, just a few are fulltime reef residents. Reptiles frequently found around reefs include the sea turtles and sea snakes.

Sea turtles move through the water with the grace of ballerinas. These large reptiles, related to the more familiar but generally smaller land turtles, are superbly adapted for sea life. Their limbs are modified as strong flippers that effortlessly push their streamlined bodies through the water.

Turtles are such expert swimmers and so perfectly designed for their lifestyles that it would be easy to think of them as big fish; however, unlike fish, turtles must swim to the surface to breathe. These animals have lungs instead of gills, and they breathe through their noses. Even though active turtles need to surface every few minutes, they are capable of staying under water for long periods of time by holding their breath. When resting in caves or on ledges, sea turtles may remain submerged up to two and one-half hours.

As true marine animals, sea turtles rarely leave the ocean. The only reason they visit the shore is to lay eggs, and then only the females emerge. Nesting females always return to the beaches where they were born to lay their eggs (see Figure below). Males accompany them only as far as the shallow water.

Sea turtles dig nests on sandy beaches where they lay their eggs.
Sea turtles dig nests on sandy beaches
where they lay their eggs.

Several species of sea turtles occasionally visit the coral reef; however, two species are reef residents: the Atlantic hawksbill sea turtle (Eretmochelys imbricata) and the green sea turtle (Chelonia mydas). From time to time they may be joined by other species of marine turtles that take time out of their migratory travels to rest and eat in luxurious reef accommodations. The green sea turtle is an impressive animal that grows up to 3.5 feet (1.1m) long and can weigh 400 pounds (181 kg). As seen in the upper color insert on page C-8, its carapace, the upper shell, is mottled in shades of dark brown on top and creamy white below. This type of dark-on-top, light-onthe-bottom coloration, called countershading, makes turtles hard to see in the water. From above, their carapace looks like the seafloor, and from below the plastron, the lower shell, blends in with the sky. Such camouflaging helps turtles get close to their prey before striking. It also helps them avoid sharks, their only predators.

Every two or three years, sexually mature green sea turtles make long journeys to mate and lay their eggs. They leave their feeding grounds and swim 600 miles or more, returning to the beaches where the females were born. During the months of March and April, mating occurs in offshore waters, then the females go ashore to lay their eggs. Green sea turtles have the ability to retain viable sperm for months after mating. The eggs laid at one mating were fertilized much earlier.

No longer supported by the water’s buoyancy, female green sea turtles drag themselves across the beach to sandy spots above the tide line. While ashore, they shed sticky tears that keep their eyes moist and free of sand. Using powerful hind legs, each female digs an egg chamber, a task that may take the entire night. When the chamber is finally finished, the female deposits 100–200 eggs, each about the size of a Ping-Pong ball, then covers the nest and returns to the sea.

Hatching begins after 60 days of incubation, usually early in July. Working together, the hatchlings scrape sand off the roof of the nest and pack it into the nest floor, a strategy that builds up the nest until it is almost even with the beach.

Using moonlight reflected in the ocean water as a beacon, all of the hatchlings scramble from the nest one evening and race toward the water. Some are picked off by birds and crabs on the beach, and others are grabbed by fish waiting for them in the shallow water.

Those that survive strike out on their own, swimming nonstop for the next 36 to 48 hours. When they get out far enough, the baby turtles are picked up by currents and carried into the open ocean. Green sea turtles remain at sea for several years, feeding on jellyfish and other invertebrates. When they are juveniles, they return to the reef areas where adults are living. There, young turtles join the colony, grazing on algae among corals and rocks.

A hawksbill sea turtle swims around the coral reef.
A hawksbill sea turtle swims around
the coral reef.
The Atlantic hawksbill sea turtle, pictured in Figure 6.2, is another reef resident. With an orange, brown, or yellow carapace that measures up to 35.8 inches (91 cm) long, the hawksbill can weigh 100–150 pounds (40–60 kg). The head of the turtle is narrow, with two pairs of scales in front of the eyes and a hawklike jaw that accounts for its name. The shape of this turtle’s mouth is ideal for reaching into the cracks and crevices of coral reef where it finds sponges, octopuses, and shrimps. The hawksbill also eats squid that are swimming in the water column. After eating, it rests on the ledges and caves of the reef.

Nest building and egg laying occur every two to three years, preceded by mating in areas of shallow water. Male hawksbill turtles can be distinguished from females by their long tails. A female nests two to four times during each egglaying season, depositing about 160 eggs in each clutch.

Renesting females will usually return to the same part of the beach, often building her second and third nests within sight of the first one. After 50 to 70 days of incubation, hatchlings climb out of the nest and make their way to the sea. Mortality rates are very high, but the ones who survive swim out to sea. Like the green sea turtles, they are not usually seen again until they return to the area as juveniles.

Sea snakes make up 86 percent of the marine reptiles, and they are primarily found in tropical waters. Some species, including the olive sea snake (Aipysurus laevis), live around reefs. With a stout, round body that averages about 3.9 feet (1.2 m) in length, the snake varies in color, ranging from dark brown to purplish brown on the dorsal side, fading to light brown on the ventral surface. The flattened tail is creamy white and has a brown ridge on the dorsal side.

Aipysurus laevis
Aipysurus laevis

The olive sea snake prefers reef waters that are 16.4 to 147.6 feet (5 to 45 m) deep where they can prey on fish, fish eggs, cuttlefish, and crabs yet surface quickly for air. During the day, the snake feeds by weaving among coral structures in search of animals that are at rest. When prey is located, the snake uses constriction to hold the victim while it injects venom with its fangs. The olive sea snake’s venom contains enzymes that begin digesting the prey from the inside.

After courtship in the open water, olive sea snake mating takes place on the reef floor. Competition for mates is fierce and several males may vie for a single female. The young snakes, which are born alive, grow quickly, maturing in four to five years.

Even though not as numerous as the olive sea snake, the turtle-headed sea snake makes it home on many reefs. Preferring lagoons to energetic parts of the reef, this snake is often found in large aggregates. The turtle-headed sea snake is a daytime feeder that slowly moves through the reef, seeking out small fish and crustaceans that it immobilizes with venom. It also feeds on the eggs of fish, such as gobies and blennies, that spawn in the lagoon, scooping them up from the reef floor with its hard pointed snout, the feature that most resembles a turtle’s head. The snake is a voracious eater and feeds every two or three hours.
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