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SHARK DISSECTION
INTRODUCTION:
The Dogfish shark is a good example of a primitive, jawed fish. Unlike most sharks, the dogfish prefers the colder
waters of the North Atlantic and North Pacific oceans. They are small sharks, ranging in size from one-half meter to
one meter in length. Adult females are slightly larger than the males. The shark exhibits all the internal
characteristics of vertebrates, such as a closed circulatory system, a large coelom, and an endoskeleton. The major
difference between the shark and other vertebrates is the shark's atypical skeletal system that is comprised entirely of
cartilage
CLASSIFICATION:
Kingdom:
Phylum:
Family:
Genus:
Species:
Animalia
Chondrichthyes
Squalidae
Squalus
acanthias
MATERIALS:
Preserved Dogfish shark
Plastic bag and newspaper
Scalpel or Scissors
Probe
Hand lens or dissecting microscope
PROCEDURES:
External Dissection:
1) Prepare a place for your shark by making a 'shark nest'. Flatten a large plastic bag on your work area and
then place several sheets of newspaper on top of the bag to absorb any fluid from the shark or the container.
2) Examine the skin of the shark by running your fingers over it lightly, from the posterior to the anterior end.
The roughness you feel is the placoid scales, also called the dermal denticles. Use a scalpel to remove a
small sample of skin and then place this sample under a hand lens or a stereomicroscope to observe the
placoid scales in greater detail. Because of its abrasive quality, shark skin was once known as "shagreen"
and was used to polish wood. Makes a sketch of these scales:
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3) Locate the mouth, eyes, nostrils (external nares), snout, and olfactory sacs (inside each spiracle). The
spiracles are actually reduced, modified gills. Most fish respire by taking in water through the mouth and
sending it through the pharynx to the gills. However, when a spiracle is present, water enters through it
also. Relatives of the shark, such as rays and skates, which live on the bottom of the ocean, use the spiracle
for water intake almost exclusively. Why might this be true?
4) The body is divided into three sections, the head, trunk, and the tail. Make a sketch of the shark and label
these 3 body parts.
5) Locate the lateral line and notice that the body is darker above the lateral line than below. This special type
of shading works to camouflage the shark as the natural light from above highlights its dorsal side and
makes shadows on the ventral side. Draw the lateral line on your drawing. What is the function of the
lateral line?
6) Locate the seven fins on your shark: anterior dorsal, pectoral (2), pelvic (2), caudal, and posterior dorsal.
Add these fins to the drawing of the shark and label them accordingly. Notice that the caudal fin is
asymmetrical. This improves the shark's stability, allowing it to ride evenly through the water. In fish
more advanced than the shark that have lungs or a swim bladder, this extra stability is unnecessary.
Therefore, such fish have a symmetrical caudal fin.
7) Open the mouth and describe the teeth.
8) Identify the gills and describe them. What is the function of the gills?
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9) Determine the sex of your shark by checking the pelvic fins for claspers. Only males have these strong,
grooved structures. They are used during mating to hold the female stationary. Make sure you observe a
shark of each sex. Locate the urinary ducts and in the male, the genital ducts that open at the tip of the
urinary papilla. This can be found just inside the cloaca, the common chamber on the ventral side of the
shark, between the two pelvic fins. The cloaca serves as an exterior opening for the digestive, excretory,
and reproductive systems.
Gender:
Internal Dissection:
1) Place the shark on the prepared surface, ventral side up. Using the diagram as a guide, take a scalpel and
begin cutting just anterior to the pelvic fins, making the cut on the right side of the midventral line.
Continue cutting anteriorly until you reach the pectoral fins. Make two transverse cuts, across the body,
one just posterior to the pectoral fins and another just anterior to the pelvic fins. The cuts should extend
from one lateral line of the shark to the other.
2) Carefully examine the layers of the body wall and locate the following structures: connective tissue,
muscle tissue, and epithelium, and the lining of the pleuroperitoneal cavity.
3) Digestive System:
At the most anterior part of the pleuroperitoneal cavity rests the liver. Count the lobes of the liver and
record.
Number of liver lobes:
What is the function of liver?
The long thin gall bladder is imbedded in the median lobe of the liver. To observe it better, scrape away
the liver tissue with a dissecting probe. Once the gall bladder is exposed, locate the attached bile duct.
What is the function of the gall bladder?
Spread the lobes of the liver to locate the esophagus and the stomach. Both have approximately the same
diameter so it is difficult to tell where the esophagus stops and the stomach begins. The posterior end of
the stomach curves anteriorly, giving the stomach a "J" shape. The rest of the digestive system turns
posteriorly to form a straight intestine that continues to the outside body opening, the cloaca.
Although associated with the digestive system, the spleen is actually part of the circulatory system. It is
found at the posterior end of the stomach. Describe the spleen. What is its function?
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Locate the pancreas. It has two-lobes, the dorsal lobe coming from the right side of the spleen on the dorsal
side of the intestine, and the ventral lobe, which is oval-shaped, and sitting on top of the beginning of the
intestine. What is the function of the pancreas?
Make a flow chart that follows food through the shark’s digestive system:
4) Urogenital System:
On either side of the anterior end of the esophagus and stomach are the gonads (testes and ovaries). Locate
them. The kidneys can be observed lying on the dorsal wall of the pleuroperitoneal cavity, one on either
side of the mid-dorsal line. They are long bands of dark tissue and often run almost the entire length of the
cavity.
A closer look at this system must be done by sex, as the male and female urogenital systems are quite
different from each other. To do this, carefully remove the organs of digestion that were observed earlier.
After the liver, esophagus, stomach, intestine, spleen and pancreas have been removed, you should be able
to locate the organs of the urogenital system. Make sure to examine both a male and female shark.
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Female Urogenital System
Find the large paired ovaries at the ends of the kidneys. Eggs can usually be removed from the ovaries.
Open an egg and examine the yolk. At maturity, the eggs can be as large as 3cm in diameter. The oviducts
appear to be long thin tubes that are enlarged at their anterior end to form a cuplike structure called the
ostium. The ostium received the eggs from the ovaries when they are mature. Posteriorly, the oviduct
enlarges to become the uterus. The uterus takes up 1/3 to 1/2 of the oviduct and is very large in pregnant
females. Embryos are often developing in the uterus. Cut it open and remove the fetuses.
Male Urogenital System
Find the large paired testes in the same position as the ovaries. Sperm are produced by the testes and
passes into the efferent duct. This duct leads to the vas deferens, a coiled tube that extends almost the
entire length of the kidney. As the vas deferens approach the excretory position of the kidney, they
straighten and enlarge into the seminal vesicle. Sperm sacs can be found near the cloaca. Unlike most fish,
the shark exhibits internal fertilization. During mating, one clasper on the male's pelvic fin is turned
forward and inserted into the cloacal opening of the female. The sperm move from the male's cloaca into
the groove on the dorsal surface of the clasper and then into the female.
Compare and contrast the male and female urogenital system:
5) Heart:
The heart is located in the head region of the shark, just above and to the inside of the pectoral girdle. The
heart is almost totally incased in the cartilaginous pectoral girdle and the muscles that surround it. The
heart lies in its cavity called the pericardial cavity. To reach the pericardial cavity, continue the original
ventral cut anteriorly through the pectoral girdle and the surrounding muscles. Make a transverse cut just
below the mouth and fold back the flaps.
Locate the heart. If you probe posterior to the heart, you should find a canal that connects the pericardial
and pleuroperitoneal cavities, showing that they are not completely separate. Carefully examine the heart
and locate the following structures: atrium, ventricle, and sinus venosus. Also, locate dorsal aorta and
ventral aorta. Make a simple drawing of the shark heart and label the parts.
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