PreLab Handout:
Dissection of the Spiny Dogfish Squalus acanthias
Background information: Sharks belong to the class Chondrichthyes; those are fishes that
have a cartilaginous skeleton. Sharks vary in size, behavior and in their methods of
reproduction. They are very successful organisms, having more than 440 different species, with
few enemies, diseases or parasites. Their cartilaginous skeleton weighs less, and makes them
more buoyant than a skeleton made with bone. They differ from Osteichthyes, the bony fish, in
that they have a large fatty liver for buoyancy, instead of a swim bladder, and they have
dermal denticles or placoid scales, giving them rough skin. They have teeth that are
continuously replaced throughout their lifetime. Sharks are slow to reach maturity and produce
relatively few eggs.
Sharks face many pressures from human commercial fishing and cultural practices. Shark fin
soup is a popular Asian delicacy and can cost up to 100 U.S. dollars per bowl. The shark is
captured, the dorsal fin is sliced off and the shark is thrown back and dies. Sharks are also
caught as bycatch in fishing nets. Pollution, trash, oil spills and toxic waste also threaten shark
populations.
The spiny dogfish is believed to be the world’s most populous shark, occurring worldwide in
temperate and sub-arctic ocean waters as well as coastal zones. They may be found anywhere in
the water column, from shallow to deep water. Due to their relatively high numbers they are the
most studied shark, used for dissections such as ours. These sharks get the name dogfish
because they occur in large schools of hundreds or thousands of fish. Some have said that like
dogs they occur in 'packs'. The dogfish are prey to whales and other larger fish, and they prey
upon smaller fish. They are commercially fished for its liver oil and as a food fish in Europe.
Distinguishing Characteristics
The spiny dogfish has some distinguishing features: rows of small white dots that run along its
sides, a sharp spine that can release a mild poison located in front of each of its two dorsal fins,
the absence of an anal fin, and the rounded, un-notched tail. The spines of the spiny dogfish, as
well as the dermal denticles, are formed from material similar to our teeth. These dermal
denticles are pointed towards the posterior making the shark more hydrodynamic. It has two
pectoral fins which help provide lift while it swims; it has two pelvic fins, two dorsal fins and a
caudal fin. Males have modified pelvic fins called claspers, one of which the male inserts into
the female cloaca during copulation. The sharks head is pointed and its body is long and
torpedo shaped giving it hydrodynamic qualities. They have large eyes and a spiracle behind
the eye, used in respiration. They exhibit countershading (darker shade on the dorsal side).
Countershading therefore, reduces the ease of detection of prey by potential predators by
counterbalancing the effects of shadowing.
Diet
The spiny dogfish is an opportunistic feeder eating whatever prey is abundant. In general their
diet is comprised of small fishes such as capelin, cod, haddock, hake, herring, menhaden and
ratfish. They also eat invertebrates such as krill, crabs, polychaete worms, jellyfish, ctenophores,
amphipods, squid and octopus.
Reproduction
Development in this shark is ovoviviparous. The gestation length is the longest known for sharks
at an estimated 22 months. Females are ovoviviparous (produce eggs that hatch within the
body).Young are born in the warmer waters off of North Carolina or New England during the
winter months. The number of young born in a litter is dependent on the size of the female,
larger females bearing more pups. However, most litters are between 2 and 16 individuals that
are approximately 20 to 30 cm (8 to 12 inches) in length. Sexual maturity in males is reached at
a size of 80 to 100 cm (31 to 39 inches) at which time they are usually 11 years of age. Females
reach sexual maturity at a later age, between 18 and 21 years at which time they are between
100 and 124 cm (39 to 49 inches) long.
Habitat
The spiny dogfish is found in cold and warm temperate oceans at temperatures between 6 and
15 degrees Celsius. However on the Scotian Shelf this shark has been caught in water
temperature between 3 and 11 degrees Celsius. The spiny dogfish is tolerant of a wide range of
salinities and can be found in estuaries. It can be located in the water column from the surface
to depths of 730 meters (2,400 feet).
Range
This shark is present in all of the world’s temperate oceans It ranges throughout the coastal
waters of the Atlantic and Pacific oceans. The spiny dogfish is a seasonal migrant into Canadian
waters. In June these sharks appear off Nova Scotia, in the Bay of Fundy and off southwestern
Newfoundland. By July they move into the Gulf of St. Lawrence and into waters off of southern
Labrador and around the rest of Newfoundland. By late fall most of the spiny dogfish migrate out
of Canadian waters and move south to waters off of North Carolina or New England.
Spiny Dogfish Dissection
Pre-Lab Questions:
1. Shark fin soup can cost up to how much per bowl in Asian countries?
2. What makes the Spiny Dogfish the most studied shark?
3. Why are they called “Dogfish Sharks”?
4. List 4 distinguishing features of the Spiny Dogfish.
a.
b.
c.
d.
5. What do the pectoral fins of a shark supply when they swim?
6. What may be the purpose of countershading in sharks and other types of fish?
7. What is the diet of a Spiny Dogfish?
8. What does the term Ovoviviparous mean in regards to the hatching of the eggs in the
female?
9. What is the range of the Spiny Dogfish?
Spiny Dogfish Dissection
I. Procedure:
a. Note its, color; can you see the spots on the fish’s body?
b. Observe its eyes; they can help reveal whether it is a surface or bottom dweller.
Which do you think it is based on the size of the eyes?
c. Can you find the spines near the dorsal fins?
d. Note if your fish is male or female
II. Getting started:
A. Turn fish ventral side up. A long incision will be made to show the inside of the
shark, the coelomic cavity, which is divided into the pericardial cavity
containing the heart, and the pleuro-peritoneal cavity containing the digestive,
reproductive and excretory organs.
B. First make a transverse incision above the pelvic girdle. This is a piece of
cartilage that joins the pelvic fins. First use the scalpel then cut upward with the
scissors. Make the cuts deep.
1. Next make a transverse cut below the pectoral girdle- a hard bar of
cartilage- that unites the pectoral fins. Do not make this cut too deep. Cut a
semi circle around to each side leaving the pectoral fins attached. Start
making the long cut from the pelvic girdle to the pectoral girdle. Do not cut
too deep but cut through the peritoneal cavity. Open up the coelomic cavity
and pin out the cut flaps.
2. Liver dissection: The liver is the dominant organ, making up to 25% of the
shark’s body weight. Pelagic sharks have larger livers than benthic sharks.
The liver may be large to give them neutral buoyancy. They do not have a
swim bladder as do bony fish. There are three main lobes of the liver –
right, left, and a smaller middle lobe.
3. First cut away the small central lobe right near the gall bladder to
reveal the other digestive organs. The gall bladder stores bile, made in
the liver, which is used to emulsify fats.
4. Cut away the liver at the corners and cut through the bile duct.
C. The alimentary Tract: Digestion begins in the mouth when the prey is captured.
Sharks do not chew, chunks are bitten off and then move down the pharynx
(throat) of the shark, then into the esophagus and the stomach- indistinguishable
on the outside as a continuous long tube. Food then moves into the intestine which
is short in length. Notice the pancreas adjacent to the intestine. It secretes
enzymes into the intestine for digestion. After the nutrients are absorbed, the
remains of the food, passes into the rectal chamber where water is further
absorbed. Food waste is excreted from the cloaca.
D. Esophagus and Stomach Dissection:
1. Make an incision the length of the esophagus and stomach and open the
cavity, or lumen.
2. The esophagus can be differentiated from the stomach because it has small
projections called papillae, which secrete mucus enabling food to pass to the
stomach.
3. The stomach has long longitudinal folds called rugae which allow the shark
stomach to distend to hold large chunks of food until digestion occurs. Strong
gastric juices break down the chitinous shells of crustaceans.
E. Intestine dissection:
1. The short intestine can be found to the posterior in the intestinal tract.
Carnivores have shorter intestines than herbivores. To increase absorption
they have a spiral valve.
2. Make a longitudinal cut to expose the spiral valve. The convoluted
shape of the valve increases the surface area of the intestine. Before cutting,
note the pancreas located at the top of the intestine.
F. Male reproduction:
1. Ventral side up. Males have one clasper for each pelvic fin.
2. With the pleural cavity open, place the liver to one side; pull the digestive
organs to the side so we can see the testes which lie dorsal of the digestive
system.
3. Remove the digestive system by cutting at the base of the esophagus,
cutting the mesentery, the thin membranes that keep the organs in place,
also cut the cloaca. We can clearly see the testes, which are paired as in
higher vertebrates.
4. In males reproduction begins with the formation of sperm in the testes.
Sperm passes into tubes called the vas deferens, the posterior portion is
called the seminal vesicle where sperm mixes with secretions from the
Leydig’s Gland and then passes into the urogenital papilla. Here semen is
stored before it is sent into the cloacal chamber where it will pass into the
clasper groove and then into a female
G. Female Reproduction:
1. Ventral side up. As in the male the digestive system overlies the reproductive
system. Move aside the left liver lobe, and the stomach, the ovary and uterus
will be visible.
2. In females reproduction begins in the ovary where ova, eggs, are formed and
then passed into the oviduct through the ostium, a small pore, where
fertilization occurs. After the egg is fertilized, it moves through a small gland,
called the shell gland at the top of the oviduct, which secretes a thin
membrane around the eggs. They then move down the oviduct and come to
rest in the uterus to undergo gestation for up to two years.
H. Uterus dissection:
1. Cut open the thin walled uterus, it will contain a structure called a candle
which is a large mass of yolk, check and see if yours have embryos attached.
2. Remove the uterus to see the embryos more clearly. Thin walls are common
in ovoviviparous species, the fertilized eggs develop within the female, the
female doesn’t give nutrition to embryo while it is growing, and the young
are born alive.
I. Ovary dissection: Ovaries are asymmetrical due to difference in their maturity. Ovaries
are thin walled and may have egg masses.
J. Last but not least:
1. Cut open the top of your shark along the dorsal length to expose the
cartilage.
2. Cut open the top of the fish’s head to find the brain.
Names of dissection partners:_______________________________________
Spiny Dogfish Dissection Lab Questions
1. What is the sex of your shark? How did you determine the sex?
2. Describe the appearance of the liver.
3. Was there liver oil present? What is the purpose of the liver’s oil?
4. What is the function of the Pancreas?
5. What is the function of the Papillae located in the esophagus?
6. Cut open the stomach, what are the contents if any in the stomach?
Names of dissection partners:_______________________________________
Spiny Dogfish Dissection Lab Questions
1. What is the sex of your shark? How did you determine the sex?
2. Describe the appearance of the liver.
3. Was there liver oil present? What is the purpose of the liver’s oil?
4. What is the function of the Pancreas?
5. What is the function of the Papillae located in the esophagus?
6. Cut open the stomach, what are the contents if any in the stomach?
7. What does the inside of the stomach look like?
8. What is the function of the many undulations or folds of the stomach called Rugae?
9. What is the function of the spiral valve?
10. What is the mesentery and its function?
11. What is the candle located in the uterus?
12. Observe the cartilage. Is it flexible? What is the color? Is the cartilage in sections or is it one long piece?
7. What does the inside of the stomach look like?
8. What is the function of the many undulations or folds of the stomach called Rugae?
9. What is the function of the spiral valve?
10. What is the mesentery and its function?
11. What is the candle located in the uterus?
12. Observe the cartilage. Is it flexible? What is the color? Is the cartilage in sections or is it one long piece?