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Marine Biology--Mr. Nelson
Phylum Chordata, Class Chondricthyes:
Dogfish Shark Dissection
INTRODUCTION:
The very mention of the word shark has, from ancient times, instilled within man an
almost irrational fear. Yet, at the same time, the cunning, strength, tenacity, and grace of
movement of the animal have never ceased to fascinate. Biologically, sharks are fish belonging
to the Phylum Chordata and the subphylum Vertebrata. However, sharks and their relatives,
the rays and skates, are unique amongst fish in that their skeletons are made entirely of
cartilage, not bone. This places them in the Class Chondrichthyes, Subclass Elasmobranchii.
They are divided into nineteen families, with five families making up 75 per cent of the known
species.
Fertilization is internal, and most shark "pups" hatch internally, to continue their
development within the uterus of the mother. After a period of gestation (up to two years in
the spiny dogfish, Squalus acanthias, the longest of any vertebrate!) they are born alive as a
smaller version of the adult. This method of reproduction called ovoviviparous. The number
of pups in a litter varies from two in some species to sixty in others. Some sharks are
oviparous, laying large eggs enclosed in shells, or egg-cases, consisting of a horn-like material.
They are usually flat and quadrangular shaped with long tendrils which serve to anchor the
eggs to seaweed or other objects.
While there are close to 20,000 living species of fish, there are approximately 250 to 350
species of sharks worldwide and only 12 species have been known to attack man. Their size
ranges from the 6-inch dwarf shark all the way up to the 60 foot plankton feeding whale shark
weighing over ten tons. Contrary to popular belief, the two largest of sharks are quite
inoffensive beasts, deriving most of their nourishment from planktonic animals (those which
float in the upper layers of the sea).
SPINY DOGFISH SHARK
The spiny dogfish, scientific name Squalus acanthias, of the family Squalidae, is our
dissection specimen. The species name "acanthias" calls attention to the animal's mildly
poisonous spines, one in front of each dorsal fin (both should have already been removed from
your shark). It is a relatively small shark, attaining about 3 feet in length and weighing about
15 pounds. The absence of an anal fin is characteristic of the entire family.
It is distributed worldwide, from the temperate to the subpolar regions, from the shallow
waters of the sea shore to depths of 100 fathoms (600 feet).
They are voracious eaters, feeding on fish, crustaceans, squid, gastropods, jelly fish, and
even red and brown algae. The spiny dogfish, as most other sharks, is omnivorous, devouring
both plant and animal matter.
It is an abundant species. On this side of the Atlantic it is infamous for its disruptive
activities to fishing operations. It is destructive of fishing gear; hook and line, nets are bitten
and their catch devoured and freed. This results in a high animal loss to the fishing industry.
Up until the early 1990’s, except as laboratory specimens, no economic use had been found for
them and, as a result, they were very inexpensive. As humans have looked to the ocean for
more food sources, dogfish shark meat has become a sought after catch to supply European fish
and chips. As a result, dogfish are now a regulated fishery and the price for our specimens is
now about $15 each!
External Anatomy
Procedure:
Obtain a shark from Mr. Nelson and rinse off any excess preservative in the sink.
Use the diagram below to label the following structures:
spiracle, anterior dorsal fin, posterior dorsal fin, nostril,
external gill slits, pectoral fin, pelvic fin, dorsal lobe of the
caudal fin, ventral lobe of the caudal fin, lateral line system
1. Take a look at the general body shape of the shark. Describe its shape in
your own words.
2. How many total fins does it have?
3. Placoid scales make up the skin of the shark. Rub your hand across the skin of the shark,
first in one direction and then in the opposite direction. Describe what you feel in each
direction.
4. These placoid scales feel like sandpaper and are very different from bony fish scales. First of
all they don't overlap like the bony fish, and they are pointed and bent backwards. The scales
are made of the same type of dentine material the teeth of the shark are made from.
5. Cut off a small piece of skin. Scrape any muscle that may remain attached to the interior
surface. Place this piece on a slide and look at it using the dissection microscope. On your
Dogfish Shark Dissection Sketches, draw one of the placoid scales.
6. Based on what you just observed, why do shark experts advise you to not
punch or slap the skin of a shark if you are being attacked?
7. Locate the mouth. The position of the mouth is underneath the shark's snout as opposed to
being at the end of the body like a bony fish. Open the mouth but be careful of the teeth they
are sharp. How many rows of teeth do you see?
8. The teeth are not fused with the jaw and the upper jaw is not fused to the skull. This allows
the shark to jut its mouth forward when feeding and the snout doesn't get in the way. The
teeth are pointed scales (both are made of the same material) which are produced in rows and
when they fall out the next row just moves forward. Have Mr. Nelson come to your bench and
do a little demonstration with the teeth. Which way do the teeth point?
9. The shark feeds by grabbing the prey, and shaking its body sideways. In addition, the prey
will try to escape from the mouth. Based on your answer above, why do you think
this design is beneficial?
10. Locate the external gill openings (gill slits). How many does it have?
11. Respiratory System: The water goes through the shark's mouth and over the gills where
oxygen and carbon dioxide are exchanged within the blood of the shark. Locate the spiracle, a
small opening located behind the eyes. The spiracle opens into the gills and is used to breath
while the dogfish it eating. List the two paths that water can take to pass over the
gills.
12. Cut out a portion of a gill. Describe its appearance.
13. How does the structure of the gills beneficial to their function?
14. Now locate the fins of the shark. Look at the caudal fin of the shark. Its shape is called
heterocercal, which means its has 2 unequal lobes. Using researched pictures, how does
this shape compare to a typical bony fish tailfin?
15. Determine the sex of your shark. If male, you should locate the male claspers. The male
transfers sperm through the grooves of these special structures to the female's reproductive
tract. The female does not have these claspers. What is the sex of your dogfish?
16. Find another group who has a shark of a different sex. Describe the
difference?
17. The shark also has several sense organs which aid it in locating food. Hearing in the shark
is very good; they have small openings located on either side of their head and can hear
sounds, such as splashing or thrashing, up to thousands of yards away. On the side of the fish
locate the lateral line. This structure picks up low frequency vibrations (like a fish injured or
splashing sound). These are pitted structures that have direct nerve contact with the brain, and
can pick up sounds several hundred feet away. On the side of the head are the eyes of the shark
which are used at a closer range, up to 50 feet away. The eyes are very well developed but the
clarity of the water determines their visibility. The eyes have a reflective surface, in the back of
the eye, which helps them see better at night. Upon attack they will roll their eyes back up into
their eye socket, preventing any damage. Their nostrils provide a keen sense of smell. It has
been said that sharks can smell a small amount of blood hundreds of yards away in the ocean
(1 part blood in a million parts of water!). Another important structure is the small pitted holes
on the head, called the ampulla of Lorenzini. These are electroreceptor structures and are used
to pick up the electrical field emitted by all living organisms even at the close range of several
inches. The last sense used is touch, when the shark comes in contact with the prey.
18. In the chart on the next page, fill in the missing distance or sense structure
used by the shark to locate its prey.
Sense chart used by the shark to locate prey
Stimulus
Distance
Splashing
50 feet
Blood
200 FEET
Thrashing water
1000 yards
Sense(s) Used
Rubbing up against the shark
EYES
Visual sighting
Close electrical vibrations of
prey
2 feet
Internal Anatomy of Shark
19. The internal dissection of the shark is relatively simple since the structures are easy to
locate. It can be a little smelly, especially if you cut too deep and puncture the liver. Put the
shark with its ventral side up and proceed to cut down from the pectoral girdle (where the gills
are) down the midline of the shark to the anus. (Tip, don't cut deeper than the muscular wall
by one-quarter of an inch or you'll damage the liver). Now make a horizontal incision to each
side of the midline near the anal area and the pectoral girdle. Cut off these flaps and discard
them in the barrel. Look at Figure 21-5, on your lab bench, for reference.
20. The first organ you'll see is the three-lobed liver which is the largest organ in the shark's
body and is very oily. Since they lack a swim bladder, the shark's liver, with its great amount of
oil, helps the shark maintain buoyancy. In the middle lobe locate the greenish gall bladder
which stores the bile from the liver for later use in the small intestine. The bile is transported
through the bile duct. The gall bladder is not a vital organ, my mother had her’s removed.
Show Mr. Nelson your shark’s liver and gall bladder. He will make a notation on your
Dogfish Shark Dissection Sketches document.
21. Remove the liver by cutting as far as possible (towards the head) where the ligament
connects it to the body. Do not cut into the liver! Now the other organs should be more
visible.
22. Digestive System: We've already located the shark's mouth where the food enters, and the
teeth for shredding. Now let's locate the esophagus, the tube leading down to the stomach. The
first portion of the stomach is called the cardiac stomach where the food first enters. Where it
makes a bend you have the pyloric stomach which leads into the small intestine. Cut the
stomach open and examine it. Is there any food inside? Can you ID that food?
23. What does a Dogfish eat?
24. Now look at the lining of the stomach, describe what it looks like. Why
does it have this appearance?
25. Now take a look at the small intestines. This is where the food is absorbed into
bloodstream. Cut the small intestine open along a section and notice the spiral layers of
membrane. This is called the spiral valve. Why does the small intestine have these
structures?
26. Finally, any food not absorbed goes into the colon (the large intestine) and exits out the
cloaca. The cloaca collects the products of the colon as well as the reproductive system. This
catch-all basin leads to the outside by means of the cloacal opening. The Latin word, “cloaca”
means “common sewer”. Why is this name appropriate?
26a. Unique Organ; The rectal gland. Functions in osmoregulation (remember that sea water
is hypertonic). It is a slender, narrowed, finger like structure that opens into the intestine via a
duct (near cloaca). It has been shown to excrete salt (NaCl) in concentrations higher than that
of the shark’s body fluids or sea water.
27. When your group is ready to point out and discuss the following
structures, call Mr. Nelson over and he will quiz you on location and function.
He will then mak a notation on your Dogfish Shark Dissection Sketches
document.
Stomach
Small intestine (duodenum and valvular intestine)
Large intestine (Colon)
Cloaca
Rectal gland
28. Circulatory System: The shark has a closed system with a two-chambered heart
(see Figure 2l-6) like the bony fish. The large ventricle pumps the blood out of the heart
through the conus arteriorsus to the gills and then to the body. The smaller atrium receives the
blood from the body through the backside collecting area called the sinus venouses and pumps
it into the ventricle. The blood is pumped from the heart to the gills, then the body, and back to
the heart again. This is very different from ours which has two of the four chambers involved
in pumping blood back and forth to the lungs. The other two chambers pump blood back and
forth from the body.
29. Cut out the sharks heart and make a cut through it lengthwise. Based on
each chambers function, why is the ventricle more muscular than the atrium?
30. Have Mr. Nelson come over to your group to show you a 4 chambered
heart, similar to yours. He will then make an annotation on your Dogfish Shark
Dissection Sketches document.
31. An accessory organ in the circulatory system is the spleen. It stores extra
blood, among other things. Locate the spleen and point it out to Mr. Nelson.
He will then make an annotation on your Dogfish Shark Dissection Sketches
document.
32. Reproductive System: The sexes are separate. Sharks can reproduce by three methods,
depending on the species of the shark. The dogfish is ovoviviparous, which means they retain
their eggs in the body of the female and consume the yolk sac. In some species, the embryos,
hatched internally (like in the dogfish) will continue to feed by eating the unfertilized eggs as
well as other developing embryos. In some species that are not cannibals, they absorb nutrients
from a creamy secretion produced by the uterine wall. Some sharks are oviparous, egg laying,
like the horrishark, and they lay them in a egg case which is deposited outside the body. These
egg cases can occasionally be found on the beach. The cases are hard and leathery. The famous
"mermaids purse," is an example of one from the skate group. The third method of
reproduction in sharks is called viviparous, which means "live bearing." These animals nurse
their young via a "yolk sac placenta" inside their body until they are ready to be born. This
method of reproduction is very similar to our own and is done by the hammerhead, basking,
and blue sharks. Which of the above methods would likely give its offspring a
better chance of survival? Why?
32. Find the reproductive organs of your shark. Point them out to
Mr. Nelson. He will then make an annotation on your Dogfish Shark
Dissection Sketches document.
33. Nervous System: The shark's brain is located on the dorsal side. To look at the brain you
must flip the shark over. Make careful incisions around the (skull) until you expose the brain.
If you wish, use Figure 21-7 of the shark's brain for reference. You might be able to identify the
parts, named in the diagram below, on your shark.
Analysis Questions:
1. Describe two examples in the Dogfish where structure is related to function?
a.
b.
2. Sharks are sometimes referred to as “eating machines”. Describe 3 adaptations sharks have
that makes them super predators.
a.
b.
c.