READ THOROUGHLY BEFORE YOU BEGIN YOUR DISSECTION!!
Shark Dissection
There are several reasons why we dissect sharks. Sharks are model vertebrates in a sense. We can lean
a lot, even about human anatomy, by dissecting a shark; many of the same systems are present, and it is
very easy to dissect a shark and see a lot of organs displayed.
Sharks are fish belonging to the Phylum Chordata, Subphylum Vertebrata, and Class Chondrichthyes.
Their skeletons are made of only cartilage. Bony fish (same phylum and subphylum, but Class
Osteichthyes) have skeletons of mostly bone and some cartilage. Bony fish also have swim bladders
enabling them to regulate their buoyancy (allowing them to float at various depths under water). Sharks
have no swim bladder. Sharks have oil, called squaline, mostly concentrated in the liver which gives
them a little buoyancy-but not enough. They are somewhat heavier than the water they displace;
therefore, once a shark ceases to move, it sinks. Coastal species rest on the sea floor in the shallow
water; however, the sharks of the deeper oceans must continue moving from the moment of birth to
the moment of death! If they were to stop swimming, they would sink and be crushed by the pressure
of the deep below.
Regulation of osmotic pressure in marine sharks differs from that of bony fish also. They retain a high
concentration of urea and other solutes in their body fluids, a concentration of salts higher than that in
the surrounding sea water. There is therefore no need for sharks to drink like bony fish do.
Fertilization is internal and most shark “pups” hatch internally and continue their development within
the uterus of the mother. After a period of gestation (up to 2 years in the dogfish) they are born alive as
a smaller version of the adult. This method of reproduction is “ovoviviparous.” The number of pups in a
litter varies from 2 to 60. Some sharks are oviparous, laying large eggs enclosed in shells, or egg-cases,
consisting of a hornlike material. They are usually flat and square-ish with long tendrils which serve to
anchor the eggs.
We will be dissecting the common dogfish, also known as the spiny dogfish. Its scientific name is
Squalus acanthius- it was described by Linnaeus in 1758. The species name acanthius refers to its
slightly poisonous spins, one in front of each dorsal fin. The spiny dogfish only gets to be about 3.5 feet
long and about 15 lbs. You will notice that this shark has no anal fin.
This is one of the most common sharks in the world. It occurs all over the world basically in temperate
waters from depths ranging from 50-900 meters. It’s a demersal shark, meaning it lives just above the
ocean floor. It’s a voracious eater, feeding on fish, crustaceans, squid, gastropods, jellyfish and even red
and brown algae. It is omnivorous.
It’s also the most commercially fished of any shark species for its meat and liver oil. It is also well known
as a pest as it is disruptive to fishing operations by destructing fishing gear-hook and line, nets, it even
eats the fisherman’s catch.
READ THOROUGHLY BEFORE YOU BEGIN YOUR DISSECTION!!
GENERAL DISSECTION HINTS
The term “dissection” means more than merely cutting your specimen apart. It is a refined method of
seeking, exposing, identifying, and studying the internal anatomy. It helps to bring into view structures
not readily seen.
A cartilaginous skeleton as found in the sharks offers certain advantages. It permits penetrating a tissue
that is ordinarily hard and bony. This requires a certain amount of skill, for cutting into the cartilage can
damage nerves and other structures irreparably. The technique of slicing think chips while holding the
scalpel horizontally must be practices. A slip of the blade may undo hours of careful work.
Use your scalpel sparingly. In the hands of a novice a scalpel can do irreparable damage to your
specimen. Blood vessels and nerves may be cut, organs removed, delicate structures destroyed without
realizing the extent of the damage caused. Improper initial dissection will render the later study of parts
very difficult.
Rely more heavily upon your dissecting needles and even your fingers. They are especially helpful in
separating muscles, in tracing blood vessels and nerves, and in clearing away connective tissue that
binds structures to one another.
When using your scissors, advance with the rounded, blunt end, not the sharp, pointed end. Your
forceps should be strong, able to hold on to thick muscle, yet fine enough to grasp narrow nerves. Move
organs aside with your fingers or with a blunt probe.
Your animals are preserved in a solution which may irritate your hands and eyes. An apron will protect
your outer clothing and it is suggested that you wear rubber gloves to protect your hands. Line your
dissection pan with paper towels in order to absorb excess fluids, as a storage for structures removed,
and to facilitate cleaning up at the close of the session.
Observe the dissections of other students in the class. Often a better preserved, a better injected, or a
larger specimen may reveal structures not seen in your shark. This is especially true in the study of the
urogenital system. If your animal is a male, observe the reproductive structures of a female specimen
and vice versa. You are responsible for learning the reproductive structure of both male and female
sharks.
At the end of each session the sharks are returned to the plastic bag. Twist the top of the bag and close
tightly with a rubber band. These procedures will prevent the drying out of your specimen between
dissections. Wrap any structures removed in paper towels and place in the bag with your shark.
Remove the paper towels lining the dissection pan and discard.
READ THOROUGHLY BEFORE YOU BEGIN YOUR DISSECTION!!
Anatomical Terminology
Some basic biological terminology should be studied at this time. Familiarize yourself with the
following words and learn to use them in referring to the location of the body parts of your
specimen.
Directions or positions of fish
Anterior (cranial) – toward the head
Posterior (Caudal) – toward the tail
Dorsal (superior) – toward the backbone
Ventral (inferior) – toward the belly
Lateral – toward the side
Medial – toward the midline
Proximal – lying near the point of reference
Distal – lying further from the point of reference