During this section of the lab you will be observing the muscular system of the shark. They are vertebrates, and therefore have a very developed and intricate muscular system. This allows them to have refined movement. Skeletal, cardiac, and smooth muscles and general muscle anatomy are some of the components of musculature. Muscles are the foundation of many functions including flexing or extending parts of the body, abducting and adducting, and rotating the body along its axis.

Skeletal muscles are a type of striated muscle, attached to the cartilaginous skeleton. They are used to facilitate movement by contraction. They generally contract voluntarily by nerve stimulation but they can also contract involuntarily. Skeletal muscles are made up of fibers. The cells are multinucleated and contract and relax quickly. Each skeletal muscle consists of numerous cells called muscle fibers. These muscle fibers are able to contract in any particular direction. They can do this because of contractile filaments. This is the reason why the myomeres can contract allowing the shark to thrust forward. The two types of skeletal muscles are somatic and branchial.

The somatic muscles are the muscles that are attached to the body axis and the appendages and as a result, are further subdivided into axial and appendicular muscles. The axial muscles form the majority of the shark's body muscle mass. This is understandable knowing that the dogfish uses most of its axial muscles to generate movement. Appendicular muscles form the muscles of the appendages. In most terrestrial vertebrates, the appendicular muscles form the bulk of the body mass as locomotion is derived from the limbs and also to support the weight of the body against the force of gravity. The branchial muscles are the muscles that are attached to the branchial arches.

Cardiac muscles are part of the circulatory system, which is the system responsible for the transportation of blood, and are restricted to the heart. As far as external appearances are concerned, cardiac muscles are similar to skeletal muscles in that cardiac muscle fibers also have well-defined striations; however, they are not under voluntary control.

Smooth muscles can be located in the many visceral organs and hollow tubes of the body including specific ducts of the urogenital system and blood vessels. Like cardiac muscles, smooth muscles are not under voluntary control, but their cells do not appear striated at the microscopic level. They do appear to have striations at the ultra structural level. This type of muscle is not related to the formation of discrete organs and is not easily dissected at a gross level.

To expose the axial muscles make a mid-dorsal skin incision that begins immediately caudal to the spine of the cranial dorsal fin and ends 10 cm caudal from the starting point. Make two vertical skin incisions on each end of the horizontal incision going towards the ventral side of the body, stopping at the ventral midline. Connect the two vertical incisions together by making another horizontal incision. Next, remove the rectangular piece of skin very carefully, ensuring that no muscle is removed. Also, remove an equivalent piece of skin just caudal to the posterior dorsal fin. The axial muscles make up the majority of the shark’s muscle mass as they are responsible for the thrust needed for the body’s locomotion through their aqueous environment. The next step is to clear the muscle that is now revealed from all extra tissues. When this is complete the direction of the muscle fibers will be clearly defined.

Axial muscles consist of many separate units called myomeres. The parts of the myomeres dorsal to the lateral line are the epaxial muscle mass and those ventral to the lateral lines form the hypaxial muscle mass. The prefix “epi” in epaxial means that these are the muscles on top and the prefix “hypa” in hypaxial means that these are the muscles on the lower side of the body. The myomeres are connected together by a tissue called myosepta. Notice that the myomeres are not straight but that they overlap each other. Also notice how the myomeres that are situated at the widest part of the body are darker in colour compared to those that lie close to the dorsal midline of the shark.

The myomeres are arranged in a pattern such that the fibers overlap each other, not in a straight pattern. Each myomere is made of thousands of small red and white muscle fibers. The red muscles fibers are situated much farther from the midline of the body compared to the white fibers. The red fibers are long and can shorten and hold for a long period of time and are far from the axis of bending of the body. The reason for the redness is because they have a high concentration of myoglobin. Myoglobin is a molecule that can bind oxygen and is present in muscles responsible for long, slow contractions. The white fibers have a low concentration of myoglobin, resulting in their lighter colour. The white fibers are shorter and can contract easily and quickly; these fibers are situated close to the axis of bending of the body.

The reason for the myomeres being situated in an overlapping pattern instead of a straight pattern is that the overlapping pattern is much more effective for mobilization. The muscles’ myomeres must be able to contract in such a way that the body and tail are able to thrust quickly and efficiently. In order for the shark to propel its way through the water it must move its tail from side to side.
The following step is to calculate the fineness ratio of the shark. To find the fineness ratio of a shark, divide the length of the shark (tip of rostrum to the caudal portion of the epichordal lobe) by the width of the shark (where the shark has a greater measurement laterally, exclude the cranial dorsal fin). An average fineness ratio is between 10 and 11. Any ratio above 4.5 will decrease resistance
Example:
Fineness ratio = length
width
= 62 cm
5.8cm
Fineness ratio = 10.7 cm

Further Information

Locomotion is essential to the shark in order to survive in its aqueous habitat. It allows the shark to manoeuver quickly in order to catch prey and escape from predators. In order for the shark to have efficient locomotion it requires four things: a system for generating thrust, mechanisms to maintain body position in the water, mechanisms to stabilize the body to create control while moving, and mechanisms to reduce resistance.

Thrust
Sharks have heterocercal tails, where the epichordal lobe is larger than the hypochordal lobe. The side-to-side movement of their tail generates the thrust required to move the shark throughout the water. When the tail strikes the water, the epichordal lobe strikes the water downward, while the hypochordal lobe strikes the water upward. The water acts equal but opposite and pushes the lobes in opposite directions. The net effect of the pushes and the tail’s rotation produces a balance forward thrust though the body’s center of gravity. This tail movement is made possible due to myomeres that contract on either side of the body

Mechanisms to Maintain Body Position
The average density of the shark’s body tissues is greater than that of the surrounding water. Thus, there is a sinking force that causes the shark to sink headfirst as it moves forward. The sharks compressed head and large flattened pectoral fins act as hydroplanes and generate lift as they move forward. Most fishes have gas bladders to raise and lower their density. Sharks don’t have these so they cannot lower their density. Their liver does aid in lowering their density by storing oils.

Stabilization and Direction Control
The shark has three different types of destabilizing forces that are necessary for efficient locomotion: roll, which causes the shark to rotate about it’s longitudinal axis; pitch, which causes the head to move up and down; and yaw, which causes the shark to move from side to side (left to right). The direction of the shark is mainly determined by the ability of the paired fins and tail.

Mechanisms to Reduce Resistance
To reduce resistance and drag there has to a minimized force between the water and the body. The aerodynamic body aids in reducing water resistance. Also the length of the body helps create less force along with the tightness of the skin and the length of the pectoral and pelvic fins. When the fineness ratio becomes greater than 4.5, resistance decreases. While cutting and teasing the skin of the musculature, you will notice that the skin is tightly attached to the underlying muscles. This close attachment reduces resistance and prevents the loss of thrust. The skin serves as an exoskeleton that receives the force of myomere contraction and facilitates flexion of the tail.