Dominic Stone
Sharks are cartilaginous fish with two types of muscles: red and white. Red muscle is aerobic, requiring oxygen to function, and is used for sustained swimming. White muscle, on the other hand, is anaerobic and enables short bursts of speed. The shape of a shark's body, with its fusiform or torpedo-like structure, also contributes to its swimming efficiency. The back-and-forth motion of the head creates varying pressure areas, with the tail playing a crucial role in providing extra motion. The tail's ability to move quickly is facilitated by the presence of cartilage instead of bones, allowing for faster propulsion through the water. The skin of a shark, covered in dermal denticles, further enhances its swimming prowess by reducing surface drag.
| Characteristics | Values |
|---|---|
| Muscle types | Red and white |
| Red muscle function | Breaking down fat in the shark's body |
| Red muscle oxygen requirement | Yes |
| Red muscle use | Cruising |
| Red muscle blood supply | Good |
| White muscle function | Using energy from the breakdown of glycogen (sugars) |
| White muscle oxygen requirement | No |
| White muscle use | Short fast sprints |
| Muscle contractions | Produce faster speeds |
| Muscle fibre direction | From the top of the shark's head to the tip of its tail |
| Muscle fibre contraction | Produces a series of movements along the body |
| Muscle fibre contraction purpose | To propel the shark through the water with its tail |
| Muscle fibre movement | Back and forth |
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What You'll Learn
Red and white muscles
Sharks have two types of muscles that enable movement: red muscle and white muscle. The red muscle is used for slow, continuous swimming, or "cruise control", and is aerobic, meaning it requires oxygen to function. It contains myoglobin, an oxygen-carrying pigment. The white muscle, on the other hand, is used for fast, sudden bursts of speed and is anaerobic, meaning it does not require oxygen.
The red muscle in sharks is positioned deeper than the white muscle and is located anteriorly, attaching to tendons that the surface-level red muscle fibres in non-thunniform fishes cannot reach. This arrangement is similar to that of tunas and lamnid sharks, which have demonstrated a remarkable convergence in anatomy, swimming mechanics, and thermal sensitivity of red muscle contraction. In thunniform swimmers, such as shortfin mako sharks and yellowfin tuna, the red and white muscles contract out of phase with each other, with the red muscle shortening and lengthening lagging the white muscle by 15-20% of a tail beat cycle. This unique physical feature is known as the hallmark of thunniform swimming.
The relative amounts of red and white muscle in sharks are related to their cruise swimming speed. Deep-sea sharks, for example, have a higher proportion of white muscle, which suggests they have a high burst capacity but are slow and listless swimmers. Conversely, shallow-water sharks have a higher proportion of red muscle, which correlates with higher cruise swimming speeds.
The red muscles in sharks also play a role in heat retention. Sharks have a modified circulatory system that helps retain heat in the red muscles. Additionally, the rigid walls of the pericardium, the membranous sac that encloses the heart, create suction to maintain blood flow. Many shark species, such as the sandbar shark, must swim continuously to circulate blood throughout their bodies due to their low blood pressure.
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Muscle contractions
Sharks have two types of muscle: red and white. Red muscle is aerobic, meaning it requires oxygen to function, while white muscle is anaerobic and does not need oxygen. Red muscle works by breaking down fat in the shark's body and has a good blood supply, helping the shark swim for extended periods. In contrast, white muscle utilises energy from the breakdown of glycogen (sugars) to enable short, rapid sprints when catching prey or evading threats.
Long muscle fibres run from the top of a shark's head to the tip of its tail. When these fibres contract, a sequence of movements are generated along the body, propelling the shark through the water with its tail. More noticeable muscle contractions result in faster speeds. However, to conserve energy, sharks will first build up speed through a series of muscle contractions and then stiffen their bodies to maintain momentum as they cruise through the water.
The shape of a shark's body and tail also play a crucial role in its ability to move swiftly and efficiently through the water. Faster shark species like the Shortfin Mako tend to have shorter, half-moon-shaped tails, while slower species like the Broadnose Sevengill Shark have longer, thinner tails. The shark's body is fusiform or torpedo-shaped, with a cylindrical form and narrowing edges, further minimising swimming effort.
Additionally, sharks possess flexible cartilage instead of bones, allowing them to move up and down in the ocean with minimal exertion. This cartilage flexibility enables them to bend more easily than bony fish, and their tails can achieve faster movements for propulsion. The shark's skin is another factor contributing to its speed and agility in the water. It is covered with millions of tiny teeth-like structures called dermal denticles, which point backward to reduce surface drag and enhance swimming speed.
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Movement and speed
The body design of sharks has been honed over hundreds of millions of years to increase swimming performance. Their body form, fins, and skin work together, enabling sharks to slice through water and execute complex manoeuvres with speed and precision.
Sharks are streamlined swimmers. Their pectoral fins on either side of their bodies are positioned and shaped to help them manoeuvre in all planes, often at "lightning" speed. The tail, or caudal fin, is perhaps the most critical tool for rapid forward locomotion. For most sharks, the tail is heterocercal, meaning it is characterised by an upper lobe that is larger and longer than the lower lobe. Because the tail is not symmetrical, it torques, or twists, the body of the shark as it beats. By controlling the motion in the two lobes of its tail, the shark can adjust the angle of propulsion. The tail creates two connected rings of moving water that help make this fish such a powerful swimmer.
Sharks also have two sets of paired fins on the sides of their bodies, in the same general position as the main wings and horizontal tail wings of a plane. The shark can position these fins at different angles, changing the path of the water moving around them. When the shark tilts a fin up, the water flows so there is greater pressure below the fin than above it, creating upward lift.
Sharks have a unique circulatory system that retains heat in the red muscles. They have low blood pressure. To circulate blood throughout their bodies, many sharks must swim continuously. Water enters the gill chambers through the mouth or spiracles and exits through the gill slits. Muscular contractions are needed to circulate their blood.
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Cartilage and flexibility
Cartilage is a flexible connective tissue that is found throughout the body. It is a crucial component of the skeletal system, providing support and flexibility to various body parts. There are three main types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage. Each type has unique characteristics and serves specific functions in the body.
Hyaline cartilage is the most common type of cartilage in the human body. It has a smooth, slippery surface and is pale blue-white in colour. Hyaline cartilage is composed of closely packed collagen fibres, making it tough yet slightly flexible. It is found in areas such as between the ribs, in the nasal passages, and in the trachea. One of its primary functions is to facilitate smooth movement by allowing bones and tissues to slide past each other with ease.
Elastic cartilage is the most flexible type of cartilage. It is responsible for supporting body parts that require a wide range of motion. This type of cartilage is capable of bouncing back to its original shape, even after enduring strong forces. Elastic cartilage is commonly found in the outer ear, the epiglottis, and the walls of blood vessels. Its flexibility and resilience make it well-suited for maintaining the shape of these structures.
Fibrocartilage, on the other hand, is the strongest and least flexible type of cartilage. It is composed of thick fibres that provide toughness and durability. Fibrocartilage is responsible for holding body parts in place and absorbing impacts. It is commonly found in the meniscus of the knee, the intervertebral discs of the spine, and in supporting muscles, tendons, and ligaments throughout the body.
The presence of chondrocytes, which are cells that reside within the cartilage matrix, is essential for the formation and maintenance of cartilage. The matrix itself is composed of fibrous tissue and various combinations of proteoglycans and glycosaminoglycans. Interestingly, cartilage does not have nerve innervation, which means that there is no sensation of pain when it is injured or damaged. However, cartilage injuries can lead to long-term issues such as osteoarthritis, causing pain and inflammation due to the loss of cushioning and lubrication in the joints.
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Conserving energy
Sharks are negatively buoyant and must swim continuously to generate enough lift over their stiff pectoral fins and body. This swimming also helps circulate blood throughout their bodies due to their low blood pressure, which requires muscular contractions to move the blood.
As obligate ram ventilators, shark gills must also extract enough oxygen to meet their metabolic needs. Coastal regions with updrafts could be very beneficial for these animals as they could provide lift and reduce the amount of energy the sharks have to spend swimming. For example, at the atoll of Fakarava in French Polynesia, grey reef sharks benefit from the updrafts in the tidally flushed channel at the south end of the atoll. During the day, they can be seen swimming with minimal effort, and they only hunt intensively at night.
Sharks may also conserve energy by selecting habitats that require less energy to move through or reside in. For instance, they may choose to reside in areas with strong currents that can carry them, allowing them to reduce their energy expenditure by 10-15%.
The climate crisis, however, is causing sharks to deviate from their normal paths and venture into unfamiliar, challenging territories. Warmer ocean temperatures increase their metabolic rates, causing them to expend more energy on essential activities like digestion and swimming. This increase in metabolic rates may lead to more frequent feeding as they absorb fewer nutrients. Unfortunately, warming oceans have also driven fish populations northward to cooler waters, reducing the availability of food for sharks.
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Frequently asked questions
No, shark muscles do not shiver. Sharks have two types of muscle: red and white. Red muscle works by breaking down fat in the shark's body and has a good blood supply, which helps the shark swim for long periods. White muscle works by using energy from the breakdown of glycogen, which provides short bursts of speed.
Long bundles of muscle fibres run from the top of a shark's head to the tip of its tail. When these contract, a series of movements are produced along the body, propelling the shark through the water.
Sharks use their fins for balance and stability, and their tails to propel themselves forward. The back and forth motion of the shark's head creates high and low-pressure areas in the rest of the body, which helps the shark move with minimal muscle usage.
