Cartilaginous fish, like sharks and rays, swim using a combination of body and fin movements, and crucially, they rely on an oil-filled liver to help control their buoyancy.
Here's a breakdown of how they swim:
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Body and Fin Propulsion: Cartilaginous fish primarily use lateral undulations (side-to-side movements) of their body and tail to generate thrust. The shape and flexibility of their body, coupled with powerful muscles, allow them to propel themselves through the water. Their fins play various roles in maneuvering, stability, and braking. For example:
- Pectoral fins: These fins, located on the sides of the body, are used for lift, steering, and braking.
- Pelvic fins: Situated near the rear of the fish, they provide stability.
- Dorsal fins: These fins, located on the back, help prevent rolling and provide stability.
- Caudal fin (tail fin): The caudal fin provides the primary thrust for swimming. The shape of the caudal fin can vary depending on the species and its swimming style. Some sharks have a crescent-shaped (lunate) caudal fin for fast, continuous swimming, while others have a more rounded fin for maneuverability.
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Buoyancy Control with an Oil-Filled Liver: Unlike bony fish, cartilaginous fish lack a swim bladder, an air-filled sac that provides buoyancy. Instead, they have a large liver filled with oil, specifically squalene. This oil is less dense than water, helping to offset the density of their cartilaginous skeleton and other tissues. This oil-filled liver helps to:
- Reduce sinking: The oil lightens the shark's body, preventing it from sinking rapidly.
- Conserve energy: By reducing the need to constantly swim to stay afloat, the oil-filled liver helps sharks conserve energy.
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Dynamic Lift: In addition to the oil-filled liver, some cartilaginous fish, especially sharks, rely on "dynamic lift." This means they must keep swimming to avoid sinking. The shape of their pectoral fins, acting like airplane wings, generates lift as they move through the water.
In summary, cartilaginous fish swim using a combination of body and fin movements for propulsion and maneuvering, while their oil-filled liver plays a crucial role in buoyancy control, reducing the energy expenditure required to stay afloat.
