Bony Fish (Osteichthyes)
* Skeleton: Have a bony skeleton, which is lighter and more flexible than cartilage.
* Swim Bladder: Possess a gas-filled sac called a swim bladder that helps with buoyancy control. By adjusting the amount of gas in the bladder, they can maintain their position in the water column with minimal effort.
* Tail Fin: Typically have a homocercal tail fin (symmetrical lobes) that provides powerful thrust for forward movement.
* Other Fins: Utilize a variety of paired fins (pectoral, pelvic) and unpaired fins (dorsal, anal) for maneuvering, stability, and braking.
* Movement: Move primarily by undulating their bodies and using their tail fin. This is efficient for sustained swimming.
Sharks (Chondrichthyes)
* Skeleton: Have a cartilaginous skeleton, which is heavier and less flexible than bone.
* No Swim Bladder: Lack a swim bladder, meaning they need to constantly swim to avoid sinking. This is why sharks are often seen moving.
* Heterocercal Tail: Possess a heterocercal tail fin (upper lobe larger than lower lobe) that provides both thrust and lift. This helps compensate for the lack of a swim bladder.
* Other Fins: Also use paired fins (pectoral, pelvic) and unpaired fins (dorsal, anal) for maneuvering.
* Movement: Swim with a more undulatory motion than bony fish, relying heavily on their tail fin and pectoral fins for propulsion.
Key Differences
* Buoyancy: Bony fish are neutrally buoyant thanks to their swim bladder, while sharks are negatively buoyant and must constantly swim to stay afloat.
* Tail Fin: Bony fish have homocercal tails, while sharks have heterocercal tails.
* Skeleton: The lighter bony skeleton of bony fish allows for greater flexibility and energy efficiency in swimming compared to the heavier cartilaginous skeleton of sharks.
In summary, bony fish and sharks have evolved different adaptations for swimming, reflecting their distinct evolutionary paths. Bony fish utilize a swim bladder for buoyancy and a more efficient tail fin for propulsion, while sharks rely on continuous swimming and a unique tail fin for both lift and thrust.