Cartilaginous Fish and Bony Fish Examples

All the true fishes have been classified into two main classes as follows:

• Chondrichthyes: These are the fishes with soft cartilaginous skeletons. Examples are sharks, rays, dog-fish, skates, sturgion
• Osteichthyes: These are fishes with bony skeleton. Examples are Tuna, Sardines, Codfish, Salmon, Trouts, Herring etc.

Fish are all aquatic animals and, as a result, have a hydrodynamic and elongated body suitable for moving underwater, without limbs and with fins. This habitat is also related to their branchial respiration.

Gas Bladder / Swim Bladder

Bony fish have a specialized organ called a gas bladder, or swim bladder, whose interior can be filled with gas released from gas glands. The swim bladder works as a hydrostatic organ, since it produces variations in the relative density of the body, thus regulating the buoyancy and the depth of the fish in water. Such swim bladders are not found in the Chondrichthyes. Due to this, they must continuously move their body to keep swimming and to maintain their depth in water. This is the reason that Sharks need to move their body to swim while bony fishes do not.

Further, when the swim bladder is filled with gas, it reduces the density of the body of the fish and, when it is emptied, this density is increased. As a result, this mechanism controls the depth of the fish under water.

Gills

Respiration in Fishes takes place via Gills, the highly vascularized organs specialized in gas exchange underwater. Apart from fishes, gills are also found in marine annelids, crustaceans, Molluscs, tadpole etc. Gills are covered in bony fishes while not covered in cartilaginous fishes.

Fish Heart

The Fish Heart has only two consecutive chambers called atrium and ventricle.

Excretion

Fish have Kidneys has excretory organs. While Bony fishes excrete ammonia, cartilaginous fishes excrete Urea as nitrogenous waste.

Lateral Line

Lateral lines of bony fish are sensory organs that extend along both sides of their body. They contact the environment by a series of specialized scales that transmit information about pressure variation and vibrations in the surrounding water.