Q. How different are the swimming strategies in chondrichthyes and in osteichthyes? Why do sharks need to agitate their body to swim while bony fishes do not?

Bony fishes have a specialized organ called, swim bladder or a gas bladder, whose interior can be filled with gas liberated from gas glands. The swim bladder works the same as a hydrostatic organ since it varies the relative density of the body regulating buoyancy and the depth of the animal in water.

Chondrichthyes do not have swim bladders and thus they must continuously agitate their body to keep swimming to maintain their depth in water. As an additional swimming aid in chondrichthyes the liver is oily and big this feature helps to reduce their body density relative to water.