Members of class Chondrichthyes have to swim constantly to avoid...

Members of the class Chondrichthyes (cartilaginous fishes) lack an air bladder (swim bladder), which is a buoyancy-regulating organ found in osteichthyes (bony fishes).

Due to the absence of this air bladder, they do not have natural buoyancy and must swim constantly to avoid sinking to the bottom.

Answer: absence of an air bladder