Bony fish have a swim bladder, and many cartilaginous fish do not have a swim bladder.

Bony fish have a swim bladder, and many cartilaginous fish do not have a swim bladder. Explain how many shark species swim up and down?

1) The swim bladder is absent in all cartilaginous fish.
2) Cartilaginous fish have a skeleton of cartilage that is lighter than the skeleton of bony fish, so they have neutral buoyancy. Thanks to this, they can easily ascend and dive with their fins. Some cartilaginous, nevertheless, lead a benthic life (rays). The first bony fish that did not yet have a swim bladder, due to their severity, had negative buoyancy and led a near-bottom lifestyle, some could briefly rise a little higher.