Most Sharks and Rays are negatively buoyant. Means, if they don’t swim, they sink.

Sharks and Rays lack what most bony fish have, a swim bladder.
Sharks rely on lift generated by their large dorsal fins, however this lift would not be enough on its own to support the animal with the size fins they have (unless moving faster), a lot of the sharks lift comes from its relatively big liver. A massive organ that is containing oil which is lighter than water and therefore floats, makes sharks slightly negatively buoyant.
This liver as opposed to a swim bladder means that sharks’ bodies are incompressible allowing them to move between very deep water and the surface with ease.

Contrary to popular belief sharks don’t have to swim constantly or they die, however they do have to swim to avoid sinking to the bottom. Also, sharks do not have gill covers, most cannot pump water over their gills and must move to breathe.

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