Here's why:
* Ram Ventilation: Sharks and rays use a method called ram ventilation to breathe. This means they constantly have to swim with their mouths open to force water over their gills. The water flow carries oxygen into the gills, allowing them to breathe.
* Lack of Operculum: Unlike most fish, sharks and rays lack an operculum, the bony flap that covers the gills. This means they can't actively pump water over their gills like other fish.
* Sinking: If a shark or ray stops swimming, they will sink to the bottom of the ocean. This stops the water flow over their gills, preventing them from getting oxygen.
While some sharks are known to rest on the ocean floor, they typically still use a minimal amount of muscle movement to ensure a slight flow of water over their gills.
It's important to note that other fish might experience difficulty breathing if they stop swimming for extended periods, but they won't necessarily suffocate immediately.