“Sharks must keep swimming or they die”

The idea that all sharks must perpetually swim to survive is a pervasive misconception, often perpetuated in popular culture and dramatic portrayals of these marine predators. This belief largely stems from observations of highly active, open-ocean species, such as great white sharks, which are frequently depicted in constant motion. For a long time, scientists assumed that all sharks relied on a breathing method called "ram ventilation," where water is forced over their gills as they swim forward, much like air is rammed into a jet engine. The lack of operculum (a bony flap covering the gills in most fish) in sharks also contributed to the assumption that they needed continuous movement to respire.

However, scientific understanding has evolved, revealing a more nuanced reality of shark respiration. While some species, known as "obligate ram ventilators," do indeed need to keep swimming to breathe, many others employ a different technique called "buccal pumping." Buccal pumping involves using specialized muscles in their cheeks and mouth to actively draw water over their gills, allowing them to remain stationary while still extracting oxygen. Bottom-dwelling sharks, like nurse sharks and angel sharks, often utilize buccal pumping, sometimes aided by spiracles—small openings behind their eyes—to draw in water even when buried in sand or resting on the seafloor.

The widespread belief in the "always swimming" myth is likely due to the iconic status of large, fast-swimming sharks that are obligate ram ventilators, such as great whites, mako sharks, and whale sharks. These species are often the focus of documentaries and media, leading to the generalization of their unique respiratory needs to all sharks. The dramatic image of a shark that ceases to move and therefore dies is a powerful one, but it overlooks the remarkable adaptability and diverse breathing strategies found across the more than 400 known shark species. In reality, most sharks are capable of buccal pumping, and some, like lemon sharks and tiger sharks, can even switch between both methods depending on their activity level and environmental conditions.