Sharks are often observed in constant motion, a behavior linked to fundamental biological processes. This continuous movement in certain species is not merely for locomotion but is essential for their survival in the aquatic environment.
Breathing Underwater: The Role of Continuous Movement
One primary reason some sharks must move continuously is for respiration. Unlike many bony fish, certain shark species rely on a method called ram ventilation. This method involves swimming forward with their mouths slightly open, forcing water over their gills. As water passes across gill filaments, oxygen diffuses into the bloodstream, allowing the shark to breathe.
Sharks that depend solely on ram ventilation are known as obligate ram ventilators. Species such as great white sharks, mako sharks, and whale sharks exemplify this respiratory strategy. For these sharks, a continuous flow of water is essential for oxygen uptake; if they stop swimming, the water flow ceases, and they risk suffocation. This constant need for movement shapes their behavior.
Other shark species, however, possess a different respiratory mechanism called buccal pumping. These sharks can actively pump water over their gills using muscles in their mouth and pharynx, even while stationary. This allows them to rest on the seafloor or in crevices without needing to move to breathe. The presence or absence of buccal pumping capabilities is a key factor in determining a shark’s need for continuous motion.
Staying Afloat: How Sharks Combat Sinking
Beyond respiration, continuous movement also assists sharks in maintaining their position in the water column. Unlike most bony fish, sharks do not possess a swim bladder, an internal gas-filled organ that helps regulate buoyancy. Without this buoyancy aid, sharks are naturally denser than water and would sink if they remained still.
To counteract this negative buoyancy, sharks rely on several adaptations. They have a large, oil-rich liver that can constitute a significant portion of their body weight, sometimes up to 25% of their mass. The oils within the liver are less dense than water, providing some lift. However, this internal buoyancy is typically insufficient to keep them suspended without additional effort.
Forward motion generates dynamic lift, much like the wings of an airplane. The unique shape of a shark’s body, particularly its pectoral fins, acts as a hydrofoil, creating upward lift as water flows over them during swimming. This continuous forward momentum allows them to stay suspended and navigate through various depths.
Beyond the Myth: Not All Sharks Move Constantly
The popular image of a shark constantly in motion is not universally accurate. While many open-ocean sharks are obligate ram ventilators, a substantial number of species can remain stationary, a distinction based on their respiratory mechanisms.
For example, nurse sharks, wobbegongs, and some carpet sharks are well-known for their ability to rest on the seabed. They can often be observed lying still in caves or on the ocean floor, using their buccal muscles to breathe. This adaptation allows them to conserve energy and inhabit environments like coral reefs or rocky seafloors where remaining still might be advantageous for ambush predation or resting.
Conversely, large, fast-swimming pelagic sharks like great white sharks, mako sharks, and whale sharks are classic examples of obligate ram ventilators. These species primarily inhabit open ocean environments where constant movement is natural for hunting and migration. Their reliance on ram ventilation means they generally cannot stop swimming without compromising their ability to breathe.
What Happens When a Shark Stops Moving?
For sharks that are obligate ram ventilators, ceasing movement carries severe consequences. If these sharks stop swimming, water no longer flows over their gills, and they cannot extract sufficient oxygen from their surroundings. This lack of oxygen leads to a state of suffocation, which can quickly become fatal. Consequently, these sharks are in a perpetual state of motion, even during periods of rest.
This fundamental need for continuous movement profoundly influences the life of an obligate ram-ventilating shark. It dictates their habitat choices, often favoring areas with consistent currents or open waters where they can maintain speed. It also shapes their hunting strategies, as they are typically active predators that pursue prey rather than ambushing from a stationary position. Some research suggests that even while “resting,” certain sharks may utilize currents to maintain water flow over their gills, allowing for periods of reduced muscle activity.