Sharks are ancient predators with unique biology, thriving in diverse marine environments. Their survival depends on specific physiological mechanisms, particularly how they breathe and maintain their position in the water. This reveals why continuous movement is so important for many, but not all, shark species.
How Sharks Breathe: Two Key Methods
Sharks, like all fish, extract oxygen from water using gills, employing two distinct methods to ensure a constant flow of oxygen-rich water over these organs. Ram ventilation is the first method, where the shark swims forward with its mouth slightly open, forcing water over its gills. This passive process relies on the shark’s forward motion for gas exchange.
The second method is buccal pumping, an active process involving muscles in the shark’s mouth and pharynx. Sharks using buccal pumping can draw water into their mouths and pump it over their gills, allowing them to breathe even when stationary. Different shark species rely on one or both methods, with some able to switch depending on their activity level.
The Critical Role of Constant Motion for Some Sharks
For a specific group of sharks, known as obligate ram ventilators, constant motion is a necessity for survival. These sharks must continuously swim forward to push water over their gills, as they cannot effectively pump water using buccal muscles. Great white sharks, mako sharks, hammerheads, and whale sharks are prominent examples.
The anatomical structure of these sharks often includes reduced or absent spiracles, small openings behind their eyes that aid in respiration for some species. This means their primary method of oxygen uptake depends on the dynamic flow of water generated by their movement.
Immediate Consequences of Stopping
When an obligate ram-ventilating shark stops swimming, the flow of water over its gills ceases or becomes insufficient. This interruption means oxygen can no longer be efficiently extracted, leading to a rapid decline in oxygen levels within the shark’s blood. The physiological consequence is suffocation, as the shark’s body is deprived of the oxygen it needs to function.
Sharks require a continuous passage of water over their gills to absorb oxygen. For species like the great white, stopping can quickly become a life-threatening event. Conversely, sharks that primarily use buccal pumping, such as nurse sharks or wobbegongs, can comfortably rest without risk of suffocation, actively pumping water over their gills.
Maintaining Buoyancy and Position
Beyond respiration, constant motion is also crucial for many sharks to maintain buoyancy and avoid sinking. Unlike most bony fish, sharks do not possess a swim bladder, a gas-filled organ regulating buoyancy. Instead, sharks rely on adaptations like a large, oil-filled liver, which can constitute up to 25% of their body mass in some species. This oil (primarily squalene) is less dense than seawater, providing significant lift.
However, this oily liver only provides static lift and does not fully counteract their tendency to sink. Many sharks also depend on dynamic lift, generated by their pectoral fins and body shape as they move through the water. This mechanism is similar to how an airplane wing creates lift. When these sharks stop swimming, they lose this dynamic lift and will gradually sink.