Sharks are some of the most efficient predators in the ocean. Their streamlined bodies and powerful tails allow them to swim continuously for long distances and hunt prey. However, sharks do need to get oxygen from the surrounding water as they lack gills. This raises an interesting question – can sharks drown if they stop moving?
Do Sharks Have to Swim to Breathe?
Sharks respire by ram ventilation. This means they force water over their gills by swimming with their mouths open. The oxygen in the water diffuses into blood capillaries in the gills. If sharks stop swimming, water stops moving over their gills and they can no longer get oxygen.
Most shark species do have to keep moving to pass water over their gills and breathe. However some sharks like nurse sharks and wobbegongs can use buccal pumping to actively pump water over their gills without swimming. This allows them to rest on the ocean floor and still respire.
So for most shark species, stopping swimming leads to suffocation as they cannot obtain oxygen from the surrounding water. But does this mean sharks actually drown if they stop swimming?
What is Drowning?
Drowning occurs when breathing is impaired due to submersion in a liquid. For people, drowning occurs when water enters the airways and prevents air from reaching the lungs. This leads to suffocation due to lack of oxygen.
For aquatic animals like sharks, the definition of drowning is a bit ambiguous. Since sharks extract dissolved oxygen from the surrounding water, merely being submerged cannot lead to drowning.
Rather, sharks experience something similar to suffocation when they stop swimming and can no longer pass water over their gills. While they may die from lack of oxygen, it does not match the definition of drowning for terrestrial species.
Shark Behavior to Avoid Suffocation
Sharks have evolved unique behaviors to avoid suffocating if they stop swimming:
Continuous Swimming
Most shark species swim continuously to ensure a constant flow of water over their gills. Some sharks like the shortfin mako are specially adapted to swim extremely fast for long periods. They have efficient gill structures, retractable eyes, and a streamlined shape to minimize drag.
Buccal Pumping
Sharks like nurse sharks and wobbegongs can actively pump water over their gills using their throat muscles. This allows them to rest on the ocean floor while still respiring.
Surface Ram Ventilation
Some fast swimming sharks like makos can briefly pump water over their gills by swimming upwards at a rapid burst. This surface ram ventilation provides temporary respiration at the water’s surface.
Tonic Immobility
When upside down, sharks enter a natural state of paralysis called tonic immobility. However their gills can still function normally allowing them to obtain oxygen even if motionless.
Can Sharks Ever Suffocate?
While active swimming allows sharks to avoid suffocation in most situations, there are some rare instances where sharks may get deprived of oxygen:
– Getting caught in fishing nets – Restricted mobility prevents sufficient water flow over gills
– Tossed onto shoreline – No water to pass over gills while out of water
– Trapped in caves/crevices – Immobilized and unable to ventilate gills
– Temporarily paralyzed by electric shock – Muscles unable to propel swimming
– Exhausted after long migration – Lack energy to continue swimming
So while sharks are well adapted to avoid suffocation in most natural situations, unusual circumstances can still lead to oxygen deprivation if they persist. However, such events are rare and it is unlikely that healthy sharks frequently die purely due to lack of oxygen.
How Long Can a Shark Survive Without Swimming?
Different shark species have varying levels of tolerance to lack of water flow over their gills:
Pelagic Sharks
– Blue shark – Large Coastal Sharks
– Tiger shark – 20-40 minutes
– Bull shark – 30-60 minutes
Large coastal sharks have lower metabolic rates and can withstand enforced immobilization for 30-60 minutes in shallow water before suffocating.
Small Coastal Sharks
– Nurse shark – 1-2 hours
– Wobbegong – 2-4 hours
Small bottom dwelling sharks like nurse sharks and wobbegongs can use buccal pumping to survive over 2 hours without swimming. However, they eventually require water flow to acquire oxygen.
Deep Sea Sharks
– Goblin shark – >1 hour
– Frilled shark – >1 hour
Deep sea sharks have extremely slow metabolisms allowing them to survive over an hour without swimming. Their oxygen demand is minimal in cold, deep environments.
Do Sharks Sleep?
Since sharks need to keep swimming to breathe, it was long believed they did not sleep like terrestrial animals. However, recent research using accelerometer tags shows some sharks do rest and sleep:
Low Speed Swimming
As nurse sharks can pump water over their gills without swimming, they can fully rest on the sea floor. Other sharks reduce swimming speed and activity levels to enter a restful state.
Unihemispheric Sleep
Sharks can sleep with one brain hemisphere at a time, similar to dolphins. This allows the opposite eye to remain open for vigilance against predators. Great whites and mako sharks display unihemispheric sleep.
Short Sleep Cycles
Sharks tend to sleep in short bursts lasting minutes to hours. They regularly wakeup to breathe consciously before going back to a resting state. Deep sea sharks may sleep longer as they don’t need active swimming to ventilate their gills.
So while sharks cannot sleep in the same way as land animals, they have evolved sleep behaviors that work within the limitations of their physiology.
Do Shark Embryos Drown?
Shark embryos develop within egg cases laid on the seafloor. The embryos obtain oxygen directly from the surrounding seawater through their gills and blood vessels in the egg case. Since they are submerged, shark embryos cannot drown as they acquire oxygen from the water.
Some key facts about oxygen intake in shark embryos:
– Embryos absorb dissolved oxygen through gills and blood vessels in egg case
– Ventilation occurs through rhythmic tail undulation within egg case
– Oxygen consumption rises towards end of gestation period
– Low oxygen environments increase mortality rates
So while shark embryos cannot drown, lack of sufficient oxygen in the egg case can impair development and reduce survival. However, most eggs are laid in well-oxygenated areas to avoid this.
Conclusion
In summary, while sharks may die from lack of oxygen if they are unable to swim, they do not technically drown due to being aquatic creatures. Their gills allow them to extract oxygen directly from surrounding water. Sharks have evolved behaviors like ram ventilation, buccal pumping and unihemispheric sleep to minimize risk of suffocation. While accidents can deprive sharks of oxygen, it is rare for them to asphyxiate under normal circumstances in their natural environment. So a true drowning event is highly unlikely although temporary suffocation is possible. The exception is shark embryos inside egg cases, which acquire oxygen safely from the surrounding seawater and cannot drown.