When sharks are observed out of water, a striking and often distressing sight is the presence of blood. This phenomenon stems from the intricate biology of shark respiration and the delicate nature of their specialized breathing organs. Understanding the underlying physiological processes reveals why these animals, adapted for an aquatic existence, experience such a dramatic response when removed from their natural environment.
How Sharks Breathe
Sharks use gills to extract oxygen from water and release carbon dioxide. They possess between five and seven gill slits located on each side of their head. Water flows into the shark’s mouth, passes over the gills, and then exits through these slits. This gas exchange process allows oxygen from the water to be absorbed into tiny blood vessels within the gills, which then transport oxygen throughout the body.
Some sharks, such as mako sharks and great whites, employ a method called ram ventilation, where they must swim continuously with their mouths open to force water over their gills. Other species, like nurse sharks, can use buccal pumping, actively drawing water into their mouths and pumping it over their gills, allowing them to breathe while at rest. The constant movement of water over these structures is fundamental for their function.
The Role of Water for Gills
The structure of a shark’s gills is adapted for efficient gas exchange in water. Gills are composed of delicate gill arches that support numerous gill filaments, with microscopic folds called lamellae. These lamellae provide a vast surface area for oxygen absorption. Water provides physical support, keeping these thin structures separated and extended.
When a shark is removed from water, the physical support provided by the surrounding fluid is lost. Without water to hold them apart, the delicate gill filaments and lamellae collapse and stick together. This collapse drastically reduces the surface area for gas exchange, impairing the shark’s ability to extract oxygen. The gill tissues are compromised, leading to stress and damage.
The Mechanism of Bleeding
The bleeding observed when a shark is out of water directly relates to gill damage. The delicate gill lamellae are richly supplied with a dense network of tiny, fragile blood vessels. These vessels are positioned close to the surface to facilitate oxygen transfer.
As the gill structures collapse and stick together outside of water, pressure and friction cause these thin-walled vessels to rupture easily. Any struggling or thrashing by the shark further exacerbates this damage. This rupture of numerous small blood vessels within the gill tissue results in the characteristic bleeding.
Additional Contributing Factors
Beyond gill damage, other factors can contribute to a shark’s injuries and distress when out of water. Physical trauma from thrashing against hard surfaces, such as boat decks or the ground, can lead to external cuts and abrasions on the shark’s skin. These injuries, while not the main source of gill bleeding, can add to the visible blood and further compromise the animal’s condition.
Physiological stress also plays a role. Capture and handling can induce stress responses, leading to internal changes within the shark’s body. While the primary source of bleeding originates from the damaged gills, stress can exacerbate internal issues and complicate recovery.