Yes, sharks do bleed. Despite common misconceptions, sharks, like most vertebrates, possess a circulatory system with red blood, essential for bodily functions.
Understanding Shark Blood
Shark blood is red due to the presence of hemoglobin, a protein that contains iron and is responsible for transporting oxygen throughout the body. This is similar to the blood of most other vertebrate animals, including humans. Sharks have nucleated red blood cells, which differ from the non-mammalian vertebrates that possess anucleated red blood cells.
The circulatory system of a shark is a single-circuit system, where blood flows from the heart to the gills for oxygenation, then directly to the body before returning to the heart. Their two-chambered heart pumps deoxygenated blood to the gills via the ventral aorta, where gas exchange occurs. Oxygenated blood then flows into the dorsal aorta, distributing it throughout the body. Red blood cells in sharks are primarily produced in the spleen and a unique organ called Leydig’s organ, found along the esophagus in some species.
Factors Obscuring Visible Bleeding
Several factors contribute to why people rarely observe sharks bleeding in the wild. The vastness of the ocean means any blood released quickly disperses and dilutes in the surrounding water. Ocean currents further spread the blood, making it difficult to remain concentrated or visible. While sharks possess an impressive sense of smell, the idea that they can detect a single drop of blood from miles away is an exaggeration; dilution and currents significantly limit the range of scent detection.
The physical characteristics of shark skin also contribute to obscuring visible bleeding. Shark skin is covered in tough, tooth-like scales called dermal denticles, which act as a protective armor. These denticles are made of dentine and are covered with enamel, similar to teeth. This outer layer helps contain superficial wounds and limit blood escape. Additionally, sharks have efficient wound-healing processes that can quickly stem blood flow, reducing prolonged external bleeding.
Shark Injury and Recovery
Sharks exhibit remarkable healing capabilities, recovering from significant wounds, including those sustained from fights or accidental impacts. Observations have shown that even severe skin and blood vessel injuries can heal quickly, with some whale sharks recovering within 35 days. Large wounds, such as those half a meter long on a shark’s back, can completely heal within six months. Great white sharks, for example, have been documented to heal extensive flesh wounds, with little more than a tight scar remaining after about a year.
The immune system of sharks plays a role in this rapid recovery. Genetic studies reveal unique modifications in shark immunity genes that may contribute to their fast wound healing and resistance to infections, even in a marine environment filled with bacteria.
The mucus layer on shark skin, for instance, has a chemical composition that resembles human mucus more closely than that of other fish, aiding wound recovery. Their biological repair mechanisms, including aspects of DNA repair and genome stability, help them survive injuries.