Sharks are often perceived as near-immortal creatures, perfectly adapted predators that have patrolled the oceans for hundreds of millions of years. This perception raises a fundamental biological question about their lifespan: Do sharks, like humans and most mammals, eventually succumb to the inevitable process of old age? The answer involves understanding how scientists measure their long lives and the unique biological mechanisms that allow some species to resist aging for centuries.
Measuring Shark Lifespan and Longevity Extremes
Scientists employ specific methods to determine the age of a shark, which is not as straightforward as counting rings on a tree trunk. For many species, age is estimated by counting the concentric bands of calcification found in their vertebrae, a process analogous to reading tree rings. These growth bands, deposited annually in the cartilaginous skeleton, provide a relatively reliable record of the shark’s life.
This method is ineffective for species with softer, less calcified skeletons, such as the Greenland shark. For these extremely long-lived species, researchers use radiocarbon dating of the eye lens nucleus. This part of the eye is formed before birth, and its proteins are metabolically inert, meaning they are never replaced, creating a time-stamped record.
This technique revealed that the Greenland shark, a slow-moving predator of the Arctic and North Atlantic, possesses the longest known lifespan of any vertebrate. The largest individual studied was estimated to be around 392 years old, with a possible range between 272 and 512 years. This extreme longevity is attributed to a very slow metabolism, an adaptation to the deep, frigid waters they inhabit.
The Biological Question of Senescence
Senescence is the biological process of aging, characterized by the progressive deterioration of function and an increased chance of death over time. Sharks, especially long-lived species, challenge the traditional understanding of this process seen in most mammals and birds. Many sharks exhibit indeterminate growth, meaning they continue to grow in size throughout their entire lives, a trait shared with many fish and reptiles.
This continuous growth suggests a different relationship with aging, as the body’s cells are constantly dividing and being replaced. Studies on the Greenland shark have shown no significant variation in muscle metabolic activity across different ages, which is a surprising absence of a traditional sign of aging. The lack of declining function in extremely old individuals is sometimes referred to as “negligible senescence.”
While sharks may have the biological capacity to live for centuries, finding one that has died purely from age-related deterioration in the wild is exceedingly rare. The slow, steady decline into old age commonly observed in humans is not a well-documented cause of death for wild sharks. Their life history traits, such as late sexual maturity (around 150 years for a female Greenland shark), make their populations vulnerable to external threats.
Primary Causes of Mortality in Wild Sharks
The scarcity of sharks dying from old age suggests that external factors overwhelmingly account for their observed mortality. The largest threat is human impact, primarily through commercial fishing activities. Sharks are often targeted for their fins, meat, and liver oil, with tens of millions killed annually.
Accidental capture, known as bycatch, is also a significant driver of mortality, as sharks are unintentionally caught in gear meant for other fish. This pressure means most sharks are removed from the population long before they approach their theoretical biological maximum lifespan.
Natural causes of death, though less common than human impact, also play a role, particularly for younger sharks. These causes include:
- Predation, especially on juveniles.
- Infectious diseases and parasites, often exacerbated by environmental stressors like pollution and changes in ocean temperature.
- Injuries from mating or territorial disputes.
- Starvation due to an inability to hunt effectively.