Sharks, ancient inhabitants of the world’s oceans, often spark curiosity about their longevity. While they do age, many factors influence how long they survive in their natural environments. Understanding their lifespan and aging processes reveals a complex biological reality.
Understanding Shark Lifespans
The lifespans of sharks vary considerably across different species. Most shark species live for 20 to 30 years in the wild. For example, blue sharks typically live 15 to 16 years, while great white sharks are estimated to live 30 to 40 years. Some species, such as the spiny dogfish and whale shark, can exceed a century, with spiny dogfish living between 25 and 100 years and whale sharks potentially living over 100 years.
The Greenland shark holds the record as the longest-lived vertebrate. Some individuals are estimated to live for at least 270 years, with one specimen found to be approximately 392 years old, potentially reaching over 500 years. A shark’s life duration is influenced by its size, habitat, and reproductive strategies. Larger, slower-growing species in cold environments often exhibit greater longevity.
The Aging Process in Sharks
Sharks experience aging, though the specific mechanisms can differ from other animals. Some long-lived species, like the Greenland shark, exhibit what is termed “negligible senescence.” This means their metabolic activity does not appear to decrease significantly as they get older, which may contribute to their remarkable longevity. Researchers are exploring whether Greenland sharks show typical hallmarks of aging observed in other animals.
Scientists primarily determine a shark’s age by examining growth rings on its vertebrae, similar to counting tree rings. These growth bands are periodically deposited and can be counted to estimate age. However, this method has limitations; the rings may not always be deposited annually, and their formation can be affected by factors like health or local conditions. For older sharks, this method might underestimate their true age because ring deposition can slow or stop.
For species like the Greenland shark, whose vertebrae are too soft for traditional ring counting, scientists employ alternative techniques such as bomb radiocarbon dating. This method utilizes elevated levels of carbon-14 from mid-20th century nuclear weapons testing, providing a timestamp in tissues. Newer, non-lethal methods like DNA methylation analysis are also being developed to predict shark ages from tissue samples.
Factors Contributing to Shark Mortality
While sharks do age, many do not reach their maximum potential lifespan, often succumbing to other causes of death. Natural factors include diseases, which can weaken sharks and make them more vulnerable. Younger and smaller sharks also face predation from larger marine animals, including other shark species, killer whales (orcas), and even crocodiles. Orcas, for example, hunt and consume even large sharks like great whites.
Environmental stressors also play a role in shark mortality. Habitat degradation, resulting from coastal development and pollution, can destroy critical nursery and feeding grounds. Climate change, leading to shifts in water temperature and salinity, can displace shark populations, forcing them into less suitable environments where food or breeding grounds may be scarce.
Human activities represent a significant threat to shark populations globally. Overfishing, driven by the demand for shark fins, meat, and other products, directly depletes their numbers. Sharks are also frequently caught as bycatch, meaning they are unintentionally captured in fishing gear targeting other species, leading to high mortality rates. Because sharks are generally slow-growing and produce few offspring, their populations struggle to recover from these human-induced pressures.