The premise that sharks are immune to cancer is a long-standing misconception circulating widely in popular culture and alternative medicine. This belief stems from the observation that tumors are rarely seen in these animals, suggesting a biological defense mechanism that could be harnessed for human benefit. Sharks, skates, and rays belong to the class of cartilaginous fishes known as elasmobranchs, an ancient lineage that diverged from bony fish over 400 million years ago. While these predators have evolved unique resistance mechanisms, the notion of absolute immunity to cancerous growth is not supported by scientific evidence. This enduring myth requires a closer look at the actual biological findings and the historical context that gave rise to the false claim.
The Scientific Reality: Cancer Occurrence in Elasmobranchs
The idea that sharks are entirely free from cancer is factually incorrect, as numerous cases of malignant and benign tumors (neoplasia) have been documented across various elasmobranch species. These tumors affect diverse tissues, including the skin, internal organs, and cartilage. Sharks have been found with melanomas, cancers of the blood system, and tumors affecting the brain.
The apparent rarity of cancer in sharks compared to mammals is likely due to observational bias. Most studies rely on examining captured or stranded sharks, which represent a small fraction of the total population. A sick or deceased shark often sinks to the ocean floor, making it difficult for researchers to perform necropsies and document internal disease. Thus, the lack of widespread documentation does not equate to the non-existence of the disease.
For decades, the Registry of Tumors in Lower Animals has cataloged cases of neoplasms in sharks, skates, and rays. Although the overall incidence rate may be lower than in many other vertebrates, this reduced frequency is attributed to factors like a very slow mutation rate and specialized anti-cancer adaptations. The scientific consensus confirms that sharks are susceptible to tumor formation.
Origin of the Immunity Myth
The misconception that sharks never get cancer gained traction in the early 1990s, largely due to a popular science book. This publication promoted the false claim of shark immunity to justify using shark-derived products to treat human cancers. Media coverage amplified this belief, presenting anecdotal evidence and flawed studies to the public seeking alternative therapies.
The pseudoscientific claim was initially rooted in legitimate, yet misinterpreted, laboratory research from the 1980s that focused on the properties of cartilage. Scientists identified that cartilage, which is naturally avascular, contains substances that inhibit angiogenesis—the process tumors use to create new blood vessels for nutrients. Since the entire elasmobranch skeleton is composed of cartilage, the incorrect leap was made that the animal’s body was saturated with a cancer-fighting substance.
This flawed logic led to a lucrative industry selling crude shark cartilage powder and capsules. Proponents ignored that ingesting the cartilage prevents active compounds from surviving the digestive process and reaching a tumor in effective concentrations. Despite a lack of compelling clinical data, the notion that shark cartilage is a cancer cure persists in the alternative medicine market.
Unique Biological Traits Under Investigation
Sharks exhibit several unique biological traits that contribute to their seemingly low rate of neoplasia, making them subjects of intense scientific interest. One area of study is their specialized immune system, which evolved separately from mammals over hundreds of millions of years. Sharks possess a unique lymphomyeloid organ, the epigonal organ, which functions similarly to bone marrow in producing immune cells.
Laboratory experiments show that compounds secreted by immune cells from the epigonal organ can selectively inhibit the growth of human cancer cell lines. Genomic studies of species like the great white and great hammerhead sharks have revealed unique evolutionary adaptations in cancer-linked genes, such as Bag1 and legumain. Modifications in these genes may alter their tendency to inhibit programmed cell death, a process the body uses to eliminate dysfunctional cells.
Sharks are also an example of Peto’s Paradox: large, long-lived animals do not have a proportionally higher cancer incidence despite having far more cells at risk of mutation. The extremely long lifespan of species like the Greenland shark, which can live for centuries, suggests highly effective cancer suppression mechanisms. Recent research suggests that sharks have the lowest known germline mutation rate of any vertebrate, meaning their cells accumulate damaging mutations very slowly. This low mutation rate, combined with enhanced DNA repair, contributes significantly to their resistance to developing tumors over long lives.
Implications for Medical Research and Conservation
Modern cancer research has moved past crude shark cartilage toward isolating and synthesizing specific compounds identified in elasmobranchs. Researchers have isolated potent substances, such as the aminosterol squalamine and the extract AE-941, which demonstrate anti-angiogenic properties in laboratory settings. Squalamine can now be synthetically produced, eliminating the need to harvest sharks for the compound.
This shift is crucial because the continued promotion of raw shark cartilage has had devastating effects on global shark populations. Demand for these products, fueled by the persistent myth, has contributed to mass harvesting and overfishing. Numerous shark species are now listed as vulnerable or endangered, with exploitation for cartilage adding considerable pressure to their survival.
The conservation of these animals is paramount, both for ecological balance and for the biomedical insights they offer. Protecting sharks allows scientists to investigate their unique biological systems, potentially leading to novel human therapeutics based on synthesized versions of their protective compounds. The true value of the shark lies not in consuming its parts, but in studying its complex evolutionary biology.