Sharks get cancer. The claim that they rarely or never do traces to a 1992 book and the supplement industry it spawned, not to any rigorous finding in marine biology. Tumors have been documented across dozens of shark species, in organs from the brain to the liver, and researchers have pointed out for decades that we do not even have the data to calculate a reliable cancer rate in wild sharks. Yet the myth has proven remarkably sticky, and the real biology underneath it turns out to be far more interesting than the sales pitch ever was.
Where the Myth Came From
In 1992, I. William Lane published a book called Sharks Don’t Get Cancer, followed by a second book in 1996. The claim was amplified by segments on the television program 60 Minutes in 1993, reaching millions of viewers. Ironically, Lane’s own books contradicted their titles: within the text, the authors acknowledged that sharks do develop cancer, while still arguing incorrectly that the disease is rare in these animals. A widely cited review in Cancer Research spelled this out bluntly, making three points: sharks do get cancer, the actual rate is unknown from existing data, and even if it were low, that would be irrelevant to whether crude shark cartilage extracts could treat cancer in humans.1Cancer Research. Shark Cartilage, Cancer and the Growing Threat of Pseudoscience
The timing mattered. Lane’s book arrived during a period of intense public interest in alternative cancer therapies, and the idea that a powerful predator might hold the key to defeating cancer was irresistible. The logic sounded simple: sharks have skeletons made of cartilage, cartilage lacks blood vessels, tumors need blood vessels to grow, so cartilage must contain something that fights cancer. Each step in that chain was either misleading or wrong, but it was compelling enough to build a multimillion-dollar supplement industry almost overnight.
Documented Tumors in Sharks and Their Relatives
The scientific record tells a different story. By the time researchers compiled a thorough catalog, they had found at least 42 cases of tumors in cartilaginous fish (the group that includes sharks, skates, rays, and chimaeras). These tumors showed up in at least 21 species across nine families and seven orders, covering 24 sharks, 16 skates or rays, and 2 chimaeras. The tumors came from nearly every organ system: nervous, digestive, integumentary, excretory, blood-forming, reproductive, skeletal, and endocrine. At least 15 of those tumors were classified as malignant, meaning they had invaded surrounding normal tissue.1Cancer Research. Shark Cartilage, Cancer and the Growing Threat of Pseudoscience
Reports have continued to accumulate. One case from the 1970s described a choroid plexus papilloma, a type of brain tumor, in a spiny dogfish.2Journal of the National Cancer Institute. A Choroid Plexus Papilloma in an Elasmobranch (Squalus acanthias) More recently, researchers documented liver cancers in three bamboo sharks living in an aquarium, including two biliary adenocarcinomas and one hepatocellular carcinoma, all confirmed through pathology.3Journal of Zoo and Wildlife Medicine. Primary Hepatic Neoplasia in Three Adult Bamboo Sharks (Chiloscyllium plagiosum and Chiloscyllium punctatum) in an Aquarium Population These are not exotic flukes. They represent the same kinds of malignancies that develop in mammals, growing in the same kinds of tissues.
Why We Cannot Say How Often Sharks Get Cancer
One reason the “sharks rarely get cancer” claim persists is that nobody has been able to calculate an accurate cancer rate for wild shark populations. Think about what that would require: you would need to examine large numbers of sharks over time, with standardized pathology workups, across species and habitats. That has never happened. Most documented shark tumors were discovered incidentally, in animals caught by fishers or housed in aquariums. A sick shark in open ocean is far more likely to be eaten by predators, sink to the seafloor, or simply die unnoticed than to be retrieved and examined by a pathologist.
This detection problem is not unique to sharks. The entire field of wildlife cancer research is constrained by the difficulty of detecting tumors in free-ranging animals using non-invasive methods.4PubMed Central. Approaches and methods to study wildlife cancer For terrestrial animals, researchers can sometimes use dart biopsies or imaging on sedated individuals. For sharks, which live in deep water, move constantly, and are logistically challenging to capture and examine, that kind of systematic screening is essentially impossible with current technology. The absence of evidence is not evidence of absence, and the low number of documented shark tumors reflects the difficulty of looking rather than a confirmed absence of disease.
The Cartilage That Could Not Cure Cancer
The logical chain that drove the supplement industry went roughly like this: shark cartilage contains proteins that can inhibit the growth of new blood vessels (angiogenesis), and tumors depend on new blood vessels to grow, so consuming shark cartilage should starve tumors. There is a grain of truth at the front end. Lab studies have isolated small proteins from shark cartilage that do show anti-angiogenic activity in test-tube and animal models. One research group identified protein fractions around 14 to 16 kilodaltons that blocked blood vessel sprouting in rat tissue assays.5PubMed. Angiogenic inhibitor protein fractions derived from shark cartilage Another team purified a protein they called SCP1, with similar anti-angiogenic properties, that turned out to resemble a protein in the parvalbumin family.6PubMed. Identification of low-molecular-weight protein (SCP1) from shark cartilage with anti-angiogenesis activity and sequence similarity to parvalbumin
But isolated proteins active in a dish are very different from powdered cartilage swallowed as a pill. Proteins are digested in the stomach. There is no credible mechanism by which swallowing dried shark cartilage would deliver intact anti-angiogenic proteins to a tumor site in the human body. And when clinical trials actually tested this, the results were unambiguous.
A phase I/II trial gave shark cartilage to 60 patients with advanced cancer. No complete or partial tumor responses occurred. About 17% of patients had stable disease for 12 weeks or more, which was similar to what happens in patients receiving only supportive care. The researchers concluded that shark cartilage as a single agent was inactive and had no beneficial effect on quality of life.7PubMed. Phase I/II trial of the safety and efficacy of shark cartilage in the treatment of advanced cancer A randomized, placebo-controlled trial by the North Central Cancer Treatment Group confirmed the finding: there was no difference in overall survival between patients receiving standard care plus shark cartilage and those receiving standard care plus placebo, and no improvement in quality of life either.8PubMed. Evaluation of shark cartilage in patients with advanced cancer: a North Central Cancer Treatment Group trial
This should have ended the story. It did not. Shark cartilage supplements remain commercially available today, and nearly half of products assessed in one labeling study had at least one instance of noncompliance with U.S. regulations, including prohibited disease claims on the packaging.9PubMed. Shark Cartilage Supplement Labeling Practices and Compliance with U.S. Regulations
What Shark Genomes Actually Tell Us
While the cartilage hypothesis was a dead end, genomics has uncovered something genuinely interesting about how sharks handle the biological risks that lead to cancer. When researchers sequenced the white shark genome, they found evidence of positive selection and enrichment in gene categories related to genome stability, particularly genes involved in DNA repair. This pattern showed up in both the white shark and the whale shark, two species that are large-bodied and relatively long-lived.10PubMed Central. White shark genome reveals ancient elasmobranch adaptations associated with wound healing and the maintenance of genome stability
The Greenland shark pushes this even further. These animals can live for several centuries, making them among the longest-lived vertebrates on Earth. Analysis of the Greenland shark genome revealed gene family expansions and signatures of positive selection in areas associated with immune function, cancer resistance, and DNA repair.11PubMed Central. The Greenland shark genome: Insights into lifespan extremes and population dynamics A preprint analysis went deeper, finding that genes specifically duplicated in the Greenland shark form a functionally connected network enriched for DNA repair, and identified a unique insertion in the conserved region of p53, one of the most important tumor-suppressor proteins known in biology.12bioRxiv. The Greenland shark (Somniosus microcephalus) genome provides insights into extreme longevity
This fits a broader pattern in biology known as Peto’s paradox: if cancer arises from random mutations in dividing cells, then animals with more cells and longer lifespans should have higher cancer rates. But they do not. Whales, elephants, and sharks all have vastly more cells than humans, yet do not show a proportionally increased cancer burden. The resolution seems to be that large, long-lived species have evolved additional layers of cancer suppression that smaller animals lack.13PubMed Central. Peto’s Paradox: evolution’s prescription for cancer prevention Sharks are not cancer-proof. They appear to have invested heavily, over hundreds of millions of years of evolution, in molecular machinery that keeps their DNA intact. That is a meaningful biological finding, even though it is far less dramatic than “sharks don’t get cancer.”
An Immune System Built Differently
Sharks diverged from other vertebrates roughly 450 million years ago, and their immune systems reflect that deep evolutionary history.14Fish and Fisheries. Consumption of shark products: The interface of sustainability, trade (mis)labelling, human health and human rights One of the most distinctive features is the immunoglobulin new antigen receptor, or IgNAR, a type of antibody found only in sharks. Unlike human antibodies, which pair a heavy chain with a light chain, IgNAR antibodies consist of heavy chains only, arranged as homodimers. Their antigen-binding regions are unusually small and stable, and structural analysis shows that the basic antibody fold is already present in these ancient molecules, even though the overall architecture is quite different from mammalian antibodies.15PubMed Central. The structural analysis of shark IgNAR antibodies reveals evolutionary principles of immunoglobulins
Structural studies suggest IgNAR may have evolved from cell-surface adhesion molecules rather than from the same antibody lineage that gave rise to human immunoglobulins. Their binding regions can adopt extended shapes that may be suited to accessing recessed or hidden parts of target molecules, something conventional antibodies struggle with.16PubMed Central. Structural evidence for evolution of shark Ig new antigen receptor variable domain antibodies from a cell-surface receptor Whether this plays a role in tumor surveillance in sharks is unknown, but the antibodies themselves have become a tool for human medicine, which we’ll get to shortly.
The rest of the shark immune system is also unusual. Sharks lack bone marrow entirely. Instead, their blood cells are produced by the thymus, spleen, and two specialized organs: the epigonal organ (associated with the gonads) and the Leydig organ (associated with the esophagus). These organs function as the shark equivalent of mammalian bone marrow, producing granulocytes, macrophages, and other immune cells at high rates.17PubMed. Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture Recent immunological characterization of the Leydig organ in elasmobranchs found that roughly 40% of its cells were actively dividing, with about 15% being myeloid progenitor cells.18PubMed. The Leydig organ of elasmobranchs: Shed light on an active and mysterious defense center with immunological characterization of its cells That is a remarkably active immune organ, and understanding how it contributes to disease resistance is still in its early stages.
A Biochemical Oddity That May Help
Sharks carry high concentrations of urea in their blood and tissues as part of their strategy for maintaining water balance in saltwater. Urea at those concentrations would damage proteins in most animals, unfolding them and disrupting their function. Sharks offset this with a counterbalancing molecule called trimethylamine N-oxide, or TMAO, which stabilizes protein structure against urea’s destructive effects. The two chemicals essentially neutralize each other: urea promotes protein unfolding and TMAO opposes it, with the combined effect being roughly additive.19PubMed. A naturally occurring protective system in urea-rich cells: mechanism of osmolyte protection of proteins against urea denaturation
Whether this protective chemistry contributes to cancer resistance is speculative, but it is one of many examples of how shark physiology has adapted to maintain molecular stability in conditions that would be hostile to mammalian cells. The theme that emerges across genomics, immunology, and biochemistry is a consistent emphasis on keeping cellular machinery intact and functional in the face of damage.
Shark Antibodies as Real Drug Candidates
The irony of the shark cartilage story is that while ground-up cartilage capsules are useless against cancer, a different part of shark biology has genuine promise in oncology. The variable domain of IgNAR antibodies (known as VNAR) is only about one-tenth the size of a conventional human antibody. That small size, combined with high stability and the ability to penetrate tissues, makes VNARs attractive building blocks for targeted cancer therapies.20PubMed Central. Shark IgNAR: The Next Broad Application Antibody in Clinical Diagnoses and Tumor Therapies?
One recent study constructed a VNAR antibody targeting TROP-2, a protein overexpressed on many human cancer cells. The researchers built a phage display library from shark-derived VNARs, identified a candidate called VNAR-5G8 with high binding affinity and the ability to get inside TROP-2-expressing cells, and fused it to a bacterial toxin to create an immunotoxin (5G8-PE38). This engineered molecule showed significant anti-tumor activity in both cell cultures and animal models.21Acta Pharmaceutica Sinica B. A novel shark VNAR antibody-based immunotoxin targeting TROP-2 for cancer therapy The work is preclinical and years from any human application, but it represents a legitimate use of shark biology in cancer research, a far cry from selling dried cartilage powder.
The Cost of the Myth to Sharks Themselves
The shark cartilage supplement industry has not been a victimless enterprise. Shark populations have declined dramatically in recent decades, with about one in three species now threatened with extinction. The primary driver is overfishing linked to demand for shark fins, but the supplement trade has added further commercial pressure on already vulnerable populations.14Fish and Fisheries. Consumption of shark products: The interface of sustainability, trade (mis)labelling, human health and human rights
Beyond population impacts, there are direct safety concerns with consuming shark products. Sharks sit at the top of marine food chains, which means they accumulate environmental contaminants over their long lifespans. A study of sharks from Arabian Gulf waters found cadmium, mercury, and other toxic metals at levels exceeding maximum permissible limits for human consumption, and documented cadmium transfer from adult sharks to their unborn pups.22PubMed Central / Marine Pollution Bulletin. Bioaccumulation of toxic elements in two shark species from the United Arab Emirates waters of the Arabian Gulf So not only do shark cartilage supplements not treat cancer, they may introduce the consumer to heavy metals that are themselves carcinogenic. The myth has managed to harm the sharks and the people buying the pills at the same time.
Peto’s Paradox and the Bigger Evolutionary Picture
Sharks are just one piece of a larger biological puzzle. The question of why large, long-lived animals do not get cancer at rates proportional to their cell counts has been explored across elephants, whales, naked mole rats, and other species. Each seems to have arrived at its own set of solutions. Elephants have extra copies of the tumor suppressor TP53. Naked mole rats have unusual cell-contact inhibition that prevents overcrowding of cells. Bowhead whales show gene duplications in DNA repair pathways.
Sharks bring their own toolkit: enriched genome-stability genes, unique immune organs, heavy-chain-only antibodies, and a biochemical environment geared toward protein stability. The lesson from comparative oncology is not that any one species has “solved” cancer but that evolution has found multiple independent routes to keeping cancer under control when the selective pressure is strong enough.23PubMed Central. What Animal Cancers teach us about Human Biology Sharks are interesting not because they are cancer-free, but because 450 million years of evolution have produced a set of defenses worth studying on their own terms, without the distraction of a disproven supplement pitch.