“Sharks don’t get cancer” is one of those facts that has been repeated so often it feels like common knowledge. Sharks have been around for an extraordinarily long time, as I found when looking into whether sharks are older than trees. They’re powerful predators with some unusual biological adaptations. But none of that makes them immune to cancer.
So, do sharks get cancer? Yes, they do. Tumours have been documented in sharks and other cartilaginous fish, including great whites off South Australia. What scientists can’t yet tell us is how often cancer occurs in wild sharks compared with other animals. And when I dug into it, the story of how the myth spread turned out to be far more interesting than the myth itself.
Where did the shark cancer myth come from?
The modern version took off with a 1992 book, Sharks Don’t Get Cancer, by I. William Lane and Linda Comac. It promoted the idea that sharks rarely develop cancer, and suggested that substances in their skeletons could be used to fight tumours. The idea gained wide attention after the US television program 60 Minutes covered it in 1993, and powdered shark cartilage was soon being sold as an alternative cancer treatment.
There was a small grain of real science behind part of the story. Laboratory experiments had found that certain substances from cartilage could slow the formation of new blood vessels, which tumours rely on to grow. The problem was the leap from that observation to the claim that swallowing a powder could treat cancer in people.
As ScienceDaily reported, Johns Hopkins biologist Gary Ostrander later described the supplements’ popularity as a triumph of marketing and pseudoscience over reason. He also noted that Lane himself acknowledged in the book that sharks do get cancer. The catchy title is what stuck.
Sharks do get tumours
Scientists have known for well over a century that sharks get cancer. Ostrander’s 2004 review in the journal Cancer Research made three points: sharks do get cancer, the rate of shark cancer isn’t known, and even a low rate wouldn’t show that cartilage products can treat cancer in people.
According to the review’s abstract, it described both malignant and benign tumours in sharks and their relatives, including two sharks with two cancers each. Wikipedia’s article on shark cartilage notes that the review even documented tumours in the cartilage itself.
Some of the most striking evidence comes from Australia. In 2013, researchers from the Fox Shark Research Foundation near Adelaide reported a great white off South Australia with a tumour about 30 centimetres long and 30 centimetres wide protruding from its mouth, along with a bronze whaler shark with a mass on its head. As Live Science reported, that study brought the number of shark species with documented tumours to at least 23.
In 2016, researchers led by Flinders University’s Charlie Huveneers obtained a tissue sample from a free-swimming white shark and carried out the first microscopic examination of such a growth. It turned out to be a benign tumour.
That distinction matters. Finding a tumour isn’t automatically finding cancer. Benign tumours aren’t cancerous, while malignant tumours can invade surrounding tissue or spread elsewhere in the body. But both are neoplasms, abnormal growths of cells, and the point for this myth is that sharks clearly can develop them.
Do sharks get cancer less often than humans?
This is where the answer becomes much less certain. It’s tempting to look at the relatively small number of documented shark tumours and conclude that sharks rarely get cancer, but that has never been shown. As Ostrander’s review pointed out, the rate simply isn’t known from the available data.
Wild sharks are extremely hard to study for this purpose. Most live in the open ocean, few are ever examined by a vet or pathologist, and cancer is often an internal disease that stays invisible until the very end. Sick sharks are also likely to be taken by predators, and dead sharks tend to sink rather than wash up on beaches the way marine mammals do, so shark cancer deaths are rarely detected.
Comparing cancer rates between any species is difficult anyway, because animals differ so much in lifespan, body size, environment and how closely they’re studied. So the honest answer is that we know sharks can get cancer, but we don’t know whether they get it more or less often than other animals. That’s very different from saying they’re resistant to it.
Do cartilage supplements treat cancer?
No. They’ve been tested, and they haven’t been shown to help.
In a randomised, placebo-controlled trial led by Charles Loprinzi for the North Central Cancer Treatment Group, 83 patients with advanced breast or colorectal cancer received standard treatment plus either a shark cartilage product or a placebo. As the trial’s abstract on PubMed reports, Loprinzi’s team found no difference in overall survival and no improvement in quality of life. A later phase III trial of a purified cartilage extract in people with lung cancer, published in the Journal of the National Cancer Institute in 2010, also found no significant improvement in survival.
It’s a useful lesson in how medical evidence works. A substance can look promising in a laboratory dish without becoming an effective treatment in people. Before a treatment can be recommended, it needs properly designed clinical trials showing that it actually helps patients and that the benefits outweigh the risks. For a look at the treatments that have actually changed cancer outcomes, see my post on the future of cancer medicine.
Why the myth caused harm
The shark cancer myth wasn’t harmless trivia. The 2004 Cancer Research review concluded that promoting these products contributed to two serious problems: a dramatic decline in shark populations, and patients being diverted away from effective cancer treatment. For someone facing a serious illness, choosing an ineffective supplement over proven treatment can have serious consequences.
It’s worth keeping this separate from other pressures on sharks. Finning and demand for shark meat and fins raise their own conservation issues; Live Science reports that humans kill around 100 million sharks a year, with many factors involved. The supplement trade was a separate part of that market and shouldn’t be confused with traditional shark fin dishes.
What’s actually unusual about shark biology?
This is where the story gets genuinely interesting. Sharks have an unusual type of antibody called immunoglobulin new antigen receptor, or IgNAR. Part of it is a small binding structure called a variable new antigen receptor (VNAR), which is much smaller and more stable than a conventional antibody. It can reach parts of target molecules that ordinary antibodies struggle to get to.
A recent scientific review describes VNARs as promising tools for future diagnostics and treatments. There’s an Australian connection, too: Melbourne-based biotech AdAlta has developed “i-bodies”, human proteins engineered to mimic the shape of these shark antibodies.
There’s an important distinction, though. Shark antibodies are an area of medical research, not proof that sharks carry a natural cancer remedy. Some experimental VNAR-based approaches have shown interesting results in laboratory and animal studies, but they’re still in development rather than established treatments. Sharks may genuinely teach us something useful about medicine, just not because they’re immune to cancer.
They’re not the only misunderstood ocean predators, either: my post on why killer whales are actually dolphins busts another popular misconception.
So, do sharks get cancer?
Yes. Tumours have been documented in at least 23 shark species, including great whites off South Australia, and even in shark cartilage itself. What remains unknown is how common cancer is in wild sharks, because there isn’t enough data to calculate a reliable rate, and dead sharks rarely leave evidence behind.
The myth became popular in the 1990s alongside claims that cartilage supplements could treat cancer. Clinical trials found they didn’t improve survival or quality of life, and the myth harmed both patients and shark populations. Sharks aren’t cancer-proof and their skeletons aren’t a treatment, but their unusual VNAR antibodies are teaching scientists some genuinely interesting things about how future medicines might be designed.
One practical word on health: if you or someone you love has cancer, talk to the treating medical team before adding any supplement or alternative therapy, and don’t swap a prescribed treatment for one. I’m not a doctor, and this article is a general explainer rather than medical advice.
Sources
- Ostrander et al: Shark cartilage, cancer and the growing threat of pseudoscience, Cancer Research (2004), Penn State research record
- ScienceDaily: Johns Hopkins biologist on the shark cartilage pseudoscience problem (2004)
- Wikipedia: Shark cartilage
- Live Science: Sharks do get cancer, tumour found in great white
- VNAR: shark single-domain antibodies for the new era of medical biotechnology (review, PubMed Central)
- Loprinzi et al: Evaluation of shark cartilage in patients with advanced cancer, a North Central Cancer Treatment Group trial, Cancer (2005), on PubMed