There are many, many pervasive myths about sharks... One which has been around for a while, which seems to really stick, is that sharks do not get cancer. You often see things on the internet about how sharks are immune to any and all diseases, or that they may even hold the cure to cancer, BUT... I hate to break it to you... this is a myth. Sharks do get cancer! In fact, scientists have recorded multiple different types of cancers in over 40 species of sharks and their close relatives! So which sharks get cancer? How does it happen? And why does this myth persist?
What is Cancer?
In normal, healthy bodies, cells are made and die continuously. They are formed by their ancestor cell dividing to form two new cells (known as "mitosis"). However, cancer is formed when cells within the body proliferate uncontrollably. This means that the normal mechanisms in place to keep cells from growing and dying, and growing and dying, stop working.
This happens because the genetic material within the cells is damaged in some way, causing it to change. This can be due to a natural mutation in the code, or due to chemical or radiation damage (or all of these things at once!). This causes the cells to lose their 'stop' or 'die' codes in their DNA. As a result, either the cell continues to make new cells uncontrollably or the cell never dies, or sometimes a combination of both. Eventually more cells than are needed grow uncontrollably until they form a mass (known as a "malignant tumour").
Cancerous growths differ from non-malignant tumours in that they are very invasive and will damage tissues around them, form their own blood supplies to feed the monster (known as "angiogenesis") and then spread throughout the circulatory system to take root in other sites in the body (this is known as "metastatic cancer").
Cancer In Sharks is Relatively Rare
Sharks are multicellular organisms (just like all the rest of us who live with the potential for cancer), so there is no reason why they would not develop such a condition.
It is true that cancer in sharks is quite rarely recorded by scientists, but it is not known whether sharks genuinely get cancer at lower rates than humans or other animals. It is possible that their lives in the open ocean means sharks are exposed to fewer chemicals which can cause cancer (known as "carcinogens"), but it is also very possible that this habitat is also the reason we erroneously believe these incidents to be rare - sharks are so wide-ranging in the oceans that it is likely that we just don't witness their cancers (Ostrander et al, 2004; Finkelstein, 2005).
Yet, scientists have studied cancer in sharks for over 150 years! Cancers of the nervous system, reproductive organs, endocrine system skeleton and nervous system have been documented in over 20 different species of sharks, including reef sharks (Carcharhinus species), dogfish (Squaliform species), catsharks (Scyliorhinus species) and tiger sharks (Galeocerdo cuvier), to name but a few (Borucinska et al, 2004; Ostrander et al, 2004; Huveneers et al, 2016; O'Connor et al, 201!
To give you an example, one research team described the cancer they had found in a blue shark (Prionace glauca) which had been caught in the north-western Atlantic ocean. The shark had a malignant tumour in its gums (known as "gingival cancer"). The scientists dissected the shark and viewed the tumour under microscopes. They discovered that the tissues had prolific growth and chronic inflammation, so they were able to confirm that the shark did, indeed, have cancer (Borucinska et al, 2004).
It Is a Myth That Sharks Do Not Get Cancer
So why are so many people convinced that sharks do not get cancer? People began repeating this myth after a book, entitled 'Sharks Don't Get Cancer' was published in 1992. The book did not claim sharks could not get cancer at all, but discussed how shark cartilage might be used as a cancer treatment, based on the fact that sharks very rarely get cancer themselves (Lane & Comac, 1992). Despite the fact that the name of the book doesn't remotely match the content within, the title is often still discussed out of context and the myth that sharks so not get cancer persists (Ostrander et al, 2004).
This myth is a perfect example of why science communication is so important... It is so easy for a scientific fact to be misquoted or to be taken out of context, so that people completely misunderstand the original research. This does not mean the general public are foolish or gullible, but that scientists and the media must improve how we share our findings with the world. It is crucial that scientists control the message when their work is published and that the media is respectful to not exaggerate a story in order to sell more newspapers (Finkelstein, 2005).
As a member of the public you also have a lot of power to make sure you are well informed. If you are reading this, then you clearly have an inquisitive mind and an interest in science. I applaud you! So use that sharp mind to be critical of things you see on the internet or read in the newspapers! NOT EVERYTHING YOU READ IS TRUE! Check the sources of the article and you can even contact the scientists directly if you are curious! Scientists are actually quite a friendly bunch and researchers are often happy to answer questions or send you their work to read if you are interested. That way, you can be certain that what you have learned has a basis in scientific fact!
There are many pervasive myths about sharks... find out if what have you heard before is actually inaccurate through my Myth Busting articles.
References
Borucinska JD, Harshbarger JC, Reimschuessel R & Bogicevic T (2004). Gingival neoplasms in a captive sand tiger shark, Carcharias taurus (Rafinesque), and a wild-caught blue shark, Prionace glauca (L.). Journal of Fish Diseases, 27, 185–191. Access online.
Finkelstein JB (2005). Sharks do get cancer: few surprises in cartilage research. Journal of the National Cancer Institute, 97:1562–1563. Access online.
Huveneers C, Klebe S, Fox A, Bruce B, Robbins R, Borucinska JD, Jones R & Michael MZ (2016). First histological examination of a neoplastic lesion from a free-swimming white shark, Carcharodon carcharias L. Journal of Fish Diseases, 39:10, 1269-1273. Access online.
Lane IW & Comac L (1992). Sharks Don’t Get Cancer. Garden City Park, NY: Avery Publishing.
O’Connor MR, Robbins MA, Poll CP &Van Bonn WG (2019). Surgical removal of a papilloma from a longcomb sawfish (Pristiszijsron). 50th Annual IAAAM Conference, Durban, South Africa. Access Online.
Ostrander GK, Cheng KC, Wolf JC & Wolfe MJ (2004). Shark cartilage, cancer and the growing threat of pseudoscience. Cancer Research, 64, 8485–8491. Access online.
By Sophie A. Maycock for SharkSpeak
