There are many, many myths about sharks that are quite pervasive and persistent. One that has been around for a while, is that sharks must keep swimming or they will die, as they cannot breathe. Some people even use this as a motivational idea; to just keep swimming - to keep pushing forward. But if, in fact, sharks can keep breathing whilst staying still, how do they do this? And where has this myth come from then?
Minority Report
The myth that sharks must keep swimming to be able to breathe has come about because it has some basis in fact. It is absolutely true that there are some species of sharks which do, indeed, need to keep swimming in order to breathe. The spiny dogfish (Squalus acanthias), for example, never stops swimming. However, this myth massively oversimplifies and overgeneralises, as the sharks that never stop swimming are actually in the minority! The vast majority of sharks do not need to keep moving to be able to breathe (Abel et al, 2020; Ebert et al, 2021).
Where human beings breathe air into our lungs to draw oxygen into our blood, sharks extract oxygen from water via their gills. As you may vaguely remember from high school biology, oxygen is vital for the chemical reaction "respiration", through which cells create energy. Many animals, including sharks, will die if they cannot get enough oxygen.
It's Rammed!
There are two main types of ventilation that sharks use to get their oxygen: "buccal pumping" and "ram ventilation". The few dozen species of sharks that must keep swimming to be able to breathe are known as "obligate ram ventilators". This means (as the myth states), that if they were forced to stop moving, they would not be able to flush oxygenated water across thier gills and they would eventually asphyxiate. This is how sharks can end up 'drowning' in water if they are caught in nets (Compagno, 1984; Abel et al, 2020; Ebert et al, 2021).
Ram ventilating sharks ensure they have a steady flow of oxygen-rich water over their gills by continuously swimming forwards. As they swim along water passes into their mouth, and runs over and back out through the gills, where oxygen is extracted. The faster they swim, the more water is pushed through their gills (Abel et al, 2020; Ebert et al, 2021).
This can only be achieved in one direction though, so if a shark is dragged backwards by the tail, they will not able to breathe properly and can even 'drown' (Abel et al, 2020; Ebert et al, 2021).
Obligate ram ventilators include the hammerhead sharks (Family Sphyrnidae) and whale sharks (Rhincodon typus). This method is especially common within the Lamniform sharks - makos (Isurus species), the great white (Carcharodon carcharias), threshers (Alopias species) and salmon sharks (Lamna nasus) all obtain oxygen by forwards motion (Abel et al, 2020; Ebert et al, 2021).
Mouth Breathers
At the other end of the spectrum are the (much more numerous) species of sharks that do not have to keep swimming to wash water over their gills in order to breathe. Instead they are able to use their "buccal" (aka mouth) muscles - by opening and closing their mouth, they create a vacuum, which draws oxygenated water in and over the gills (Abel et al, 2020).
There are many, many species of sharks that use this method. In fact, it would be quicker to list those that don't! Buccal pumping occurs across many taxonomic groups of sharks, ranging from nurse sharks (Ginglymostoma cirratum) to Port Jackson sharks (Heterodontus portusjacksoni), and from lesser spotted catsharks (Scyliorhinus canicula) to zebra sharks (Stegostoma tigrinum) (Abel et al, 2020; Kelly et al, 2020; Ebert et al, 2021).
There are also many species of sharks, including tiger sharks (Galeocerdo cuvier), lemon sharks (Negaprion brevirostris) and sandtiger sharks (Carcharias taurus) that can switch between these two methods depending upon the situation they are in. If they are moving fast enough, they breathe via ram ventilation and then swap to buccal pumping when they are resting on the bottom (Compagno, 1984; Abel et al, 2020; Ebert et al, 2021).
Going Snorkelling
Another way that sharks obtain oxygen, common amongst the buccal pumping gang, is the use of "spiracles". Found on all sharks except for requiem sharks (Family Carcharhinidae) and hammerheads (Family Sphyrnidae), these paired openings can be seen on each side of the head, just behind the eyes. Don't be fooled, these are not ears! They are for respiration (Abel et al, 2020; Ebert et al, 2021).
Spiracles supply highly oxygenated blood directly to the optic nerve and the brain. In species that lie on the substrate like angel sharks (Order Squatiniformes) and wobbegongs (Family Orectolobidae), the spiracles act like a snorkel to draw oxygenated water over the gills, without sucking up sand and debris. This is what allows these sharks to be such successful ambush predators - as they can lie in wait on the bottom (Compagno, 1984; Abel et al, 2020; Ebert et al, 2021).
The spiracles are especially large and well-developed in the shark's batoid relatives. These critters use their spiracles to actively pump water over their gills whilst they are buried in the sand (Compagno, 1984; Abel et al, 2020; Ebert et al, 2021).
Keep Talking
This myth is interesting because it is partly true, but also equally inaccurate. So it is an excellent example of how scientific knowledge can easily be taken out of context and be misunderstood when it is not properly communicated.
In the past, scientists and the public were somewhat separate - with scientific knowledge locked up in inaccessible literature and prestigious (intimidating!) institutions, and the effort made to communicate new findings to the general pubic was woefully limited.
Today, we now understand that it is vitally important to empower people through education. If someone is interested in a scientific topic, why can't they be free to learn about it!? So now scientists are starting to focus more on reaching out to the public and improving our communication (that is exactly why I write these articles!). If scientists do not make the effort to form a relationship and a dialogue with the general public, not only do we have no hope of getting people interested in our work, but we run the risk of misunderstandings or out-right ignorance causing bizarre myths to continue to perpetuate.
References
Abel D, Grubbs RD & Guttridge TL (2020). Shark Biology and Conservation: Essentials for Educators, Students, and Enthusiasts. Johns Hopkins University Press.isbn: 9781421438368.
Compagno LJV (1984) Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. FAO Species Catalogue. Access online.
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Kelly ML, Murray ER, Kerr CC, Radford CA, Collin SP, Lesku JA & Hemmi JM (2020). Diverse activity rhythms in sharks (Elasmobranchii). Journal of Biological Rhythms, 35;5, 476-488. Access online.
By Sophie A. Maycock for SharkSpeak
