The mighty basking shark (Cetorhinus maximus) can reach lengths of 11 metres and weigh over 5,000kg. They are relatively sedate and slow-moving, cruising through the water, filtering their microscopic food-stuffs into their enormous, open mouths. And yet, basking sharks are known to “breach”, meaning they jump out of the surface of the water! In fact, they often breach several times in a row… Quite a sight! So why on earth do they do this? Why bother? Does breaching have a function?
Jump Around
It might surprise you to learn that there are actually many different sharks that breach out of the water: great whites (Carcharodon carcharias), threshers (Alopias species), makos (Isurus species), spinner sharks (Carcharhinus brevipinna), as well as many different rays, to name a few (Johnston et al, 2018; Rudd, 2020). To learn more check out Jumping the Shark
But unlike their faster-moving relatives, basking sharks do not hunt using surprise attacks, which would require sudden, great bursts of speed. In fact, they usually cruise at a leisurely 3 km per hour. With their mouth open up to one metre across, they sift 500 tons of water per hour through their “gill rakers” (small cartilaginous projections from the gill arch, which form a finger-like sieve) to filter out their “zooplankton” food (tiny animals such as crustaceans, and eggs and larvae of larger animals) (Compagno, 1984).
Scientists have many theories about why basking sharks breach. Currently, there is a debate about whether it might be for for the removal of parasites or maybe for communication with other sharks whilst mating or forging. We have more work to do to be certain of the function of breaching and it is very possible that the behaviour has several functions depending on the context (Rudd, 2020).
Right Hand Man
But what is interesting about basking shark breaching is that they seem to favour a particular side of their body when jumping. Tagging studies have found that the majority of basking sharks preferred to direct their breaches into a right-sided roll. This "lateralisation" is similar to how a person tends to favour one hand when playing tennis or opening a door, so it is possible basking sharks are right-handed (or right-finned may be more apt!) (Rudd, 2020).
It is possible that basking sharks favour different sides when they are using their breach for different functions. If breaching is used to dislodge parasites, the breach would be directed towards the side where parasites were attached to the skin. However, if breaches are used to communicate with "conspecifics" (other individuals of the same species), it may be that the direction of the breach says something different (Rudd, 2020).
Lateralisation has also been found in several species of marine mammals, and has been reported in fish across different contexts; including in relation to aggression, for communication, when foraging, during anti-predatory manoeuvres, to reduce the cost of movement, and in response to climate change (Rudd, 2020).
Right Place, Right Time
In British waters, if luck is on your side and you know where to go, you can witness these spectacular basking shark breaches in person! Current research is focusing on an area in the Sea of Hebrides, off the coast of the islands of Coll and Tiree, where breaches are relatively common. If you would like to see a basking shark jump, head over to Ireland Wildlife to learn when, where and how you can spot them on your next Irish holiday!
Acknowledgents
Great thanks go to much to Miss Jessica Rudd for contributing her masters thesis for this article!
References
Compagno LJV (1984). "CETORHINIDAE – Basking sharks". In:Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations.
Johnston EM, Halsey LG, Payne NL, Kock AA, Iosilevskii G, Whelan B & Houghton JD (2018). Latent power of basking sharks revealed by exceptional breaching events. Biology Letters, 14:9, 20180537. Access online.
Rudd JL (2020). ‘Fantastic breaches and where to find them: first insights into basking shark breaching behaviour’. In: The role of accelerometry in the conservation of two coastal marine vertebrates. Masters degree thesis, University of Exeter, Exeter, UK, pp. 24- 58.
By Sophie A. Maycock for SharkSpeak
