Updated: 5 days ago
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 do they do this? That is the function of breaching?
Unlike their faster moving relatives which breach (including white sharks (Carcharodon carcharias), shortfin makos (Isurus oxyrinchus) and common thresher sharks (Alopias vulpinus)), 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 1 m 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).
There are many hypotheses for why basking sharks breach. Currently, scientists are debating whether breaches are used for communication in mating or forging, or for the removal of parasites. 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.
But what is particularly interesting is the recent discovery that basking sharks favour a particular side of their body when breaching. This is termed "lateralisation". Recent tagging studies have found that the majority of basking sharks preferred to direct their breaches into a right-sided roll. This 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!).
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 wold 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).
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!
Great thanks go to much to Miss Jessica Rudd for contributing her masters thesis for this article!
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.
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.