Giant Basking Sharks' Strange Swimming Circle Formation Explained [Study]

The scientists finally cracked the code of the swimming circle formation of basking sharks. The swimming formation was a courtship dance referred to as the "speed dating" style.

The endangered species are considered the second largest fish species next to whale sharks. They have occasionally been spotted in strange mass circle dances.

Researchers from the Marine Biological Association (MBA) collaborated with the Irish Basking Shark Group to find an answer as to why the sharks do the strange circles.

Courting in Circles: The Speed Dating Style

The gentle giants only gathered together to mate. However, there are not much studies about their mating process.

They have been seen swimming in circles and were suspected of mating-related behavior. However, at the time, the exact reason for the activity is not yet confirmed as of the moment.

The scientists observed 19 separate groups of circling basking sharks in the waters off County Clare in Ireland between 2016 and 2021. With the help of underwater cameras and aerial drones, the researchers figured out that each circling group was composed of between six and 23 sharks slowly swimming at the surface, while another group was also swimming in a circle below them. The circles measured between 55 to 130 feet in diameter and 52 feet in depth.

MBA senior research fellow David Sims said that alone basking sharks finding a mate in the ocean's vastness has added to the current mystery about the mammals. The University of Southampton professor also added a statement that courtship circle is not only for slow motion "speed dating" but also an assessment of potential mates in one go.

They also observed that the number of sharks involved in the "toruses" was equally composed of sexually mature males and females. Once the connection has been built, the couple will undergo gentle fin-to-fin and fin-to-body touching.

The paper was published in the Journal of Fish Biology.

(Photo : Getty Images)
Considered as endangered, basking sharks' mating circle observed by scientists.

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Basking's Reproduction Before Extinction

Other sharks also have mystical terms of mating and courtship rituals because they spend all their time underwater. On the other hand, the great white sharks have never observed mating.

The basking sharks were listed as endangered on the IUCN Red List of Threatened Species.

For more than 200 years, these aquatic giants were widely fished because of their liver oil. The oil is used in lamps, perfumes, cosmetics, and lubricants.

According to IUCN, over 100,000 Cetorhinus maximus were fished and killed between 1946 and 1997 alone.

Many countries globally banned the fishing of basking sharks in the 1990s. Few young and sexually mature sharks will be a candidate for mating - around 12 to 16 years for males and about 20 years for females. That means their population is struggling to recover.

The study took place on the northeast Atlantic coast, which was considered a critical habitat supporting the reproductive behavior of endangered basking sharks.

According to National Geographic, basking sharks can grow up to 33 feet long and weigh up to 4.5 tons. They are known for their big, gaping mouths, which, despite their frightening look, are used only to passively filter-feed planktons from the seawater as they swim.

These placid sharks are scattered all over the world and are totally harmless. They only eat microorganisms called zooplankton. Their three-foot-wide mouth agape lets them take water and filter out the planktons using their gill rakers, a special body part that prevents their food from escaping through their gills.

Marine biologists believe that their teeth-which are not used in feeding--are part of their mating process.