Giant Sharks Swarm to Swim Round in Circles and Scientists Finally Know Why

Endangered basking sharks swim around multiple others in a "speed dating" style courtship dance, scientists have found.

Basking sharks, the second largest species of fish in the world after the whale shark, have been occasionally spotted performing these strange mass circle dances, but in a paper published in the Journal of Fish Biology, scientists from the Marine Biological Association (MBA) and the Irish Basking Shark Group have worked out why they do it.

Basking sharks can measure up to 33 feet long and weigh up to 4.5 tons, according to National Geographic. They are famous for their huge, gaping mouths, which despite their terrifying appearance, are used only to passively filter-feed plankton from the water as they swim.

Usually solitary, these giants only come together to mate, although mating has never been observed. They have previously been sighted swimming in circles, and while that was suspected to be mating-related behavior, the precise reasons for the activity had not been confirmed until now.

The researchers observed 19 separate groups of circling sharks in waters off County Clare, Ireland, between 2016 and 2021. Using underwater cameras and aerial drones, they found that each circling group was made up of between six and 23 sharks swimming slowly at the surface, with others were also swimming in the circle below them, in a three-dimensional donut formation. These shark circles measured between 55 and 130 feet in diameter, and went to depths of about 52  feet.

"How usually solitary basking sharks find a mate in the ocean's expanse has been an enduring mystery," David Sims, a senior research fellow at the MBA and a professor at the University of Southampton, and the lead author of the study, said in a statement. "Incredibly we now find that a courtship torus not only forms but acts like a slow motion 'speed-dating' event for assessing lots of potential mates in one go."

The scientists also found that the sharks involved in these "toruses" were made up of equal numbers of sexually mature male and females, with individual females and males only interacting with most other members for a few minutes of the hours-long torus formation. During their brief associations in this "speed dating" session, the males and females underwent gentle fin-fin and fin-body touching.

"Additional unifying characteristics were the presence of breaching behavior, dynamic assorting of females and males within a torus and female–male interactions such as touching and rolling/diving behavior. Collectively, the results strongly suggest a courtship function for the basking shark torus," wrote the authors in the paper.

"It is astonishing that this wonder of the natural world has remained hidden for so long, presumably because circles most often form at depth away from surface observation, which could explain why mating itself has never been seen," Sims said.

Many other sharks are also a mystery in terms of mating and courtship rituals, due to the fact they spend all their time underwater. The great white shark has also never been observed mating.

Basking sharks are listed as "endangered" on the IUCN Red List. Over the past 200 years, these giants were widely fished for their liver oil, which was used in lamps, cosmetics, perfumes, and lubricants. According to the paper, over 100,000 basking sharks were caught and killed between 1946 and 1997 alone.

Many countries began banning the fishing of basking sharks in the 1990s. They have very few young, and only reach sexual maturity at around 12 to 16 years in males, and about 20 years in females, according to National Geographic, meaning that their populations have struggled to recover.

"This study highlights northeast Atlantic coastal waters as a critical habitat supporting courtship reproductive behavior of endangered basking sharks, the first such habitat identified for this species globally," the authors wrote in the paper.

According to the Florida Museum, basking sharks continue to be fished in China and Japan, primarily for their fins and their livers. The fins are sold as the base ingredient for shark fin soup, and the livers are thought to be an aphrodisiac.

"Our discovery of important basking shark courtship grounds in coastal waters off western Ireland makes it even more urgent that this species gains protection in Irish waters from potential threats, such as from collisions with marine traffic and the impact of offshore renewables," co-author Dr Simon Berrow of the Irish Basking Shark Group and Atlantic Technological University, Galway, said in a statement.