Although plankton-eating basking sharks are the second largest filter-eating sharks in the world after whale sharks, much of its life is unobserved. Now, however, researchers have determined that these often solitary animals find mates by "gathering" and surrounding each other, forming an underwater shark circle.
Over the past 40 years, scientists have occasionally observed this circumference on the North Atlantic coast of Canada and the United States. The three-dimensional donut-shaped shape formed by the shark's swimming pattern is called a toroidal surface.
Previously, people did not fully understand the purpose of these "gatherings". To find out, scientists from the Irish Basking Shark Group and the Marine Life Society of England used underwater cameras and aerial drones to study basking shark populations off the coast of Clare, Ireland. From 2016 to 2021, the researchers recorded a total of 19 "circling groups" at nine sites in the region. The number of sharks in each group ranges from 6 to 23.
According to preliminary observations, these animals did not eat when they engaged in this behavior, so food is clearly not a motivating factor. However, other findings suggest that the sharks are rapidly checking each other and then choosing a partner to pair with.
First, each group consists of sexually mature males and females in roughly equal numbers. In many cases, female sharks temporarily exhibit a paler body color than normal, which is thought to be associated with the willingness of other shark species to mate.
In addition, although this situation lasted for hours to days, each shark in each group "socialized" with most of the other sharks in the group within a few minutes. These interactions—initiated by both males and females—take the form of fin-to-fin and fin-to-body gentle contact. Male sharks will also roll over, exposing their bottoms (i.e., their reproductive organs) to the female sharks that follow.
Lead author of the study, Professor David Sims of the Marine Life Association, said: "How basking sharks, who usually live alone, find a mate in the vast oceans has been an enduring mystery. Incredibly, we now find that courtship rings not only form, but also resemble a slow-motion 'fast dating' event that evaluates many potential partners at once. Surprisingly, this wonder of nature has been hidden for so long, probably because rings most often form deep away from surface observation, which could explain why mating itself has never been seen. ”
Now, it is hoped that the scientists' findings will help strengthen basking shark conservation efforts in the region. The paper on the study was recently published in the Journal of Fish Biology.