Rarely observed circling formations of basking sharks in Irish waters have been explained as 'speed dating' displays in a new study.
Research by marine biologists from the UK's Marine Biological Association (MBA) and the Irish Basking Shark Group reveals that the circles of basking sharks seen in recent years off the Co Clare coast are animals engaged in courtship behaviour, the first place in the world where this has been verified.
The research team captured footage of 19 circling groups using underwater cameras and aerial drones off the Clare coast from 2016 to 2021.
They found each group comprised between six and 23 sharks swimming slowly at the surface, with others below them deeper down, in a three-dimensional ring structure the researchers termed a 'torus'.
The team found that the sharks in circle formations included equal numbers of sexually mature male and females, and were not filter-feeding.
Some females had a paler body colour than males, a difference seen during courtship and mating behaviour in other shark species.
Professor David Sims, Senior Research Fellow at the MBA and University of Southampton said: "How usually solitary basking sharks find a mate in the ocean's expanse has been an enduring mystery. 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.
"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."
Basking sharks feed on microscopic animals called zooplankton and can grow up to 12 metres in length.
They were hunted for liver oil and fins throughout much of the 20th Century in the northeast Atlantic.
They remain endangered in Europe as populations continue to recover.
Dr Simon Berrow of the Irish Basking Shark Group and Atlantic Technological University, Galway who co-led the field research, said: "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."
Legislation to protect basking sharks in Irish waters was drafted earlier this year. If signed into law it will be illegal to hunt, injure, interfere with or destroy their breeding or resting places.