Scientists Discover Endangered Sunfish Swimming in Circles for “Speed Dating” to Mate.
The sunfish, the second largest fish species in the world after the whale shark, occasionally swims in large circles in a peculiar manner. In a paper published in the journal Fish Biology, scientists from the Marine Biological Association (MBA) and the Sunfish Conservation Organization in Ireland have uncovered the reason for this behavior, Newsweek reported on September 9.
Research team captures an image of a sunfish circle. (Photo: MBA)
The sunfish can reach lengths of up to 10 meters and weigh as much as 4.5 tons, according to National Geographic. They are known for their extremely large mouths. Despite their intimidating appearance, they only feed on plankton by passively filtering water as they swim. Although they typically live solitary lives, these massive fish gather only when mating. Previous research had noted their circular swimming behavior, and while there was suspicion that this behavior was related to mating, the exact reason for this activity had not been confirmed.
The research team observed 19 distinct schools of sunfish swimming in circles off the coast of County Clare, Ireland, from 2016 to 2021. Using underwater cameras and aerial drones, they discovered that each school consisted of 6 to 23 individuals swimming slowly at the surface, while others swam in circles below them, forming a doughnut shape. They swam in circles with diameters ranging from 17 to 40 meters, and depths of up to 16 meters.
“How sunfish, which are typically solitary, find mates in the vast ocean remains a mystery,” said David Sims, a researcher at MBA and a professor at the University of Southampton, who led the study.
The scientists also noted that the sunfish formed groups with an equal number of males and females. Male and female sunfish interacted with most other members for only a few minutes. During these “speed dating” sessions, they often rubbed their fins or gently brushed against each other. The findings revealed the courtship function of the sunfish circles. “It’s surprising that this natural wonder has remained hidden for so long, possibly because the circles primarily form at depths difficult to observe from the surface. This might explain why mating has never been observed,” Sims added.
The sunfish is classified as endangered in the International Union for Conservation of Nature’s Red List. Over the past 200 years, sunfish have been hunted for their liver oil, which is used in lamp production, cosmetics, perfumes, and lubricants. According to research, more than 100,000 sunfish were caught and killed from 1946 to 1997. Sunfish reproduce very infrequently and only reach sexual maturity between the ages of 12 to 16 years for males and 20 years for females, according to National Geographic.
