Meet the megamouth shark (Megachasma pelagios). Despite being one of the biggest fish in the ocean, this species is famously elusive and little is known about its biology and life history. In fact, there have only been 273 total sightings since 1976 and two megamouth sharks have never been recorded together, until now.

On September 11, 2022, off the coast of San Diego, California, USA, two megamouth sharks were spotted together from a recreational fishing boat. Previously, only five free-swimming megamouth sharks have been reported (i.e. not as fishing bycatch or stranded) and this event marks the first time two of these ocean giants have been seen together.

The people fishing took short videos of their encounter and via social media, the sighting was passed on to scientists who analyzed the videos and interviewed the people on the boat. 

When they were first spotted, the two sharks at the surface were around 45-60 meters (148-197 feet) from the fishing boat and the larger one, measuring around 4.6 meters (15 feet), appeared to be circling the smaller one, which was 3.7 meters (12.1 feet) long. The smaller shark, identified as a male, remained circling deeper under the water as the boat approached but then tailed the larger shark (sex unknown) towards the boat at a different depth. 

The sharks were not observed feeding during this time, and the larger shark made several passes near the boat while the smaller one remained mostly at depth except for one slow appearance at the surface near the side of the boat.

The team suggest two main theories for why the two megamouth sharks were together during this time. The first is that the sharks were observed together to forage. Though little is known about the diet of megamouth sharks, they are believed to be filter feeders that consume mainly krill, but also squid, and other gelatinous organisms. While no feeding behavior was observed by the people filming, the appearance of the fishing boat could have disturbed the sharks. The people on the fishing boat also reported an abundance of mola and gelatinous organisms. Mola and megamouths are often recorded together in fishing bycatch reports, and both feed on gelatinous organisms. 

The second theory is that the megamouths were engaging in a courtship event. The researchers think the trailing behavior of the male shark to the larger shark is similar to courtship behaviors seen in other shark species. They also suggest that mating opportunities can arise in other shark species when there is a feeding aggregation. No copulation attempts were made during the encounter with the boat, but the researcher noted that observations of sharks mating, particularly the filter feeding species, are extremely rare. 

The researchers conclude that based on the video evidence and previous insights, the two sharks were most likely engaging in pre-mating behavior. They further suggest that with the rise of social media, increased awareness, and ocean traffic, reports like this may become more common and shed more insight into the lives of these elusive megamouth sharks.

The study is published in Environmental Biology of Fishes.