The megamouth shark – a sluggish but arresting-looking filter-feeder that basically puts the head of a majorly oversized frog on the body of a mackerel shark – only emerged on the scientific radar in 1976. That's an astonishingly late date for such a big animal, thought to reach at least five metres in length and possibly more. Since then, fewer than 300 megamouths have been documented, most of them dead, stranded or net-tangled specimens – which makes the following footage, taken in Taiwan on June 7, 2024, so remarkable:
The diver having the once-in-a-lifetime encounter was shark biologist and documentary filmmaker Bee Smith. Working in collaboration with local researchers, she’d been riding aboard the same kind of Taiwanese drift-net fishing boat that, through accidental capture, has yielded the world’s most prolific megamouth records in hopes of finding what’s otherwise an incredibly elusive shark. As it happened, Smith – who recounted the search, on a tight timetable, in a TikTok video journal – lucked out with not one, not two, but three megamouths, including this large female.
Having such an intimate, underwater look – something only a handful of people have likely enjoyed – at a rarely seen marine giant made an impression, to say the least. “When I saw the first megamouth, it didn’t seem real,” Smith told me by email. “I had seen a dead specimen before, and of course pictures and videos, but it looked so different. The first things I noticed were its chin covered in dark spots, its beautiful brown eyes, and its large floppy pectoral fins – so unlike any other shark.”
The expedition was part of a documentary project: Smith, who researched shark bycatch at the University of Oxford for her master’s degree, is working on a film about the subject in Taiwan. In July 2020 – only a month after no fewer than six megamouths snarled in drift nets off Hualien County in the space of a few days – the country instituted a ban on the capture of the species (as well as great whites and basking sharks). The new ruling required fishers who accidentally catch the sharks to immediately release them, whether they’re alive or dead. “My documentary is about this policy’s effectiveness in terms of whether it sufficiently protects megamouths,” Smith told me, “and it is also about the negative impacts the policy has had for megamouth fishers.”
Smith told IFL Science she and her team were accompanying drift-net operators in order to document their industry, help cut free and film any caught megamouths, and gather tissue samples from and affix acoustic tags to the sharks.
Primary targets of the drift-net fishery off eastern Taiwan are sunfishes or molas, which are primarily sought at night from April to August. It’s suspected that molas and megamouth sharks share a basic daily pattern of vertical movements, spending the day at depth and moving closer to the surface at night to track prey (an up-and-down journey technically termed “diel vertical migration”). It’s possible molas – munchers of jellies, salps, small fish, and other pint-sized snacks – and megamouths, mainly plankton-eaters, also more generally mingle around upwellings and other favourable, nutrient-rich areas. These overlapping movements and geographies may make megamouths particularly vulnerable to entanglement in the nets of mola fishers.
The mola drift-net fishery in Taiwan intensified after about 2000, and in the decades since has seen these waters log more than half of all recorded megamouths. A 2019 IUCN Red List of Threatened Species assessment classified the megamouth as a species of “Least Concern,” but noted that the issue of bycatch warranted further study and mitigation measures.
So little is known about megamouths that it’s not clear whether these Taiwanese waters are a legitimate global hotspot for the species or simply tally relatively many sharks because of the intense deep-water fishery here. Some data hints at relatively low genetic diversity in Pacific megamouths, suggesting the sharks may be cosmopolitan seafarers.
A 2021 study looking at the spatial-temporal distribution of megamouths theorised the sharks might travel along the mighty swirl of the North Pacific Gyre across the course of the year, with some – perhaps especially males – crossing the ocean basin from Asia to the western coast of the Americas in the late summer and fall and possibly returning westward via the North Equatorial Current. This proposed scheme integrates a spring-summer megamouth feeding zone in the middle Western Pacific, maybe driven by the Kuroshio Current and associated upwellings, which includes the continental shelf off eastern Taiwan. The authors stress the need for more research, such as tagging studies, in order to confirm such movements. As with all things megamouth, after all, our knowledge thus far is fragmentary at best.
One of the more intriguing recent megamouth observations came out of the waters of Southern California, where – potentially supportive of the yearly megamouth rangings hypothesised in that 2021 paper – the vast majority of sightings have been logged between September and December. On September 11, 2022, as reported in a 2023 Environmental Biology of Fishes paper, fishermen filmed a pair of megamouths near the surface about 40 kilometres offshore San Diego. The sharks – an approximately 4.6-metre individual and a 3.7-metre one that, the footage showed, had the claspers giving it away as a male – were circling in close proximity, “creating a disturbance in the surface water” that initially clued the fishermen in.
The study authors analysing the video concluded that the sighting may have documented “pre-copulatory behaviour,” given the up-close interaction between the two animals and the fact that the larger shark, likely a female, bore marks similar in appearance to the mating scars well-documented in many shark species. (Shark sex is, shall we say, on the rough side.)
The researchers noted some potentially interesting associated observations: The fishermen who filmed the megamouth pair also reported 20 or more molas in the vicinity that day, and whale sharks – filter-feeders like megamouths that are also thought to undergo diel movements and congregate hungrily at upwellings – were spied on multiple occasions off the San Diego coast that same September. Besides the potential base-level seasonality of megamouth occurrence in this part of the Eastern Pacific, the authors noted that the recent nearby passage of post-Hurricane Kay’s remnants could possibly have induced upwellings that drew the filter-feeders as well as the sunfishes to these whereabouts.
Smith – who plans to return to Taiwan next year for more megamouth work – told me that seeing living megamouths was a coup, not only because of the obvious value for her forthcoming film, but also, more fundamentally, on account of a longtime passion for the more mysterious, “eccentric” members of the shark clan. “I fell in love with sharks as a child,” she said, “and it all started with the weird, deep-sea species, especially the goblin shark, so to now be a researcher and filmmaker who successfully led an expedition that found three megamouths meant a lot.”
Header image: Penny Bielich
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