UPDATE (May 24, 2017): Just a day after it was rescued from fishing nets, the megamouth shark featured in this article was found dead on the seabed. More updates to this story follow below.
The megamouth shark is so rarely seen that its existence eluded us until 1976. To this day, the species is officially known from just 102 specimens. We've yet to discover where these creatures give birth, exactly where they live, or even how many of them are out there. But just recently, divers off the coast of Japan managed to free one of the lumbering giants after it swam into a fishing net.
This amazing clip was filmed by local resident Hiroyuki Arakawa during a dive in the waters off Chiba, a prefecture of Japan located on the island's east coast. While the sizeable shark looks scary, Arakawa and his fellow divers were in no danger here.
Like so many of the ocean's largest inhabitants, megamouth sharks (Megachasma pelagios) use their 50 rows of tiny teeth to sieve krill, small jellies and other plankton from the surrounding water – and those dietary preferences likely had something to do with how this individual landed in a midwater net.
The colossal fish are known to dive up to 1,500 metres beneath the surface, but their krill prey regularly migrate from deep to shallow water. Scientists suspect that megamouths follow suit: spending the daytime hours in the depths and ascending to mid-water at night to feed. The composition of their liver oil, which helps sharks stay buoyant, also supports the hunch that they clock ample time in the shallows.
This particular shark was first seen by fishermen in the early hours of the morning, so it likely became trapped during a midnight snack gone awry.
After news of the entanglement began circulating locally, fish researcher and TV personality Sakana-kun, who is also a professor emeritus of the Tokyo University of Marine Science and Technology, instigated rescue efforts. The stunning female – estimated to be between five and six metres (about 20ft) long – was moved to a safe area of open ocean using an underwater guide cage.
More so than any other country, Japan is the place to spot megamouth sharks – and even so, only about 20 sightings exist on record for the region. Most of these cases were similar entanglements, and very few of those sharks survived. In fact, only two living megamouth specimens have ever been studied by researchers, so this successful rescue is a big deal for science.
"My dream has come true," Sakana-kun told local news outlet Asahi. "I could see her vigorously swimming."
Unlike great whites, megamouths don't need to keep swimming in order to breathe: instead, they can move oxygenated water through the body by gill pumping. In entanglement situations, that certainly improves their chances of survival, but these incidents can still prove fatal if the sharks become really stuck. We also don't know how long the sharks can run on gill pumping alone.
Back in 2015, a 5.5-metre megamouth was killed by a fishing net in Albay's Marigondon port. That encounter marked the 15th sighting for the Philippines – but whether or not the area is a true "hotspot" for the sharks remains unclear.
When it comes to popular megamouth hangouts, some point to the western Pacific – but it could be that sightings in this region are simply more common due to its topography and extensive local fisheries. Specimens have been found in all major oceans over the years, so it's quite likely that these animals are cosmopolitan travellers.
UPDATE (May 24, 2017): Just a day after it was rescued from fishing nets, the megamouth shark featured in this article was found dead on the seabed. Sakana-kun, a professor emeritus of the Tokyo University of Marine Science and Technology, confirmed the death with local news outlet Asahi. "It is regrettable,” he said. "Having seen her on the previous day I thought she was going to become energetic."
UPDATE (May 25, 2017): After speaking with several local marine biologists, we can confirm that the megamouth shark's body has been collected for preservation. None of the information we have received thus far supports allegations that this shark was moved into a guide pen strictly to provide entertainment for divers. The exact hold-time and specific location of the shark's death, however, remain hazy.
"It was transferred to the offshore pen, and filmed and photographed there," says Dr Kazuhiro Nakaya, a professor emeritus at Hokkaido University, who wasn't directly involved in the relocation efforts. "The body has already been frozen by Kamogawa Sea World for scientific investigation, and we are going to make some investigation in near future."
Analysis of the shark's tissues may reveal a clearer cause of death. The stress of capture in the net and pen may certainly have been a contributing factor, but that is speculation at this point.
Some online commenters have surmised that time in the pen prevented the large shark from breathing properly. Nakaya explains that while this is possible, we really don't have the information to make that claim. As noted in this article, data from one megamouth shark suggests that the animals can – to some extent – breathe without swimming forward.
"The megamouth shark is a member of the salmon sharks in the broad sense," he says. "And they are generally adapted to offshore waters, always swimming. They take the water for breathing in mostly by swimming [but] also partly by buccal [gill] pumping. The 6th megamouth shark survived under roped condition, and so I think the megamouth shark can survive for some period of time only by buccal pumping."
We'll continue to update this post with any details regarding the hold, so watch this space.
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Top header image: Hiroyuki Arakawa/Facebook
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