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Revisiting the Zuiyo-maru "Sea Monster"

Mamerto Adan is a feature writer who is back in college once again. Science is one of his favorite topics.


Few people realize this, but the ocean is a mysterious place. We probably know more about the surface of Mars than what is going on in that great abyss. We have gone a long way since early sea explorers bring back stories of voyages in the world’s end, and how they encounter monstrous creatures along the way. Even today at the age of great scientific discoveries, our knowledge of our bodies of water is still inadequate. A friend once joked that all he knows about what lives there were limited to sea foods.

With that said, we can’t blame the land dwellers if they freaked out upon seeing deep sea dwellers making a visit to the surface. Those vast unexplored regions of the ocean may harbor some yet to be known creatures, as what the sightings of megamouth sharks showed us. And when wild imaginations got mixed with lack of knowledge, we may perceive any peculiar shapes swimming in the ocean as monstrous.

Once, the crews of a Japanese fishing trawler got nothing in mind but hauling catches one April day. But as they pulled in their net, they got more than what they hoped for. A large rotting carcass of what appeared to be a sea monster.

The Zuiyo-maru

The Zuiyo-maru, docked in Vancouver.

The Zuiyo-maru, docked in Vancouver.

It all happened one morning of the 25th of April, 1977. During that day, the Japanese trawler Zuiyō-maru went to sail east of Christchurch, New Zealand to harvest mackerels. But 300 meters below the surface, something else got caught in their net. And when the net was pulled back, and its content examined, it became clear that what they got wasn’t the usual haul of mackerel. Instead, was this large carcass, weighing 4000 lbs, or two tons. The problem here was that this dead animal was in an advanced state of decomposition, no more than a hunk of foul-smelling flesh and some bones. To pull in a rotting sea creature was one thing, but knowing what exactly the thing was another. For much to the crews’ surprise, it never resembled their usual idea of a regular sea creature. In fact, they were not sure what it was.

The photo of the mysterious carcass.

The photo of the mysterious carcass.

The mysterious carcass measured ten meters, from the “nose” to “tail”. The remaining flesh were whitish, or greyish, with the surface covered with dermal fibers. It got fat-like tissues on the back, and remnants of reddish muscles. Its overall shape was described as a fleshy mass, with spine and a long 1.5-meter neck that ends in a cranium. It got large flippers. No internal organs or dorsal fins remained.

The crews assumed that it might be a dead whale, or a turtle missing its shell. But it also bore a resemblance to a long extinct prehistoric marine reptile. But the captain Akira Tanaka decided to dispose of the carcass back to the sea, before it fouls the catches. But not before photographs, sketches and pieces of the carcass were taken as samples.

Relative of Nessie?

A skeleton of plesiosaurus.

A skeleton of plesiosaurus.

Nevertheless, the discovery caused an excitement in Japan once the news came out. The thing here was that the corpse so resembled a prehistoric reptile known as the plesiosaurus, from the elongated neck, to the flippers. Officially, the creature was long gone, but how come they found its rotting remains somewhere in the waters of New Zealand? Could it be that it died just last month, and more like it lurks? If that was the case, it confirmed the existence of sea monsters. People reckoned that sea monster legends came from sightings of relic populations of plesiosaurs. Plus, it may help explain why there was something reptilian peering out from a lake in Scotland.

From the fossilized remains, the plesiosaurus is how we expected what a sea monster looks like. It was not a dinosaur however, but a prehistoric marine reptile with long neck, short tail, flippers and flattened body. Some body proportions might differ, like the kronosaurus, which had short neck but an enormous skull. They range from five feet, to forty-nine feet in length. And unlike the classic reconstructions, plesiosaurs don’t raise their head in a swan-like posture.

Going back to the carcass of the Zuiyo-maru, the excitement built when members of the Japanese scientific community support the plesiosaurus hypothesis, like Professor Tokio Shikama from Yokohama National University and Dr. Fujiro Yasuda from Tokyo University of Marine Science and Technology. They based their findings on photographs taken before the carcass was dumped back to sea.

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But other scientists were not convinced.

But Not So Fast...

Close-up of the rotting carcass.

Close-up of the rotting carcass.

Other scientist doubt that the rotting hunk of flesh was even a plesiosaurus remains. Hans Christian Bjerring, a Swedish scientist once pointed out that the difference on the environment back then, when the plesiosaurus was around raises doubts. Ove Persson, another Swedish scientist was equally critical, as how a large air breathing reptile escaped detection that long?

Plesiosaurus evolved to adapt in its own environment. And the condition of the sea today, from the marine environment to the prey item might not fit to sustain a plesiosaur population. But again, who knows? Creatures like the coelacanth managed to survive across many prehistoric time periods until being discovered recently. And that was after going undetected. This raises the possibility that other relic population of marine life might be lurking elsewhere, and the vast undiscovered ocean holds many possibilities.

The problem here is that a plesiosaur is an air breathing animal that needed to surface. It never stayed in the deep for too long, hence not easy to miss. If that was the case, sightings of these large long necked creatures should not be limited to tall tales and legends.

Thankfully, the crew of the Zuiyo-maru brough flesh samples of the strange carcass for testing. And the result of amino acid analysis revealed an underwhelming conclusion.

A Dead Basking Shark

An illustration showing how a dead basking shark degrades. Source,

An illustration showing how a dead basking shark degrades. Source,

As it turns out, the rotting carcass was no more than dead fish, a basking shark to be exact as what the biochemical test showed. But it can’t be! The whole dead thing never resembled any living basking shark. Just look at the obvious plesiosaurus shape.

But the prehistoric looking condition of the carcass was also explainable by simply understanding how a basking shark degrades after dying.

The first thing to fall off is the lower jaw, the gill area, and the dorsal and caudal fins. Being a cartilaginous fish, the highly calcified spine and the braincase would remain, producing the “head” and the long neck.

And the result is disfigured shark carcass that took on a plesiosaurus appearance.

Decomposing Animals Are Unrecognizable

A dead whale.

A dead whale.

This applies to all animal, whether land dwelling, or sea dwelling. A dead animal could be passed for something else. This is especially true for marine vertebrates, like sharks and whales. With a lot of cartilaginous parts that could fall off, sharks could look like something else. In fact, the Zuiyo-maru incident wasn’t the first time a dead basking shark was passed for sea monster. Back in the 19th centry, a commotion happened upon the discovery of the Stronsay Beast. Even today, the internet is full of pictures of sea monster remains, that turned out to be dead whales.


  1. Borrell, James (23 September 2016). "Have you heard about the mystery of the Zuiyo-maru?" Animalogic.
  2. Kuban, Glen (n.d.). "Sea Monster or Shark?".
  3. Aamoth, Doug (31 March 2010). "Cryptids: New Nessie"

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