The Zuiyo-maru Carcass: Japanese Plesiosaur Catch

On April 25, 1977, the Japanese fishing trawler Zuiyo-maru was operating in the Pacific Ocean about thirty miles east of Christchurch, New Zealand, when its nets brought up something extraordinary. From a depth of approximately 300 meters came a massive carcass unlike anything the experienced crew had ever seen. With its long neck, small head, four flippers, and long tail, the creature bore a striking resemblance to a plesiosaur—a marine reptile that supposedly went extinct 66 million years ago.

The Discovery

The Zuiyo-maru was fishing for mackerel when the nets surfaced with their unexpected catch. The carcass weighed approximately 1,800 kilograms (about 4,000 pounds) and measured roughly 10 meters (33 feet) in length. It was in an advanced state of decomposition, with much of the flesh having rotted away, leaving stringy tissue clinging to the skeletal structure.

The crew gathered around the strange catch. The creature had a small head at the end of a long neck—estimates ranged from 1.5 to 2 meters for the neck alone. It had four large flippers and a long tail. Fat was still attached in places, and the overall appearance was nothing like any known fish.

Documentation

Michihiko Yano, a company representative aboard the ship, had the presence of mind to photograph and sketch the carcass. He collected samples of the tissue for later analysis. These photographs and drawings would become crucial evidence in the subsequent debate.

Despite the scientific value of the catch, the crew threw the carcass back into the sea. The stench was overwhelming, and they feared it would contaminate the valuable fish in their hold. The company’s catch was worth millions of yen, and the smell threatened to ruin it. Though Yano argued for keeping the specimen, he was overruled.

Initial Reaction

When the Zuiyo-maru returned to Japan, Yano’s photographs caused a sensation. The images appeared to show a creature matching descriptions of plesiosaurs from prehistoric times. Japanese media exploded with coverage of a possible living prehistoric creature.

The Taiyo Fishery Company, which operated the Zuiyo-maru, convened a panel of scientists to examine the photographs and tissue samples. Initial excitement was high—a surviving plesiosaur would be one of the greatest zoological discoveries in history.

Scientific Analysis

The tissue samples Yano had collected were subjected to analysis. Scientists examined the amino acid composition, seeking to determine what kind of animal the carcass had come from.

The results were disappointing for those hoping for a plesiosaur. The amino acid ratios were consistent with a basking shark, not a reptile. The protein analysis showed clear shark characteristics.

Further examination of the photographs supported this conclusion. The “long neck” could be explained by the decomposition pattern of basking sharks. When basking sharks decay, they lose their gill arches, lower jaw, and much of the head, leaving the spine and cranium protruding in a way that resembles a long neck. The four “flippers” corresponded to the positions of pectoral and pelvic fins. The overall shape was consistent with a badly decomposed shark.

The Basking Shark Explanation

Basking sharks are the second-largest living fish, commonly reaching 10 meters in length. They are found in temperate waters worldwide, including the area where the Zuiyo-maru made its catch.

When basking sharks die and decompose, they create what marine biologists call “pseudoplesiosaurs”—carcasses that superficially resemble long-necked marine reptiles. The phenomenon has been documented multiple times. Several supposed sea monster carcasses washed up over the years have been identified as decomposing basking sharks.

The gill arches of a basking shark are enormous, designed to filter plankton from vast quantities of water. When these rot away, along with the distinctive gill slits, the remaining body takes on a dramatically different appearance.

Continuing Controversy

Despite the scientific consensus, controversy continued. Some researchers noted apparent discrepancies between the Zuiyo-maru carcass and known basking shark anatomy. The fat content, the number of fin rays, and certain proportions did not perfectly match expectations.

Cryptozoologists argued that the photographs showed a genuine unknown animal, possibly a surviving marine reptile or an undiscovered species. They questioned whether the tissue samples had been contaminated or misidentified.

A minority of scientists have remained open to the possibility that the carcass was something unusual, if not necessarily a plesiosaur. The creature’s identity, they argue, cannot be definitively determined from photographs and limited tissue samples.

Cultural Impact

The Zuiyo-maru incident had significant cultural impact in Japan. A postage stamp commemorating the discovery was issued. The creature became known as “New Nessie” in reference to the Loch Ness Monster. The fishing company that made the catch used the image in marketing.

The incident sparked renewed interest in sea monsters and living prehistoric creatures. It demonstrated the possibility that unusual animals might still exist in the ocean depths and the challenges of identifying decomposed marine carcasses.

Scientific Legacy

Whatever the Zuiyo-maru carcass was, it provided valuable lessons. The incident demonstrated how decomposition can dramatically alter an animal’s appearance, creating “monsters” from ordinary creatures. It showed the importance of preserving specimens rather than discarding them. It illustrated how public excitement can outpace scientific analysis.

For marine biologists, the case became a textbook example of how basking shark carcasses can be misidentified. For cryptozoologists, it remained an intriguing case of a possibly unusual animal discarded before proper examination.

Conclusion

The truth of the Zuiyo-maru carcass likely lies somewhere between the extremes. It was probably not a plesiosaur—the tissue analysis strongly suggests a shark. But whether it was a typical basking shark or something slightly unusual may never be known. The carcass lies somewhere on the Pacific floor, beyond reach of further examination.

The incident reminds us that the ocean holds secrets and that even experienced fishermen can be startled by what their nets bring up. Whether those secrets include surviving prehistoric reptiles remains one of cryptozoology’s most compelling questions.