A sea creature mentioned in 13th-century Old Norse manuscripts, which historians thought was a kraken-like mythological monster, is actually a whale using a hunting strategy known as trap or tread-water feeding, a new study finds.

Scientists only described this feeding behavior around a decade ago, after they spotted humpback whales (Megaptera novaeangliae) and Bryde's whales (Balaenoptera brydei) waiting with their mouths wide open in a motionless, upright position at the surface of the water. Unsuspecting shoals of fish perceive the gaping jaws as shelter and swim straight into the lethal trap.

A clip of a Bryde's whale performing this tactic went viral on Instagram after featuring in a BBC documentary series in 2021.

"I was reading some Norse mythology and noticed this creature, which resembled the viral whale feeding behavior," John McCarthy, a maritime archaeologist in the College of Humanities, Arts and Social Sciences at Flinders University in Australia, told Live Science. "Once we started to investigate a bit further, we noticed the parallel was really quite striking."

Marine biologists, archaeologists and medieval literature and language experts teamed up to investigate the similarities between the behavior of the medieval monster, named "hafgufa" in Old Norse manuscripts, and this whale feeding strategy. The study was published Tuesday (Feb. 28) in the journal Marine Mammal Science.

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A detailed account of the hafgufa, which translates to "sea mist," appears in a 13th-century manuscript called "Konungs skuggsjá" (the "King's Mirror") written for the Norwegian King Hákon Hákonarson, who reigned from 1217 to 1263. But researchers have traced references to the hafgufa back to a second-century A.D. Alexandrian text called "Physiologus," which contains drawings of a whale-like creature, referred to as "aspidochelone," with fish leaping into its mouth.

According to the researchers, medieval mariners probably knew that the hafgufa was a type of whale and not a fantastical sea monster. "Norse people were huge seafarers. Most of the trips people would take in the Middle Ages in Scandinavia were fishing trips, so they had a very high level of knowledge concerning the tides, the currents, the wave patterns, as well as the fish," Lauren Poyer, an assistant professor in the Department of Scandinavian Studies at the University of Washington, told Live Science.

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Nevertheless, some medieval accounts suggest that mariners landed their ships and lit fires on the hafgufa's island-like back. Only in the 18th century, however, did writers equate the creature to a leviathan, the kraken or even a mermaid. "I would call it a misuse of the medieval sources," Poyer said.

"The tendency is to dismiss medieval accounts of the natural world as misinformed and inaccurate," McCarthy said. "In fact, although their framework of knowledge was quite different, they were able to give a precise description of this type of whale in the 13th century. It was later on, in the absence of being aware of this feeding phenomenon, that 18th century writers invented this sea creature and made these errors."

In the Old Norse manuscripts, the hafgufa emits a perfume that attracts fish into its mouth. According to the new study, this special scent could refer to the "rotten cabbage" smell associated with whale feeding. Humpback and Bryde's whales also produce a distinct smell when they regurgitate their food to lure more prey into their stationary jaws.

So why did modern scientists only recently find out about it? One explanation is that technology such as drones enable us to watch whale populations more easily than before, McCarthy said. The second explanation is that "whale populations are just beginning to recover towards their natural, pre-whaling size and their behavior is changing as their numbers go up."

"If we take these as medieval eyewitness accounts, then it's not the 21st century when we first observe [the whale behavior], it's actually at least 1,000 years ago," Poyer said.