In the case of the Plesiosaur-shaped creature off Cornwall related in Tim Dinsdale's book Monster Hunt (above), the creature was also seen fighting seals from above, and in a similar tradition relating to the Icelandic Skrimsl, the monster was characteristically said to decapitate seals. Putting these observations together, it seems that Longnecks can feel antagonistic (territorial? defensive?) against seals and sea lions, and it deals with them by throwing them up into the air to break their necks.
snap!
Since a common type of Sea serpent sighting seems to be a fishing posture striking down from above, it is not too unusual that the usual depiction of Longnecks attacking humans shows the creatures bearing down on the humans from above and grabbing them headfirst by their mouth
This also seems to be the usual attack mode of the congo dragons. Fairly early on I read in On The Track of Unknown Animals that such creatures as the Mokele-Mbembe were often said to kill humans but not eat them and to leave the "bodies" untouched. I wondered then if the peculiar emphasis on leaving the bodies untouched meant that they bit the heads off. Later on I did get confirmation from other traditions in other parts of Africa which stated explicitly, "He takes the head off and then hurls the rest of the body away from himself in disgust, since he wants nothing more to do with it" This is putting the matter rather more plainly and I suppose the more common version wants to put the matter more delicately than that because it is a matter distressing to the listeners. However back in the 1970s when I first noticed the odd statement I also noticed that the Icelandic water monster the Skrimsl (Comparable to the Loch Ness Monster) was stated by Heuvelmans characteristically to "decapitate seals and sink ships" There seemed to me to be a connection between the way the Longnecked creatures treated seals and the way they treated people.
Mesoamerican Khan (Snake) and Quetzalcoatl depictions also showed them attacking people by taking them in the mouth headfirst. Yet for all of the intimations that Longnecks are dangerous creatures one thing seems to be true: "They can't eat people or more precisely, they cannot swallow them, because their throats are not large enough to get a person down". And they can't eat things like seals, whales or manatees either, which probably is the reason why the Congo dragons are also said to kill any number of other things like crocodiles, hippos and even elephants but not eat them. It was a sort of a blanket rule in some places and probably got confused with other kinds of animals, too. As another line of evidence confirming this, many kinds of dragon depictions, both from the Orient and from Europe, show the dragons with human heads or skulls ("ONLY") in their mouths. What I think is happening is this: similar to great white sharks and some other large predators, Longnecks do not see much difference between humans and seals, since they are about the same size and shape in the water. Because of that, the Longneck's preferred method of attack against humans is to seize them headfirst and hurl them violently through the air to break their necks. This apparently can lead to decapitation of the victim. In the case of either seals or people, the Longneck thereafter cannot actually "eat" the body and so then ignores it. which leads to the very peculiar reputation the Mokele-Mbembe and other Congo Dragons have of regularly killing things but not eating them Longneck Seal-killing move as applied to the human body can lead to accidental decapitation
Bonus: chart of the seals, sea lions and walruses of the world for reference.
