Contrary to the suggestion that merfolk myths were caused by creatively observed (and very wayward) manatees and/or dugongs, those seen by the Norse in the 13th century were probably spyhopping marine mammals distorted by atmospheric refraction. The original accounts of the mermen describe huge objects rising from the sea with shoulders, no arms, and tapering bodies which were seen before storms. It so happens that a warm front moving over cold surface air can create such an illusion. In the 18th century, the phenomenon was reported on very warm and calm days, where strong temperature gradients and the subsequent refraction can also occur. It could be possible that some of Heuvelmans’ Long-necked and Ambiguous Periscope sightings were also caused by similar refraction… although unfortunately this wasn’t discussed by Lehn and Schroeder.
Lehn, W. & Schroeder, I. The Norse merman as an optical illusion. Nature 289(5796) 362–366.
* It is very annoying how so many people discussing strange phenomena don’t even bother to look up the actual reports. Next time I hear somebody claim Sea Serpents are all plesiosaurs, oarfish or giant squid, I swear I’m going to rage-vomit.