According to some tales, the isonade is a shark that uses its long tail to drag people in the boat into the sea, where it devours them. Is it possible that the isonade is an actual thresher shark and not a youkai? The following are some information about the thresher shark that somewhat also describes the isonade:

• The thresher shark has a thresher-like tail or caudal fin which can be as long as the body of the shark itself.
• The thresher shark are active predators that uses its tail as a weapon to stun prey.
• Thresher sharks are one of the few shark species known to jump fully out of the water making turns like dolphins.

However, these sharks are rare n the Central and Western Pacific and are not threat to humans, although some divers have been hit with the upper tail lobe. There is also a dubious account of a fisherman being decapitated by a tail swipe as the shark breached.

Weird. I was just reading about the Isonade in my Yokai Encyclopaedia a few days ago. It's said to have barbs growing backwards on its tail to swipe at people on the beach or hook them on a boat.

What about the spirits of thresher sharks?