This is a threadsail filefish (Stephanolepis cirrhifer) in the Sea of Japan. Some fish, like this species, rest at night in a state that can be described as sleep. Their eyes remain open though, and the fish maintain some level of awareness even as they doze. You can think of it as being something like semi-aware daydreaming.
Like other filefish, this species is not a particularly strong swimmer. So when they rest at night, they often look for something to bite onto, a holdfast to keep from being swept away.
Shortly after I took this photo, this fish latched onto some thick line tied to the bottom, safe and secure for a good night’s rest.
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