Threadsail Filefish

Found singly, close to sandy sea floors, amongst corals and seaweeds.
Juveniles found drifting with sargassum rafts.
Western Pacific - Hokkaido to East China Sea

Filefish can alter their colour and pattern to match their surrounds and so deter predators.
Although closely related to the Triggerfish, their "trigger spine" cannot be locked into place.