American Whitespotted Filefish

10 yr. volunteer at South Fl. Costal /marine Science Center with emphasis on costal environmental education, research, conservation, sea turtle rehab. facilitly and an afilated university lab on location (gumbolijmbo.org). We recently got a Scrawled Filefish exhibiting a strange behavior. With a adult/sub adult porkfish near the Filefish, a thin type dorsal fin pops out from the head of Filefish when Porkfish is around. Is this a defense mechanism or warning sign of some sort? does anybody know of this?

Yes this is a warning to keep away! As with the triggerfish particularly the Titan Triggerfish when these are nesting they warn you with their trigger then spine that pops up to keep away and woe betide a diver that doesn't as they will come at speed and try to take a chunk out of you! Filefish aren't so aggressive though!