Filefish and triggerfish are closely related and very similar in appearance. Both have a long dorsal spine (shown retracted here on a filefish). In the filefish, it appears the base of the dorsal spine is located above the eyes, but in triggerfish the spine seems to be located more mid-body, toward the tail fin. Filefish usually have six large teeth in the outer series, and four in the inner series, fewer than the eight outer and six inner of the triggerfish.
Though I have not observed a filefish use its spine defensively, I have regularly seen triggerfish use their spine to lock themselves into a familiar hole in the reef after plunging in headfirst. A smaller second spine (triggers have three, filefish two) serves to lock the first spine in the extended position. The powerful jaws are also sometimes used to grasp the coral wall of the hole, providing additional security. Frequently the holes are just large enough to accommodate their bodies; sometimes the fish must roll horizontally to enter.
This foot-long (30 cm) filefish has smooth, tough skin with barely
visible scales. Though it may appear to be smiling, it was probably
unhappy to have the light from my strobe compound the disorientation of
swimming in the night sea.
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