Boxfish (Cofferfish)

Yellow boxfish (Derek Keats / CC BY)

Boxfish (Ostraciidae) are characterized by bony polygonal plates that give them the appearance of a box or suitcase. They have small mouths with conical, stout teeth. Boxfish are poor swimmers. Acanthostracion quadricornis can reach a length of 60 centimeters (24 inches). Most species are smaller, though. Ostracion lentiginosus and some other boxfish species secrete a highly toxic substance (ostracitoxin) when disturbed, so divers should avoid touching them.

skull and crossbones
Some species venomous.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.