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The order is named for the four “teeth” in the outer jaws of pufferfish; these teeth are actually sharp‐edged jaw bone. They lack ribs. The skin is thick, tough, and covered by scales that are modified into spines, bony plates, or ossicles. The first dorsal spine is prominent, particularly in the triggerfish and filefish. The base of the second spine fits into a groove in the first spine, locking it. They can create grunting noises via vibration of the swim bladder by the pectoral spines. The eyes can rotate independently. Puffers can inflate their bodies with water (or air, if exposed) as a defense. They also have toxins in their tissues (tetrodotoxin).

Figure A1.22 Sexual dimorphism in brook trout (Salvelinus fontinalis); a female is at the top of the image.

Source: Image courtesy of Catherine Hadfield, National Aquarium.

The ocean sunfish (Mola mola), seen in some commercial aquariums, have a unique shape thanks to the clavus that replaces the caudal fin. They also lack pelvic fins and a swim bladder, which is unique for this order. They have more cartilage than other teleosts, particularly in the skull and fin girdles. The spinal column has fewer vertebrae than any other fish. Females produce the most eggs of any vertebrate (>300 million) (Helfman et al. 2009; Nelson et al. 2016).