Which external features distinguish tetraodontiform fishes from many other fishes?

Tetraodontiform fishes are distinguished from many other fishes by several specific external features resulting from evolutionary reductive tendencies. Key characteristics include:1

• Small Openings: They possess a small mouth, usually featuring massive teeth, and a gill opening restricted to a relatively short slit on the side of the head.12
• Modified Scales and Skin: Their scales are typically highly modified into overlapping or sutured plates, or into sharp, projecting spines. In some species, the skin is thickened and hardened by deep layers of connective tissue.12
• Fin Structure: They lack anal fin spines, and their dorsal fin spines are either absent or reduced to no more than six. Similarly, the pelvic fin is either entirely absent or significantly reduced.12

These features, combined with a generally reduced number of skeletal elements, define the order's unique morphology compared to the generalized perciforms from which they evolved.13