Several characteristics distinguish this group of fish (Order Tetraodontiformes) from others: inflexible bodies, slow swimming speeds, various defensive strategies, a particular gill structure, and teeth that are bony plates. The bony plates in the mouth, coupled with very strong jaws, enable these fish to prey upon shelled animals like crustaceans or shelled molluscs. Some even gnaw on coral. With inflexible bodies, fish in this group primarily use their pectoral fins to swim, rather than their tails. The tails function more like rudders, or are only used to dart away from predators. Fish in this group can also be recognized by their gills, which are single slits located very near their pectoral fins. The Order includes about 420 species with several very different body plans and defensive strategies.
One unique defensive strategy utilized by these fish is to ingest water and inflate themselves to much larger than normal size, making themselves more difficult to swallow. The typical pufferfish (Family Tetraodontidae, 173 species) and porcupinefish (Family Diodontidae, 24 species) use this strategy. Both of these types of fish have spines on their bodies, with those of the porcupinefish being large and very visible, but those of the pufferfish being smaller and visible only when the fish is inflated. As indicated by the names of the Families, the porcupinefish have two bony plates in their mouths, while typical pufferfish have four. In the latter, the bony plates in the upper and lower jaws are divided into two. As another defensive strategy, these fish also have a very powerful toxin in their flesh or internal organs, so they are very poisonous to potential predators.
The boxfish have scales fused into a rigid body structure that appears square or rectangular in cross-section. They rely upon this bony armor, as well as secreting toxins from their skin into the water, to deter predators. Variously called boxfish, cowfish or trunkfish, they might also have horny projections in front or protruding mouths. There are 25 species of typical boxfish (Family Ostraciidae), along with 13 more primitive, deepwater species (Family Aracanidae).
This Order also includes the triggerfish and filefish. These fish have tough, leathery skins, so some of these may be given the name leatherjacket. As a defense against predators, these fish have a large, strong dorsal spine that can be raised and locked into place by a second, smaller dorsal spine. The triggerfish (Family Balistidae, 43 species), have a wide variety of colors and striping. Filefish (Family Monacanthidae, 106 species) have similar large dorsal spines, but differ from the triggerfish by having more spikes on the surface, as well as a deeply keeled lower body.