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The Tetraodontiformes are an of highly derived ray-finned fish, also called the Plectognathi. Sometimes these are classified as a suborder of the Perciformes. The Tetraodontiformes are represented by ten families and approximately 360 species overall; most are marine and dwell in and around tropical coral reefs, but a handful of species are found in freshwater streams and estuaries. They have no close relatives, and descend from a line of coral-dwelling species that emerged around 40 million years ago.

Physical Characteristics

Various bizarre forms are included here, all radical departures from the streamlined body plan typical of most fishes. These forms range from nearly square or triangular (boxfishes), globose (pufferfishes) to laterally compressed (filefishes). They range in size from Ru darius excelsus, measuring just 2 centimetres (0.79 in) in length, to the Ocean sunfish, the largest of all bony fish at up to 3 metres (9.8 ft) in length and weighing over 2 tonnes.^[1]

They are ostraciiforms, meaning the body is inflexible, and undulation during movement is limited to the paired and median fins. Because of this, they are slow-moving and rely on their pectoral, dorsal, anal, and caudal fins for propulsion. However, movement is usually quite precise; dorsal and anal fins aid in manoeuvring and stabilizing. In most species, all fins are simple, small, and rounded.

The tetraodontiform strategy seems to be defense at the expense of speed, with all species fortified with scales modified into strong plates or spines — the latter sometimes retractable and able to lock in place (the triggerfishes) — or with tough, leathery skin (the filefishes and ocean sunfish). Another striking defensive attribute found in the pufferfishes and porcupinefishes is the ability to inflate the body to greatly increase its normal diameter: this is accomplished by sucking water into a diverticulum of the stomach. Many species of the Tetraodontidae, Triodontidae and Diodontidae are further protected from predation by tetrodotoxin, a powerful neurotoxin concentrated in the animals' internal organs.

Long-spine porcupinefish, Diodon holocanthus. On the right is a blue-spotted grouper, Cephalopholis argus.

Tetraodontiforms have highly modified skeletons, with no nasal, parietal, infraorbital, or (usually) lower rib bones. The bones of the jaw are modified and fused into a sort of "beak"; there are visible sutures which divide the beaks into "teeth". This is alluded to in their name, derived from the Greek words tetra meaning "four" and odous meaning "tooth" and the Latin forma meaning "shape". Counting these teeth-like bones is a way of distinguishing similar families. For example the Tetraodontidae ("four-toothed"), Triodontidae ("three-toothed"), and Diodontidae ("two-toothed").

The jaws are aided by powerful muscles and many species also have pharyngeal teeth to further process prey items. This is because Tetraodontiformes prey mostly on hard-shelled invertebrates such as crustaceans and shellfish.

The Molidae are conspicuous even within this oddball order: they lack swim bladders and spines, and are propelled by their very tall dorsal and anal fins. The caudal peduncle is absent and the caudal fin is reduced to a stiff rudder-like structure. Molids are pelagic rather than reef-associated and feed on soft-bodied invertebrates, especially jellyfish.

Families

Ocean sunfish.

The Honeycomb cowfish is part of the Ostraciidae family.

• Aracanidae
• Balistidae — triggerfishes
• Diodontidae — porcupinefishes
• Molidae — ocean sunfish and relatives
• Monacanthidae — filefishes
• Ostraciidae — boxfishes
• Tetraodontidae — pufferfishes
• Triacanthidae — triplespines
• Triacanthodidae — spikefishes
• Triodontidae — three-toothed puffer

Fossil Families

• †Bolcabalistidae
• †Cretatriacanthidae
• †Eoplectidae
• †Eospinidae
• †Moclaybalistidae
• †Plectocretacicidae
• †Protobalistidae
• †Protriacanthidae
• †Spinacanthidae