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The Tetraodontiformes are an order 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 nine 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.
Various bizarre forms are included here, all radical departures from the streamlined body plan typical of most teleosts. These forms range from nearly square or triangular (e.g. boxfish [Ostraciidae]), globose (pufferfish [Tetraodontidae]) to laterally compressed (filefish [Monacanthidae]). They are all slow-moving and most rely on their pectoral and stiff 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 (e.g. triggerfish [Balistidae])—or with tough, leathery skin (e.g. filefish and ocean sunfish [Molidae]). Another striking defensive attribute found in the Tetraodontidae and porcupinefish (Diodontidae) 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 tetraodotoxin, a powerful neurotoxin concentrated in the animals' internal organs.
Tetraodontiformes 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 the name Tetraodontiformes, derived from the Greek words tetra (four) words and odous (tooth) and the Latin forma (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.
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