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Triacanthidae (triplespine, tripodfish; subclass Actinopterygii, order Tetraodontiformes) A small family of marine, tropical fish that have a deep and compressed body, a small mouth, two dorsal fins, and a narrow caudal peduncle. The first dorsal fin spine and the two pectoral spines are capable of being locked firmly in an upright position when the fish is threatened. Triacanthus brevirostris (shortnosed tripodfish), 24 cm, is fairly common in inshore and estuarine waters. There are about seven species, distributed in the Indo-Pacific region.