Beloniformes are typically elongate fishes, with dorsal and anal fins situated posteriorly on the body and the lateral line situated ventrally. Additional characteristics of the group include fusion of the toothed 5th ceratobranchials into a lower pharyngeal jaw and an open nasal pit. Belonids, aptly called needlefishes, are sleek and garlike piscivores, with very long upper and lower jaws studded with sharp teeth. They can achieve lengths of up to 3.3 ft (1 m). A small number of needlefishes have a reduced upper jaw, and like halfbeaks, feed on plankton and insects. Scomberesocids, of which the largest are about 1.65 ft (0.5 m) long, can be distinguished from belonids by the five or six finlets behind their dorsal and anal fins. The diminutive scomberesocid Cololabis adocetus, at 3 in (7.5 cm), is the smallest fish in the surface waters of the open ocean.
In most species of hemiramphids, or halfbeaks, the lower jaw is much longer than the upper. The front margin of the upper jaw is triangular in shape, the scales are large and cycloid, and fin spines are lacking. Exocoetids, the flyingfishes, are torpedo shaped with greatly enlarged pectoral fins, and the lower lobe of the caudal fin is stiffened and much larger than the upper. Interestingly, the most primitive flyingfish genera, Oxyporhamphus (once included with hemiramphids),
Fodiator, and Parexocoetus, have elongate lower jaws reminiscent of halfbeaks. More derived flyingfishes have acquired oversized pelvic fins in addition to large pectoral fins, and are called four-wingers.
Adrianichthyids, the most basal among the Beloniformes, are superficially unlike other members of the group. Most of the species are in the genus Oryzias, and are small, relatively deep-bodied fishes with large eyes, upturned mouths, and a long anal fin base. Noteworthy in the family is the duckbilled buntingi (Adrianichthys kruyti), which has a bill-shaped mouth with the upper jaw overhanging the lower. Xenopoecilus species also have a bill-shaped mouth, and the carry their eggs at the base of the pelvic fins by way of filamentous attachments.
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