An order of eel-like fishes in the class Actinopterygii, commonly called swamp eels. Synbranchiforms are among the most highly specialized fishes. They are characterized by a usually scaleless eel-like body lacking pelvic fins with pectoral fins present or absent; gill openings entirely ventral and often confluent across the breast; premaxillae lacking ascending processes and nonprotrusible; ectopterygoid bones enlarged; endopterygoids reduced or absent; swim bladder present or absent; and gills poorly developed, with respiration in some species accomplished in part by highly vascularized buccopharyngeal pouches. The order has no fossil record and comprises three families, 15 genera, and about 99 species, all but three of which are freshwater inhabitants. Synbrachiformes are thought to form a monophyletic group with the smegmamorphs.