Synbranchiformes

Synbranchiformes, often called swamp eels, is an order of ray-finned fishes that are eel-like but have spiny rays, indicating that they belong to the superorder Acanthopterygii. No synbrachiform fossil is known. The Mastacembeloidei were removed from the Perciformes and added to the Synbranchiformes after a phylogenetic analysis by Johnson and Patterson. These authors consider the Synbranchiformes to be part of a monophyletic group called Smegmamorpha, also containing Mugilimorpha, Atherinomorpha, Gasterosteiformes, and Elassomatidae. There are a total of about 99 species divided over 15 genera in three families. There are two suborders: Synbranchoidei and Mastacembeloidei, or Opisthomi. The Synbranchoidei has one family, the Synbranchidae; four genera; and 17 species. The Mastacembeloidei has two families: Chaudhuriidae, with four genera and five species, and Mastacembelidae, with three genera and about 26 species. These eel-like fishes range in size from 8–48 inches (20–150 cm). Although they are eel-like, they are not related to true eels (Anguilliformes). The premaxillae are present as distinct bones and are nonprotrusible. The gills are poorly developed, and their openings are usually single, small, confluent across the breast, and restricted to the lower half of the body. Oxygen is absorbed through the membranes of the throat or intestine. The dorsal and anal fins are low and continuous around the tail tip. Pelvic fins are absent. Scales are either absent or very small. They lack a swim bladder. These fishes are distributed in tropical America, tropical Africa, southeastern and eastern Asia, East Indies, and Australia. The three families each have a somewhat different distribution: The Synbranchidae are found in Mexico, Central and South America, West Africa (Liberia), Asia, Hawaii, and the Indo-Australian Archipelago. The Mastacembelidae are found in Africa and through Syria to Maritime Southeast Asia, China, and Korea. The Chaudhuriidae are found in northeastern India through Thailand to Korea (including parts of Malaysia and Borneo). All except three species occur in fresh water. They usually are found in swamps, caves, and sluggish fresh and brackish waters. When found in pools, they typically are associated with leaf litter and mats of fine tree roots along the banks. Swamp eels are capable of overland excursions, and some can live out of water for extended periods of time. Some species are burrowers. Four species are found exclusively in caves: Monopterus eapeni and M. roseni from India, Ophisternon candidum from Australia, and O. infernale from Mexico. One species, O. bengalense, commonly occurs in coastal areas of southeastern Asia.