The Asian swamp eel (Monopterus albus), also known as rice eel, ricefield eel, rice paddy eel or white rice-field eel, is a commercially important, air-breathing species of fish in the family Synbranchidae. It occurs in East and Southeast Asia, where it is a very common foodstuff sold throughout the region. It has been introduced to two areas near the Everglades in Florida and near Atlanta in Georgia.
Taxonomy
The Asian swamp eel is a freshwater, eel-like fish belonging to the family Synbranchidae (swamp eels).
Some work indicates that the species should be split into three geographical clades or cryptic species, although these were not given nomenclatural names, as the taxonomic synonymy was too complex to sort out at the time. The populations in the Ryukyus are distinct, the populations in China and Japan belong to another clade, and the rest, the original M. albus, belong to the third group. Although these groups are too phenotypically similar to tell apart morphologically, they exhibit different brooding behaviours. In the Japanese/Chinese form, the males wait until the fertilised eggs hatch in their foam nests, and then keep the larvae in their mouths until they can breathe their own air. The Ryukyuan populations also make foam nests, but do not keep the larvae in the somewhat narrow buccal cavities in their mouths. The most widespread clade does not make foam nests, but lays the eggs among the roots of floating plants, and shows no parental care. Larvae use their pectoral fins to increase water flow and thus oxygen intake across their skin. This last form shows the most genetic diversity and may belong to numerous cryptic species. This form is also the type that has been introduced to the USA.
M. albus is not an "true" eel' in the order Anguilliformes; it belongs to the order Synbranchiformes.
Description
The Asian swamp eel has a scaleless, anguilliform body that grows to a meter or less, typically 25 to 40 cm as an adult. As a swamp eel, it has a tapering tail and blunt snout, and lacks pectoral and pelvic fins. The dorsal, anal, and caudal fins are rudimentary, with the caudal fin often absent. These fins serve to protect the swamp eel against rolling, and assist in sudden turns and stops. Its gill membranes are fused, but one v-shaped gill is located beneath the head. Such a shape prevents reverse flow.
Their colour is variable, but generally olive or brown, with irregular dark flecks. Individuals in Florida usually have a dark body and head, with dark olive or brown dorsal coloring and light orange ventral coloring. Individuals caught in Florida can be more colourful, indicating breeding for the pet trade.
The mouth is large and protractile, and both upper and lower jaws have tiny teeth for eating fishes, worms, crustaceans, and other small aquatic animals.
Similar species
M. cuchia, a related species also commonly eaten, has also been introduced to the USA. When it breaths, the throat expands on sides of head, as opposed to ventrally in M. albus. This species also has "suprapharyngeal air chambers", which M. albus does not, and a few scales, which are entirely absent in M. albus. M. cuchia has a single row of teeth, as opposed to two rows.
Distribution
It has a wide distribution. Monopterus albus is native to much of East and Southeast Asia, ranging west as far as India. M. albus is native to the tropical and subtropical areas of northern India and Burma to China, Japan, and Indo-Malayan Archipelago, possible populations in Far East Russia and northeastern Australia might belong to different cryptic species. It is a common fish in India, Southern China to Malaysia and Indonesia.
The populations in most of Japan (Honshu and Kyushu) are likely introduced from China. The population in the Nara Basin was introduced from Korea in the beginning of the 20th century. Its distribution in Japan is discontinuous, which also indicates that it is introduced within the last millennium or so. The eels found in Taiwan appear to belong to two different species, a Japanese form was introduced in 1940, but a Southeast Asian form is also common and may have also been introduced or be native. The eels were first introduced to the Oahu in Hawaiian Islands around 1900, where they still occur. Earliest record of the fish in the Philippines is in 1918 from a collection by the Commercial Museum of Philadelphia where it has become an invasive species.