Ricefield Eel

Ricefield Eel

　　The ricefield eel (Monopterus albus) belongs to the order Synbranchiformes, family Synbranchidae, and genus Monopterus. It is also known as the yellow eel, shan yu, luo yu, or snake fish.

The order Synbranchiformes includes about 15 species of slender, eel-like fish. They are not closely related to the true eels of the order Anguilliformes but are more closely related to the order Perciformes. These eels are found in tropical freshwater or brackish water environments. They typically measure about 20–70 cm (8–28 inches) in length, have no scales or very small ones, and possess very low dorsal and anal fins that are continuous around the tail. They usually have only one external gill opening at the throat. Some species have very small gills and rely on oxygen absorbed through the mucous membranes of the throat or gut. In the East, they are valued as food fish and are often raised in ponds or rice paddies.

Yellow eel (Monopterus albus): Scaleless, capable of raising the front half of its body upright in shallow water to breathe air from the surface using its mouth, storing the air in its mouth and throat, which causes the throat area to appear swollen. Widely distributed across Southeast Asia, it is a common freshwater food fish found everywhere except the northwestern highlands. It inhabits ponds, small rivers, rice fields, etc., often hiding in mud burrows or rock crevices. It forages at night. Its reproductive behavior is quite unique: juveniles are female, and after one reproductive cycle, they transform into males. This phenomenon of sex change is called protandrous hermaphroditism.

Characteristics:

The body is elongated and snake-like, cylindrical at the front and laterally compressed toward the rear, with a pointed tail. The head is long and round. The mouth is large, terminal, with the upper jaw slightly protruding and well-developed lips. Both jaws and the palate bear fine teeth. The eyes are small and covered by a thin membrane. The left and right gill openings merge into a single "V"-shaped opening on the ventral side. The gill membranes are connected to the gill arches. The body surface is generally coated with a slippery mucus to aid in escape and lacks scales. There are no pectoral or pelvic fins; the dorsal and anal fins are vestigial, reduced to skin folds without spines, and are connected to the caudal fin. The body color is typically yellowish-brown, light yellow, or orange-yellow with dark gray spots, though some eels are white, commonly referred to as "white eels."

The ricefield eel is a tropical and warm temperate fish that leads a bottom-dwelling lifestyle. It has strong adaptability and can survive in rivers, lakes, ditches, and rice paddies. During the day, it prefers to burrow in muddy sediments rich in organic matter or take shelter in rock crevices along the water’s edge. It is mostly inactive during the day and comes out to forage at night. Its gills are not well developed, so it relies on the inner skin lining of the mouth and throat as auxiliary breathing organs, allowing it to directly breathe air. It can survive even in waters with extremely low oxygen levels. Once removed from water, it can live for several days as long as its skin remains moist. It is an omnivorous fish that feeds on various small animals. It is highly greedy and most active in summer, while it can go without food for extended periods in cold seasons without dying.

The ricefield eel's breeding season is around June to August. Throughout its individual development, it exhibits hermaphroditic sex reversal — meaning it is female from the embryonic stage until its first sexual maturity (i.e., individuals under 35 cm in length have ovaries exclusively). After laying eggs, the ovaries gradually transform into testes. At lengths of 36–48 cm, partial sex reversal occurs, with nearly equal numbers of females and males. Individuals over 53 cm are predominantly male. Juvenile fish in their first year only grow to about 20 cm. Females reach maturity in their second winter at a minimum length of 34 cm. The largest individuals can reach up to 70 cm in length and weigh 1.5 kg. The eel lays eggs near the entrance of its burrow. Before spawning, it secretes foam to build a nest, and the fertilized eggs develop on the water’s surface, buoyed by the foam. Both male and female eels exhibit nesting-guarding behavior.

The eel’s cylindrical body is well-suited for a burrowing lifestyle, minimizing friction when entering or exiting its hole. It has something of a “hermit” temperament — it lacks any specialized offensive abilities or strong defensive weapons. Its only skill is “when in danger, escape is the best strategy.” It has neither dorsal nor pelvic fins, and even its dorsal and anal fins are reduced to tiny skin folds. Its scales are so tiny they are barely visible to the naked eye. However, its entire body secretes an extremely slippery mucus, allowing it to easily slip out of human hands. The mucus on the eel’s body serves several key functions: preventing bacterial and pathogenic infections, reducing illness; deterring parasitic organisms, aiding growth; and providing a slick surface that allows it to move effortlessly through mud.

The ricefield eel is female during its first sexual maturation but becomes male starting from its second sexual maturity. In other words, the eel serves as both mother and father in its lifetime. This gender transition process is biologically referred to as sex reversal.

Most fish breathe through gills, but the ricefield eel is unique — its gills are degenerated, and it breathes air directly through the fine blood vessels on the surface of its throat. Therefore, to keep a ricefield eel alive, it must be kept moist and stored in a cool place.

Distribution:

Found in Southeast Asia. In China, it is found in all water bodies except the western highlands. Internationally, it is present in western Korea, southern Japan, the Philippines, Java, and Myanmar.

The ricefield eel is widely distributed across lakes, rivers, reservoirs, ponds, ditches, and other water bodies throughout China. Records exist in all regions except the northwestern highlands, especially in the Pearl River and Yangtze River basins, which are major production areas. Internationally, it is mainly found in Thailand, Indonesia, the Philippines, and also in India, Japan, and Korea.

Value:

The flesh of the ricefield eel is tender and flavorful, with high nutritional value. It is rich in DHA and lecithin, which are primary components of cell membranes in human tissues and organs, and essential nutrients for brain cells. According to U.S. research data, regular intake of lecithin can improve memory by 20%. Thus, eating ricefield eel meat has brain-nourishing and health-enhancing benefits. It contains a special substance called “eel essence,” which helps lower and regulate blood sugar, making it beneficial for diabetes treatment. Coupled with its very low fat content, it is an ideal food for diabetics. The eel also has astonishingly high levels of vitamin A, which improves vision and promotes skin membrane metabolism. Per 100 grams of eel meat, there are 17.2–18.8 grams of protein, 0.9–1.2 grams of fat, 38 mg of calcium, 150 mg of phosphorus, and 1.6 mg of iron. It also contains various vitamins such as thiamine (vitamin B1), riboflavin (B2), niacin (vitamin PP), and ascorbic acid (vitamin C). The ricefield eel is not only served as a delicacy to entertain guests but in recent years has been exported live, selling well abroad. Frozen eels are also exported to markets such as America. Eels are available year-round, but those caught around Minor Heat (Xiaoshu) are considered the most plump. There is a folk saying: “Xiaoshu eel is as nourishing as ginseng.”