The Question: I live in Kolkata (Calcutta), India. We have a few natural ponds here and I found this eel-like species. I’ve seen people here keep it in aquariums. But the people here don’t know it’s scientific name or English name. The reason I think it’s not an eel is because it has no fins in the front or on its tail. I’d like to know what it is.

Submitted by: Debashis, India

The Short Answer: Two Indian fish experts identified your pictures as Monopterus cuchia, which goes by the common name Swamp eel, Rice eel or Mud eel. These common names are also used throughout Asia for several closely related species in the genus Monopterus. M. cuchia is found in freshwater in Pakistan, India, Nepal, Bangladesh and Myanmar.

More Information: As you correctly suspected, the Asian Swamp eels are not true eels from the family Anguillidae, they are members of another family, the Synbranchidae. In fact, despite the similarity in appearance, Swamp Eels and the true eels are found in different orders (Synbranchiformes versus Anguilliformes), and are therefore not closely related at all.

The Swamp eels are notable for several things. For example, they have only vestigial fins. (In the original version of this article, I stated that M. cuchia doesn’t have scales, but in fact, they do.  My apologies.)  But the most unusual thing about these fish is that they breathe air. M. cuchia, for example, has small gills, but can’t get enough oxygen from water to survive. So the fish swallows air, increases its heart rate to rush as much blood as possible through the many small blood vessels of the throat, and then expels the air again. The throat acts as a rudimentary lung allowing the fish to absorb oxygen directly from air. M. cuchia goes one step further in that it has pouches in its throat lined with blood vessels. This increases the surface area for absorption of oxygen.

Swamp eels are a food source and cash crop in many parts of Asia. They have the advantage of being able to thrive in water with little oxygen. They often dig burrows in mud where they hide. Swamp eels eat insects, other fish, and small vertebrates such as tadpoles. They grow to a length of about 60 cm (two feet).

Trivia: Swamp eels are protogynous hermaphrodites, which means they begin life as females and become males as they get older.

Sources: My sincere thanks to Dr. K. Rema Devi of the Zoological Survey of India for passing on this question to Indian fish experts Marcus Knight and Beta Mahatvaraj, who generously identified the fish from the photos.

The transition to air breathing in fishes v. comparative aspects of cardiorespiratory regulation in Synbranchus marmoratus and Monopterus albus (Synbranchidae). Graham J.B, Lai N.C., Chiller D, and Roberts J.L. The Journal of Experimental Biology 198, 1455–1467 (1995).

Structure of the air breathing organs of a swamp mud eel, Monopterus cuchia. Munshi J.S.D., Hughes G.M., Gehr P., Weibel Ewald R. Japanese Journal of Ichthyology, vol. 35, no. 4, 1989.

Investigation on health condition of a freshwater eel, Monopterus cuchia from Ailee beel, Mymensingh, Bangladesh. G. U. Ahmed, M. N. Akter1, S. A. Nipa and M. M. Hossain. J. Bangladesh Agril. Univ. 7(2): 421–426, 2009 ISSN 1810-3030.

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