The Question: I found these creatures in mud. They were approximately 30 cm (1 ft). The head is 4-5 mm (0.25 inch) in diameter. They don’t look like worms. I wonder if they might be snakes. What are they and what do they eat?

Submitted by: Madhumita, Chhattisgarh, India

The Short Answer: This is definitely a synbranchid fish, most likely Monopterus digressus.  The synbranchid eels are known collectively as “Asian swamp eels.”  The Asian swamp eels are famous for their ability to survive and thrive in environments that most of us wouldn’t think of as suitable for fish.  Monopterus digressus has been found in newly dug underground wells, which seems impossible, but it’s believed they live in and swim through the small underground channels through rock that connect subterranean waters.  Like many other animals that live underground or in caves, they don’t have functional eyes.  They also lack fins.

I have previously written about another Asian swamp eel, Monopterus cochia:

Monopterus albus, another synbranchid eel, has been introduced to Hawaii and Florida and is of concern as a possibly destructive invasive species:

More Information: As to your question about what they eat, it’s difficult to study these creatures in their natural habitat, but they are definitely predatory and probably would eat worms, insects, and any other animals small enough to fit into their mouths.  An investigation of their feeding habits involved feeding them both chopped earthworms (Oligochaeta) and brine shrimp (Artemia).  The eels seem to locate prey by smell and then, when close enough, by touch.  The instant a prey object touches the fish’s head, it immediately engulfs it.  If you still have this fish, I suggest you try different live foods until you find a few that it will eat regularly.

Sources:  Thanks to Marcus Knight, Moncey Vincent, GK Bhat, and Mark Wilkinson for help in identifying this fish.

Vincent, M and Thomas, J. (2011). Observations on the foraging behavior of a subterranean fish Monopterus digressus (Synbranchiformes: Synbranchidae). Ichthyology Research, 58:95-98.

Vincent, M.  (2012).  Occurrence, distribution and troglomorphisms of subterranean fishes of peninsular India.  Current Science, Vol. 102, No. 7, 1028-1034.

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