Biologists at the University of Manchester were on a field trip to the French Alps when they found a duck egg in a small pond. Noticing its movement, they cracked it open, to discover three live minnows inside.
The scientists, despite their combined knowledge of the natural world, are at a loss as to how the fish got inside the visibly uncracked egg.
Dr Matthew Cobb, a lecturer in animal behaviour at the university, said: "As 21st century scientists rather than 17th century antiquarians we think it's unlikely this represents a hitherto unknown mode of fish reproduction."
Dr Cobb and his colleague have written to the magazine New Scientist to see if any reader can come up with the solution.