[1] It is found in freshwater fishes in Asia and Europe and known for its complete monogamy.

This parasite is commonly found on the gills of European cyprinid fishes.

It is usually around 0.7 centimeters long (approximately the size of a fingernail) and has bilateral symmetry.

According to The New York Times: Diplozoon paradoxum [is] a flatworm that lives in gills of freshwater fish.

"Two immature hermaphrodite individuals meet each other, and their bodies literally fuse together, whereupon they remain faithful until death," Dr. [David P.] Barash said.