Everyone wants a window seat

Through chance and unfathomable lengths of time, evolution has shown remarkable creativity, yet some creatures still defy imagination. The digenetic (requires more than one host) trematodes definitely fall into that category. Childhood imagination could not provide a more bizarre life cycle than that evolved by some of these helminths that bedevil so called higher creatures. Diplostomum spathaeum illustrates that phylum with an ophthalmic twist.

The life cycle of D spathaeum begins as an egg in the faeces of a piscivorous bird, such as a gull or a pelican, as a definitive and unaffected host. It is in this host that the sexual phase occurs and the adult parasite lays its eggs in the bird’s gastrointestinal tract. The excrement containing the eggs is deposited in a fresh water lake or stream. As the faeces drop to the floor of the lake or stream, perhaps on the northern Canadian shield where they are prevalent, the eggs embryonate for approximately 3 weeks and then hatch when exposed to light. The photonegative and short lived miracidia exit the eggs, find their way to a snail, often of the Lymnaea genus, and penetrate the flesh. In the snail, the miracidia penetrate the hepatopancreas and undergo metamorphosis into sporocysts …