|Left: Adult P. toshimai in a fish's gut, Centre: Adult P. toshimai in a frog's gut, Right: Larval P. toshimai from a woodlouse|
Photos from the graphical abstract of the paper
So while the fish's gut is a hospitable enough environment for the parasite to grow into an adult worm, it is lacking a certain je ne sais quoi that the female worms need to start producing eggs and complete the life-cycle. It is not entirely clear what exactly that might be - it could be that the fish's gut does not produce the right type of nutrients for egg production, or there is simply not enough mating opportunities for the parasite in the gut of a fish - since they are not as commonly nor heavily infected as the amphibians. Either way those salmonids are ultimately dead-end hosts for P. toshimai. So how are the worms ending up in those fish in the first place?
This is where we have to consider the other animal involved in the parasite's life-cycle which is the woodlouse. Woodlice - also known as slaters - are terrestrial crustaceans commonly found under rocks and among leaf litter. As mentioned above, P. toshimai uses a species of woodlouse as intermediate host, where their eggs develop into larval stages known as cystacanths. Since those crustaceans are commonly eaten by frogs and salamanders, they also act as a vehicle to transport the parasite to its final host.
The researchers noticed that P. toshimai is only ever found in fish from one particular stream which is surrounded by bushes. These bushes are habitats for woodlice and amphibians which are the usual hosts for P. toshimai, and provide the necessary conditions for the parasite to complete its life-cycle. But every now and then, instead of getting eaten by a frog or a salamander, an infected woodlouse would fall into the stream, and become a tasty snack for a hungry fish. Indeed, the researchers did find a few woodlice in some of the fishes that they caught.
Nakao, M., & Sasaki, M. (2020). Frequent infections of mountain stream fish with the amphibian acanthocephalan, Pseudoacanthocephalus toshimai (Acanthocephala: Echinorhynchidae). Parasitology International 81: 102262.