|Image from Fig 2a of the paper|
The researchers found that clams that are below a size threshold of 11 mm appear to be free of this parasite, but once they grow above that size, infection rates steadily increase and almost half the clams above 11 mm in size are infected. This resembles a pattern that has previously been found for other bivalve parasites (for example see this study and this study). The asexual stages of the P. borealis are made up of interconnected, branching modules that extend throughout the clam's body like aggressive roots (see the picture above), invading various internal organs to draw out nutrients to fuel the production of hundreds of cercariae.
Extending from the back of each cercaria are a pair of long filaments that can reach more than six times the length of the cercaria itself (see the picture below). When they are released into the water, those filaments unfurl and fully extend and the cercaria resembles a "V" as it floats in the water. While it can't swim, it can use its tail to hang passively in the water, carried along by the current. In related species, scientists have observed the cercariae attaching to each other by the end of those extended filaments so that together they form a floating "net of cercariae".
|Image from Fig 2b, 2c of the paper|
Gadids like cod, haddock, whiting, and pollock are generally fairly large fish, so if that is not the final destination for P. borealis, then the last host must be a pretty voracious predators that can eat a whole cod for lunch. Enter the monkfish Lophius piscatorius.
Lophius piscatorius is a fish with a big mouth, big appetite and indiscriminate taste, and that suits P. borealis just fine. A monkfish has no trouble when it comes to eating a fairly sizeable cod as you can see in this video. In fact, it has little hesitation when it comes to dining on a lot of things, including the occasional sea bird. And it is in the intestine of this fish that the adult stage of P. borealis spends the rest of its life, nestled in a nutrient-filled highway of muscle and laying eggs that are carried out into the sea with the rest of the host's...waste traffic, where it can then go on to infect clams and begin the cycle anew.
So if you have ever seen an unfortunate cod ambushed by a well-hidden monkfish, then what you just saw was not just a cod being eaten by a big ugly fish with an alarmingly large mouth, you have also just witnessed another few dozen P. borealis completing their life cycle.
Eydal, M., Freeman, M. A., Kristmundsson, A., Bambir, S. H., Jonsson, P. M., & Helgason, S. (2013). Prosorhynchoides borealis Bartoli, Gibson & Bray, 2006 (Digenea: Bucephalidae) cercariae from Abra prismatica (Mollusca: Bivalvia) in Icelandic waters. Journal of Helminthology 87: 34-41