These parasitic flatworms are dorso-ventrally flattened animals[1] characterized by a bilaterally symmetrical body enclosed within a syncytial tegument.

Rapid proliferation of sporocysts results in a knotted white mass of tubules, which is found primarily in the gonads of the mussel.

As infection intensifies, the sporocyst develops branches through connective tissue passages, emerges from the gonads, and can spread into other body regions.

Such secondary sites of infection have been previously reported to occur in the digestive glands, the gills, the bundles of adductor muscle, and the mantle epithelium lining the interior of the shells.

[4][8][9] The study found that the digestive glands of infected bivalves appeared to be relatively normal when compared to full bodied, uninfected specimens.

It was also observed that heavy infection of the parasite led to host castration, which left the entire gonadal space often occupied by the sporocysts.

[10] The location of the sporocyst (primarily in gonads), its overall shape, irregular branches, and the morphology of its cercariae with a bifurcated tail, distinguish B. polymorphus from other trematode parasites of zebra mussels in histological sections.

[11] A study by Lajtner et al., which surveyed the zebra mussel population in the Drava River in Croatia, found a prevalence of 21.3%.