A parasitic bivalve affects the interaction between a native and an invasive salmonid

Abstract: The glochidia larvae of the freshwater pear mussel parasitize on juvenile brown trout during their maturation into tiny mussels. The parasitation has several negative impacts on the trout, one of the less studied being behavioral effects. As such, this study aimed to determine if the mussel infestation had any negative impacts on the brown trout’s dominance performance when encountering larvae free brown trout and invasive brook trout. I hypothesized that the general dominance behavior, the number of strikes per individual fish and the proportion of strikes per trout in pairwise interactions would decrease with increasing infestation. It was also hypothesized that infested brown trout would perform differently when encountering either non-infested brown trout or brook trout, that infested fish in an isolated group would have a lower number of initiated aggressions and that infested brook trout would eject their glochidia larvae before the latter complete their metamorphosis. Most of the hypotheses were tested in two experiments where the behavior was recorded with cameras. The brook trout’s infestation was tested separately. No glochidia larva completed their metamorphosis on these fish. The isolated group consisting of infested brown trout had a lower average number of initiated aggressions. In the pairwise interactions, the larvae infestation only significantly decreased the general dominance behavior and the proportion of strikes. The number of strikes varied greatly between the fish. The infested brown trout’s performance did not differ between interactions with either non-infested brown trout or brook trout. These results portray a problematic scenario for the freshwater pearl mussel. As their hosts are more prone to take subordinate roles and since invasive brook trout does not act as a functional host, the latter will have a greater opportunity to dominate streams inhabited by the bivalve, increasing the likelihood of the mussel’s local extinction.