SURVEY OF UNIONID MUSSELS NATURALLY INFESTING WALLEYE IN THE ST. CROIX RIVER

Nathan W. Juergens, Macalester College
Sarah L. Boyer, Macalester College
Mark C. Hove, Macalester College
Jordan B. Eckstein, Macalester College
Jessica N. Ramirez, Macalester College

Freshwater mussels in the family Unionidae have an intricate life history in which their larvae parasitize the gills of vertebrate hosts in order to metamorphose and disperse to new habitats. Many host-mussel relationships are unknown and at the parasitic stage the animals are too small to easily morphologically distinguish between species. Therefore, we utilized "DNA barcoding" of the ND1 mitochondrial locus to identify juvenile mussels collected from naturally infested St. Croix River walleye (Sander vitreus). We successfully sequenced 62 unknown juvenile mussels, and identified 42 as being of the species Lampsilis cardium, 17 as either Lampsilis higginsii or Actinonaias ligamentina, two as Pyganodon grandis, and one as Lasmigona compressa. Phylogenetic analysis identified all of the juveniles with high enough posterior probability values to definitively say that they grouped with their indicated species, but those that grouped with either L. higginsii or A. ligamentina, could not be distinguished from one another, due to high genetic similarity between the two species. Of the Lampsilis cardium juveniles, 31 were collected from one walleye individual, and interestingly, all 31 of these sequences were identical. This is consistent these juveniles being the offspring of one mother, but two or more genetically identical mothers cannot be discounted. Our reference data set of 76 L. cardium adult sequences had 22 different haplotypes, so clearly there is some genetic diversity within this species in the St. Croix. The suspected Lasmigona compressa individual is curious because adult L. compressas are not found where the walleye was caught, but rather much further upstream in the St. Croix. That said, L. compressa have been shown to be generalists when it comes to metamorphosing on a fish-host, and walleye are known to travel several kilometers in the spring when this fish was caught, so our result is within the realm of possibility.