Observations on giant American brook lampreys (Lampetra appendix)

The American brook lamprey (Lampetra appendix) does not feed after metamorphosis from the larval stage, and is typically much smaller than lampreys that feed as parasites or predators of other fishes. However, relatively large giants have occasionally been reported from Great Lakes tributaries, and one hypothesis is that they have fed parasitically after metamorphosis. I describe here two additional giant specimens from tributaries of Lake Michigan and Lake Huron. The presence of acanthocephalan parasites in the hindguts of both giants supports the hypothesis that they fed parasitically, because parasites of this group are known only from lampreys that have fed after metamorphosis.