Also known as mussels, clam populations at Isle Royale exhibit rather low diversity but very high abundances in select inland lakes. While not all scientists agree on taxonomic designations, four species have been identified in park lakes: Lampsilis luteola, Lampsilis radiata, Pyganodon cataracta, and Pyganodon grandis. Specialists believe that the profusion of clams, along with a broad assembly of age classes and the presence of many reproductive individuals, make island populations similar to healthy and vigorous populations that have been extremely uncommon in the Midwest in recent times. The number of clams estimated to reside in Chickenbone Lake alone is at least 6.4 million!
While clams in some island lakes thrive in densities not seen since pre-settlement times, other lakes have no clams at all. Streams, which on the mainland often contain large numbers of clams, appear to be devoid of clams on the island. Theories for this patchwork distribution include the lack of sufficient sand or gravel substrate in streams, which clams burrow into during winter, and that some lakes may have natural chemical profiles that inhibit clam survival. Copper deposits, which clams are highly sensitive to, may be a problem, while some lakes may have never been colonized by clams.
Isle Royale clam populations are becoming increasingly important and rare, as clams around the Great Lakes are lost to suburban sprawl, impacts from herbicides and pesticides, and introductions of non-native and rapidly expanding clam species such as the zebra mussel. A non-native fish, the round goby, could also produce major deleterious impacts on inland lake ecosystems. Visitors can help reduce these threats by cleaning boats, canoes, and kayaks prior to visiting the park.