Freshwater mussels are unique among bivalves in that most require a host fish to complete their life cycle. Unlike male and female marine bivalves, which release sperm and eggs into the water column where fertilization takes place, fertilization of freshwater mussels takes place within the brood chambers of the female mussel. The female freshwater mussel carries the fertilized eggs in the gills until they develop into a parasitic stage called glochidia. Female freshwater mussels then release the glochidia into the water column where they must come into contact with the suitable host fish species. Once the glochidia are released, some survive for a few days, while others survive less than 24 hours if they do not successfully attach to a host fish.
After successfully attaching to the host fish for a few weeks, the glochidia metamorphose and drop to the substrate to become free-living juveniles. The time required for a glochidium to metamorphose varies with water temperature and among mussel species. Properly encysted glochidia will metamorphose, with little change in size, into free-living juveniles, displaying adductor muscles, gill buds, and a ciliated foot with protractor and retractor muscles.
Newly transformed juvenile freshwater mussels will pedal feed using their ciliated foot for several months before burrowing into the substrate and beginning filter feeding. Cilia-lined membranes move the water through a series of sorting organs towards the stomach. During the pedal feeding phase, the juvenile freshwater mussel maybe more susceptible to water pollutants because the shell is gaped, filtration organs are not fully developed, and the mussel may ingest pollutants with sediment particles.
Once a juvenile freshwater mussel transitions to filter feeding, it will burrow into the substrate and spend months or years burrowed below the substrate surface until the sub-adult stage. Faster growing species like the Floaters could start siphoning at the substrate surface at 1-3 years of age, whereas slower growing species, like the Western Pearlshell and Western Ridged Mussel, may not siphon at the surface until 4+ years of age.
The mussel-fish relationship is usually species-specific, where only certain species of fish can serve as suitable hosts for a particular freshwater mussel species. Some mussel species have a very restricted number of host fish species (“specialists”) while other freshwater mussels utilize a wide range of fish species (“generalists”). Knowledge of the reproductive biology of many mussels remains incomplete. Only 25 percent of the 304 known freshwater mussel species in North America have had their host fish relationships identified by way of field and/or laboratory experiments, although recent studies are gradually expanding this number. Recent research has indicated non-native fishes, like bass, are poor hosts for western freshwater mussels (Maine and O’Brien 2022). As the rivers warm the encroachment of non-native fishes may reduce already diminished freshwater mussel reproduction. Non-native fishes can also feed on native fishes and have also been known to consume juvenile freshwater mussels.
