The first difference is the salinity tolerance, “Increasing osmotic gradients for water uptake from marine through estuarine into freshwater habitats required that the external epithelia of bivalves became increasingly impermeable to water with their evolutionary transition through these environments” (McMahon and Bogan 353). Some mussels can survive in the intermittent
estuarine waters, while others cannot. All mussels have internal
osmoregulation that maintainstheir water content,
pH, and salinity levels. If
homeostasis cannot be sustained, then the mussel cannot survive.
Another contrast between the marine and freshwater varieties is accentuated by shell morphology. Freshwater mussels possess more elliptical-shaped shells whereas typical marine mussels have nearly circular or fan-shaped shells (Rehder 25). The elliptical shape greatly benefits the burrowers by allowing them to use almost pointy shell to ease into the sediments. The outside of the shell has fewer ridges than marine mussels and reduces the friction. The rounded marine mussels can use
byssal threads and attach to substrate without digging into the sand. Most freshwater mussels belong to the subclass
Paleoheterodonta, whereas the marine mussels claim the
Heterodonta subclass. Within the shell class, the paleoheterodonts have a single row of hinge teeth and the marine subclass has two rows.
Next, marine mussels are typically consumed by humans as seafood (FDA). Freshwater mussels could be eaten, but that is not very common today. “Indians who once occupied eastern North America, especially certain prehistoric cultural groups, are known to have utilized tremendous numbers of freshwater mussels” (Parmalee and Klippel 421). Although most species are edible, freshwater mussels are not as tasty as their saltwater relatives. Some attribute that to the saline conditions of the marine mussels, which add a degree of salt to the flesh. Both freshwater and marine mussels are long-lived filter feeders, and pollutants can easily settle and build up inside them, making them distasteful and potentially unhealthy for human consumption. For this reasonthe Food and Drug Administration closely monitors metal accumulation, pollution spills, and red tide outbreaks as they is relate to the mussel fishery. Another consideration comes from the endangered status of so many freshwater species. A very harsh penalty would be served to any person poaching an endangered species.
The last dissimilarity is apparent during reproduction. Some marine mussels have mobile offspring known as
veligers that are free-living. These larvae flow with the changing tides until they find a suitable
substrate to which they can attach. They utilize a
byssal thread to stay adhered to rocks, docks, boats, or other shells. In contrast, the majority of the freshwater mussels have a larval form known as a
glochidium which requires a host to survive (Ruppert et al 399-401).
From here on out, only freshwater mussels will be discussed.
Go To Reproduction