Freshwater mussels are virtually non-mobile inhabitants of the benthic infaunal community of many waterbodies. They feed by filtering particulate organic matter (i.e., algae, bacteria, and protozoans) along with indigestible components (i.e., silt and sand) from the water column. Water enters through an incurrent siphon, where the mucus-lined gills separate digestible from indigestible material. Food particles of an appropriate size are transported into the mouth, while silt and sand are expelled. Nutrients are absorbed in the intestines and wastes exit the excurrent siphon in the form of pseudofeces (Pearse and others 1987). Lack of mobility and reliance on filter feeding has resulted in the demise of many marine and freshwater bivalve taxa (Bogan 1993, Harris and others 1997).
Many researchers associate the decline of freshwater mussel resources with the anthropogenic alteration of habitat, such as construction of dams (Bogan 1993, Vaughn and Taylor 1999), dredging, water drawdown for irrigation, and increased water pollution from runoff (NNMCC 1998). Dams interfere with a fish’s ability to move up and downstream in a river, which may decrease the likelihood of reproductive success for a mussel. Dams may also have a role in habitat alteration by converting riffle-run habitat into lentic environments. Dredging can dislodge, or even bury entire mussel beds. Water drawdowns can leave mussels' littoral zones exposed leading to desiccation. Increased pollution in the form of heavy metals (i.e. mercury, lead, and cadmium), herbicides and pesticides can interfere with nutrient uptake; high sediment loads from runoff may clog siphons and bury larger immobile unionids.
There is no doubt that some mussel species are better adapted than others to survive stressful environmental conditions. All freshwater mussels are covered and protected by a hard shell, but shell thickness varies among species. For example, the shell of Megalonaias nervosa (Washboard) is much thicker and heavier than that of Leptodea fragilis (Fragile papershell). As a consequence, thicker-shelled species may be less vulnerable to abrasion, displacement, and handling than thin-shelled mussels.
Most benthic invertebrates require a dissolved oxygen (DO) concentration exceeding 5 milligrams per liter for basic metabolic processes. Freshwater mussels are no exception. Some species thrive in well-oxygenated riffle habitat. Other species have adapted to living in pools, or even hypoxic conditions (e.g. Quadrula metanevra: in Parmalee and Bogan 1998). According to Johnson et al. (2001), riffle species (species typically found in habitats with greater flow and DO concentration) and the federally endangered species collected in their survey were much more intolerant of low DO (< 5 mg/L) than other species. Low DO concentrations, although not always lethal, should definitely be considered a major biological stressor.
Sediment deposition from terrestrial runoff is inevitable. However, vegetation along and adjacent to the streambank (riparian zone) can inhibit surface flow and erosion during heavy rainfall events. In recent years, the emphasis (especially with landowners) on maintaining the riparian zone has waned. The need for affordable lumber, produce, and durable goods outweighs the need to keep sediment out of streams. The limited mobility of mussels can make them very susceptible to increases in suspended sediments and deposition. There are a few exceptions, but in most cases freshwater mussels do not tolerate areas with high silt concentration. Mussels like Pyganodon grandis (Giant Floater), Anodonta suborbiculata(Flat Floater), and Leptodea fragilis are much more likely to be found in high silt areas than a riffle species like Epioblasma triquetra(Snuffbox).
Mussels have been negatively affected by environmental changes linked to anthropogenic activities. The major reasons for the decline are habitat alteration, environmental pollution, and disruption of the unionids reproductive cycle through impoundment. As bleak as the outlook is for freshwater mussels, there are a few hardy species that can adapt to almost any environment. As long as basic needs are met (i.e. adequate DO, low siltation, and an opportunity to reproduce), then more robust species should survive. Typically, rare and endangered species are less tolerant of environmental perturbation. If the interest is to conserve all mussel species, then a variety of habitat types must be considered for preservation. If the interest is only to conserve the rare and threatened species, then more research needs to done to document the needs of the individual mussels on a species by species basis.