Native Freshwater Mussels January 2007 Fish and Wildlife Habitat Management Leaflet Number 46 Introduction Freshwater mussels belong to the phylum Mollusca, the second most diverse group of animals in the world in terms of number of described species. The phy- lum consists of approximately 100,000 freshwater, marine, and terrestrial species and includes mussels, snails, octopi, squid, as well as several other less fa- miliar groups. Although freshwater mussels are dis- tributed throughout the world, they reach their great- est diversity in North America, east of the Mississippi River. United States mussel populations have been in Virginia Department of Game and Inland Fisheries decline since the late 1800s for a number of reasons. Although freshwater mussels are found throughout Currently, nearly three-quarters of North America’s much of the world, they reach their greatest diversity native freshwater mussel species are considered en- in North America. dangered, threatened, or species of special concern, and some researchers believe that as many as 35 spe- cies (12%) are already extinct. >80 species The objective of this leaflet is to raise awareness 71–80 species about the decline of freshwater mussels in North 61–70 species America, their life history requirements, and the im- 51–60 species 41–50 species portant ecological role they play in aquatic habitats. 31–40 species In addition, this leaflet provides a number of practi- 21–30 species cal habitat management considerations to help pro- 11–20 species tect freshwater mussel populations. Freshwater mus- 1–10 species sels can also be referred to as freshwater clams or Adapted from presentation of Kevis S. Cummings, bivalves. However, for the sake of consistency, they http://clade.acnatsci.org/mussels/graphics/presenta- are referred to as freshwater mussels throughout this tions/granum_salis_lg.jpg Freshwater mussels are found in 49 of the 50 United leaflet. States. Values of freshwater mussels This, coupled with loss and degradation of freshwa- ter habitats associated with the America’s rapid indus- Cultural and economic importance trialization, contributed to the first major declines in freshwater mussel populations in the United States. Freshwater mussels were once an important natu- By 1912, nearly 200 button factories were operating in ral resource for Native Americans, particularly the towns all over the country; pearly shells of harvested mound-building tribes of the Midwest. While it seems mussels were used for buttons and their soft tissues that they were gathered primarily for use as a food for livestock feed. The button industry declined with source, their shells were also valued and used for the advent of plastics in the 1950s. tempering pottery and making tools, utensils, and jew- elry. It was not until the late 1800s, however, that the By the 1950s, however, the Japanese had found a new commercial value of freshwater mussels was recog- market for freshwater mussels as a source materi- nized by the newly born American button industry. al for cultured pearls. Mussels harvested for this pur- Native Freshwater Mussels pose are sorted and steamed or cooked to remove ple. Males and females of some species can be distin- the soft parts. The shells are then cut and finished guished by their shell size and shape. into beads for insertion into oysters to serve as nu- clei for cultured pearls. Thousands of tons of mussel Due to their sedentary lifestyle, mussels rely on a shells are processed and exported to Japan each year unique reproductive strategy to colonize new areas. to supply the cultured pearl industry. Additional com- Freshwater mussels have three basic life stages: larval mercial and medical uses for freshwater mussels are (or parasitic), juvenile, and adult. When water temper- under consideration. For example, recent research atures and other environmental variables reach ideal suggests that some mussels may be resistant to cer- conditions, male mussels release sperm into the wa- tain types of cancer and that the extraction of cancer- ter column. The female mussels draw the sperm into curing drugs from mussels may be feasible in the fu- their shell cavities as they filter water. After fertiliza- ture. tion, females brood the young from the egg to larval stage in their gills. The larvae, called glochidia, may Ecological role mature and be released the same year or may be re- tained in the gills over winter and released the follow- Freshwater mussels are an important part of the food ing spring. Species that release glochidia in the same web in aquatic ecosystems. Adults are filter-feeders year are called short-term brooders, whereas species and consume phytoplankton, diatoms, and other mi- that retain their glochidia over winter are called long- croorganisms in the water column, as well as detri- term brooders. tus and bacteria. As juveniles, mussels are deposit feeders and use their ciliated foot to obtain nutrients. Once released, glochidia must attach to the gills or Mussels are, in turn, consumed by muskrats, otters, fins of an appropriate fish host to complete their and raccoons, and young mussels are often eaten by metamorphosis to the juvenile stage. For many spe- ducks, herons, and fishes, as well as other inverte- cies of mussel, the host is limited to a single species brates. of fish, and their survival is wholly dependent on the presence of that species in the ecosystem. Subject As natural filter feeders, freshwater mussels strain to the availability of fish hosts, only a small percent- out suspended particles and pollutants from the water age of the 75,000 to 3,000,000 glochidia released from column and help improve water quality. Some mus- a female may survive to the juvenile stage. Several sels can filter up to 10 gallons of water per day, which species of freshwater mussels have fascinating ad- helps to improve water quality for other animals, in- aptations that serve to increase the chance that their cluding humans. Mussels are commonly used as indicators or biologi- cal monitors of past and present water quality condi- tions in rivers and lakes. A sudden increase in mor- tality of freshwater mussels is a reliable indicator of toxic contamination. The disappearance of freshwa- ter mussels usually indicates chronic water pollu- tion problems. Moreover, biologists can measure the amount of pollutants found in mussel shells and tis- sue to determine the type, extent, and even timing of water pollution events in streams and lakes. Life history Freshwater mussels are easily recognized by their hinged shell; however, shape, size, thickness, and col- or of shells vary greatly among species. Shell surfaces vary in color from yellow or green to brown or black; they also may contain distinctive ridges, rays, bumps, and textures. Many species have colored rays or chev- ron marks on their shells. The interior of the shell is Virginia Department of Game and Inland Fisheries composed of pearly nacre that varies in color from Freshwater mussels have three basic life stages: lar- pure white to shades of pink, salmon, gray, and pur- val (or parasitic), juvenile, and adult. 2 Native Freshwater Mussels glochidia will come into contact with the appropriate Habitat requirements fish host. For example, female plain pocketbook mus- sels (Lampsilis cardium) wiggle a modified soft tis- Most species of freshwater mussels are adapted to sue part resembling a small minnow to lure the host life in streams and rivers, although they can also be fish, a smallmouth bass. When the smallmouth bass found in artificial flow areas (ditches) and wetlands attempts to bite the minnow, the female releases her with persistent standing water (lakes). Most species glochidia. The oyster mussel (Epioblasma capsae- of freshwater mussels prefer medium to large bodies formis) opens its shell wide and flashes a brilliant of water in areas with depths less than 3 feet. The ma- blue mantle. The fish is lured towards the mussel, and jority are found along the shallow edges of waterbod- the mussel slams its shell shut on the fish’s head and ies where warmer temperatures and additional light holds on tight while the glochidia are released and at- generally provide them with more food. tach to the captured fish. Currently, little is known about environmental factors Glochidia generally remain attached to host fish for that have the most significant effects on mussel com- 2 weeks to 7 months, depending on the species. The munities. However, available information suggests mobile nature of fish allows glochidia to be trans- that mussels prefer well-oxygenated water flowing ported to other waterbodies and far stream reaches. over a stable substrate, usually comprised of sand and Following metamorphosis, juveniles drop from the gravel with some silt. The base water flow sustained fish and take up life as sedentary filter feeders. by ground water discharge or surface runoff must be adequate to moderate seasonal changes in streamflow As adults, mussels have very limited mobility. Mussels and water temperature, dilute contaminants in sur- have a muscular foot that protrudes out between the face water runoff, and withstand periods of drought. shells, wedges into the substrate, and contracts to Waterborne contaminants, such as sediments and pull the animal a short distance. Mussels can use this pollutants, must be minimal, and host species of fish method to move either sideways or vertically. Some must be available for glochidia to develop. Preferred are fairly active, while other species may remain in sources of food for adults and juveniles include many the same location for the duration of their lifespan. forms of algae, zooplankton, bacteria, and detritus. It is estimated that only about 1 in 1,000,000 glochidia develop into juveniles. The juvenile stage, or period of Habitat loss and degradation sexual immaturity, ranges from 2 to 12 years, depend- ing on the species.