Found on every continent except Antarctica, freshwater mussels are most diverse in eastern North America, where they number nearly 300 species. Their large shells make mussels the most conspicuous mollusks in the waters of the Midwest.
Freshwater mussels (also referred to as clams, naiades, or unionids) spend their entire life partially or wholly buried in mud, sand, or gravel in permanent bodies of water. The vast majority of species are found in streams, but a few are present in ponds or lakes. Although they can be found in almost any type of stream bottom, mussels are usually absent from, or rare in, areas of shifting sand or deep silt.
In most species of freshwater mussels the sexes are separate. Males release sperm into the water, the sperm enters the female via the incurrent siphon, and the eggs are fertilized internally. The fertilized eggs develop into an intermediate larval stage, termed glochidia (singular, glochidium). The glochidia are stored in the female's gills, which function as a brood chamber as well as a means for obtaining oxygen.
In the spring or summer, the glochidia are expelled into the water to begin the parasitic phase of their life cycle (Figure 1). The glochidia attach to an appropriate host, usually a fish, and form numerous cysts. Some species, in fact, possess a "lure" for attracting potential host fishes. Depending on the species of mussel, the glochidia are either internal parasites, on the gills, or external parasites, on the fins. Although some species are host-specific, others can use a wide variety of fishes as hosts.
While encysted, the larvae change form and begin to resemble adults. After metamorphosis, the small, young mussels break free from the cysts and drop to the stream or lake bottom to begin an independent life. The period of attachment varies from about 1 to 25 weeks depending on the host, location of attachment, and water temperature.
Freshwater mussels continuously pump water through their bodies. Water enters via the incurrent or branchial siphon and exits via the excurrent or anal siphon. During this pumping process, the mussel filters food from the water. The food consists of detritus, which is organic matter found on the stream or lake bottom, and plankton, composed of microscopic plants and animals
Mussels are long-lived, with many species living more than 10 years and some reported to live more than 100 years. Thin-shelled species (floaters and papershells, for example) grow much faster than thicker-shelled species (threeridges and mapleleafs, for example). In many species, the surface of the shell has distinct black lines or ridges, which are believed to represent winter rest periods. The rest periods, or growth rings, are often used to estimate the age of a mussel.
Mussels are an important food source for many animals, including muskrats, minks, otters, fishes, and some birds. Large piles of freshly cleaned mussels, called middens, can be found along the banks of a river or lake where muskrats are actively foraging. These middens often contain many species and can be one of the best places to find shells.
Freshwater mussels were collected and used in various ways by Native Americans, particularly the mound-building tribes of the Midwest, long before Europeans set foot in North America. Mussels were not only eaten but also used for tempering pottery and for making utensils, tools, and jewelry (Baker 1930, Matteson 1953).
In the late 1800s and early 1900s, enormous numbers of freshwater mussels were harvested to make pearl buttons for clothing, and button-making was a multimillion-dollar industry. By 1912, nearly 200 factories were operating in towns such as Muscatine, Keokuk, Peoria, and Beardstown (Coker 1919). The pearl-button industry collapsed, however, with the invention and widespread use of plastics in the 1940s and 1950s, and some freshwater mussel populations subsequently recovered.
By the 1950s, the Japanese had found a new use and market for freshwater mussels: cultured pearls for jewelry. Once harvested, mussels are sorted and steamed or cooked out to remove the soft parts. The shells are then cut and finished into beads for insertion into an oyster to serve as nuclei for cultured pearls. Today thousands of tons of mussel shells (particularly those of washboard and threeridge mussels) are harvested each year and exported to Japan to supply the cultured pearl industry.
Freshwater mussels are one of the most endangered groups of animals in North America. Surveys conducted over the past few decades have documented significant declines in mussel populations across the continent. Among the factors thought to be responsible for the decline are overharvest; siltation of their habitat from agriculture, poor land management, channelization, and impoundments; competition from exotic species such as the zebra mussel; and pollution by herbicides, pesticides, and other chemicals. One result of the status surveys has been the designation of many mussels as state-endangered or federally endangered species. In the United States, 42 mussels are listed as federally endangered or threatened, and another 70 have been proposed or are candidates for listing (USFWS 1991a, 1991c).
Before collecting mussels it is advisable to contact the Department of Conservation or the Department of Natural Resources to find out whether there are any restrictions and to obtain any permits that may be required. Because of the rarity of many of the native species, live mussels should never be collected without prior permission. One can still build a nice collection by
Perhaps the best place to begin looking for shells is along the bank of a medium-sized or large river when the water is at its lowest level (usually July to September). Although a few species can withstand some dessication, most are found in permanently flowing streams or lakes that contain water year-round.
Mussels can be found in a variety of habitats but are most abundant on shoals, where they live in gravel or a mixture of sand, mud, and gravel. A wide variety of shells can often be found along the shore in piles or "middens" left by muskrats or raccoons. The simplest and possibly the most effective method of collecting mussels is by hand-picking along the shore or in the stream. A small net bag or old potato sack makes a good container for holding shells in the
With a little practice, you can learn to see mussels in the shallows of clear streams. Usually the only part visible is the posterior end, with the incurrent and excurrent siphons protruding from the stream bottom. A mussel can often be spotted at one end of the meandering trail it leaves in the sand as it moves from place to place.
For your specimens to have scientific as well as aesthetic value, you need to keep accurate labels and records of field observations. After specimens are collected, a label should be made immediately and placed in the bag with the specimens; it should include the following information: the name of the body of water, road or bridge crossing, distance and direction from the nearest town, the county and state, the date, and the name of the collectors. Other information, such as water temperature, depth, current velocity, bottom type, and time spent collecting, can be recorded in a field notebook. Locality data should be written in pencil or india ink on a good grade of label paper so the label will not mold or disintegrate in the bag. Specimens without sufficient locality data are essentially worthless, so it is extremely important to accurately label specimens.
Once collected, the shells should be cleaned with warm water and a brush or teflon scrub pad to bring out the true colors and other markings needed for identification. After cleaning, locality data or a numbering system used to tie that specimen to a particular locality should be written directly in the shell with a pencil or india ink. If, after cleaning, you still have trouble identifying your specimen, you can often send it to a specialist for verification. Prior arrangements should be made with the curator of a museum before sending specimens for identification.
By keeping well-organized and curated collections, many amateurs have made significant contributions to malacology (the study of mollusks) over the years. Most of the major museums of the world contain donated private collections that are available for study by specialists for years to come. See van der Schalie (1941) for additional information about collecting shells.