New York Metropolitan Region and New Jersey Freshwater Mussel Identification Handbook
Introduction to Freshwater Mussels Taxonomy and Biology
The following topics provide a brief overview of the taxonomy, biology, and diversity of mussels.
The worldwide superfamily Unionoidea contains the group of bivalved mollusks commonly referred to as freshwater mussels. Various arrange- ments have been proposed for the families of the Unionoidea. Three families- Hyriidae, Margaritiferidae and Unionidae- were described, and Heard and Guckert (1970) added a fourth, the Amblemidae. Davis and Fuller (1981) reclassified the unionoids, placing the Margaritiferinae and Ambleminae as subfamilies with the Anodontinae in the family Unionidae. Synonymies are also widespread at the species level. Margaritifera margaritifera has over one hundred available synonyms. Proliferation of names was common prior to modern phylogenetic practice and before nomenclatural standards were adopted requiring references to published descriptions and locality data. Several current works propose to address these issues through the use of molecular data and cladistic models (Lydeard et al., 1996; Hoeh et al., 1998). This handbook follows the classification recognizing the families Margaritiferidae and Unionidae. Also included in the handbook are two families not part of the Unionoidea, the Corbiculidae and the Dreissenidae. Several species of these families occur as invasives within the metro area.
Diversity and Distribution
North America: The Margaritiferidae and Unionidae are the only unionoid families that occur in the United States. They include largely sedentary, filter-feeding bivalves that range in size from four to thirty centimeters (Williams, et al., 1993), and live in lakes, ponds, and streams throughout the world. Their association with major river drainage systems with relatively long geologic histories has allowed for the evolution of a large number of genera with highly specialized, endemic species (Burky, 1983; Burch, 1975; McMahon, 1991). North America has the greatest diversity of freshwater mussels in the world with more than 300 currently recognized species and subspecies in the United States and Canada (Williams et al., 1993; Turgeon, et al., 1998). Although there is no recent, comprehensive North American field guide, partial lists of species distribution have been published by Clarke (1973; 1981) for Canada and by Burch (1975) for the United States. There are also distribution keys for many regions, states and drainages.
New York Area: The New York State mussel fauna is much richer in species than its eastern seaboard neighbors. This is due in part to the state's size and geographic position- westward from the Atlantic across the Appalachian and Allegheny ranges to the Interior Basin, and northward to the Great Lakes, St. Lawrence, and the Canadian border. Other important factors, in addition to the presence of large, stable river drainages in all regions of the state, include the effects of glaciation cycles, and human influences such as the development of the St. Lawrence seaway and the Erie Canal. The state fauna thus incorporates species assemblages from streams on the Atlantic slope, flowing eastward into the Atlantic Ocean between Nova Scotia and Georgia, and also ones from the Great Lakes, and the Ohio and Mississ- ippi River watersheds in the Interior Basin. New York therefore records 51 species, compared to neighboring Connecticut and New Jersey with 12 each. Thirteen unionoid species occur in the metro area and all, with the possible exception of Strophitis undulatus and Utterbackia imbecilis, are derived from the Atlantic Slope. For a detailed discussion of the complex origins and distribution of New York's unionoid fauna, see Strayer and Jirka (1997).
Unionid sexes are typically separate (mussels are dioecious). Some species may become hermaphrodites capable of self-fertilization when population density is low (van der Schalie, 1970; Kat, 1984; Bauer, 1987; Downing et al, 1993). In females the ovaries lie within the visceral mass just dorsal to the foot, and they are connected to the suprabranchial chamber by tube-like ducts (Lefevre and Curtis, 1910; Pennack, 1989; Oesch, 1995). Unfertilized eggs stored in the ovaries pass through the oviduct to the chamber. During spawning, males release sperm directly into the water and, with luck, they are drawn through the incurrent siphon of a sexually mature female of the same species and pass into the chamber where fertilization occurs.
Fertilized eggs are captured by the inhalant siphon of the female (Coker et al., 1921; Matteson, 1948; Lynn, 1994) and carried against the water currents by means of beating cilia into the gills. Here they are stored in the water tubes of marsupial gill pouches (=modified brood pouches, or marsupia) where the larvae, called glochidia (Coker, et al., 1921; Pennak, 1989), are held together during development by an adhesive membrane. The degree of gill modification has been used as a character in mussel classification; either all four gills (two on either side of the foot and visceral mass), just the outer gills, or only specialized portions of the outer gill may be utilized.
The period of larval development in the marsupia varies from days to months. From several thousand to more than three million larvae are expelled through the exhalant siphon into the water, depending on the species. Bradytictic (long-term brooding) species fertilize in summer or early fall, and glochidia are released the following spring. Tachytictic (short-term brooding) species fertilize in spring and glochidia are released later the same summer. A mature glochidium ranges in size from 0.05 to 0.50 mm in diameter, and bears a thin shell with two valves that can be drawn together by a single adductor muscle (Coker et al., 1921).
In the days after their release glochidia must avoid predation and, despite being unable to swim, locate and attach to their host (usually a fish) as an external parasite. Unless the proper host is found they will be sloughed off by a fish's natural defenses, and many unionids rely on obligate host relationships for dispersal. If not attached to a fish host without a very short time the glochidia die. One species, Simpsonaias ambigua, has been documented to utilize the mud puppy, Necturus maculosa, as a glochidial host (Howard, 1915; Pennak, 1989). Recent studies (Watters, 1997; Watters and O'Dee, 1998; Wicklow and Brisheim, 1998) have revealed that for species such as Utterbackia imbecilis other amphibians may serve as potential hosts in the laboratory. And some metro area species- Utterbackia imbecilis, Lasmigona subviridis, and Strophitus undulatus are reported to undergo glochidial transformation without a fish host.
There are various dispersal strategies. Glochidia are sometimes released unattached in bursts, enclosed in a membraneous packaged sheath (Barnhart and Roberts, 1997), or suspended on long strands of mucus latticeworks or gelatinous packages called conglutinates (Matteson, 1955). Some species release conglutinates in the form of "lures", including a long "superconglutinate" consisting of all its glochidia compacted into a mass that mimics the appearance and movement in water currents of a small fish or insect larva (Hartfield and Butler, 1997). In female Lampsilis, the edge of the mantle is developed into a "lure" that resembles a fish (Williams et al., 1993), while in Ptychobranchus occidentalis , the sheathed packet is colored and shaped like a fish lure (Barnhart and Roberts, 1997).
In other species the "superconglutinate" lure is used to attract fish while remaining attached to the adult female by means of a transparent musci- laginous line. This eventually becomes detached by water currents or the actions of attracted fish (Hartfield and Butler, 1997). When a fish strikes the "lure" the glochidia within are released in a cloud, clamping onto the host fish to begin their next phase of development (Lefevre and Curtis, 1910; Coker et al., 1921).
The glochidia are of two types, hooked and hookless. Hooked glochidia typically attach to fish by clamping their shell, sometimes adorned with sharp hooks and attachment threads, to the body or fins, while hookless forms attach to the gills (Arey, 1932; Matteson, 1948). At the time of attachment glochidia typically possess a shell, adductor muscle, and a rudimentary foot, mouth and intestine (Coker et al., 1921; Oesch, 1995).
The glochidium encysts beneath the host tissue as an ectoparasite for varying lengths of time, from six to one hundred sixty days, depending on species and environmental conditions, during which the single adductor muscle is replaced by two and the digestive system forms as the glochidia metamorphose into juvenile mussels. Typically little or no increase in size occurs during encystment (Coker et al., 1921; Matteson, 1948; Pennak, 1989; Oesch, 1995), and the glochidia remain more an irritation to the fish than a danger. Some fish deaths have been reported from encystment by a large number (100 or more) of glochidia (Matteson, 1948; Smith, 1976; Pennak, 1989).
When metamorphosis is complete, the juvenile mussel drops from its host to the bottom. This is a vulnerable time for, if not quickly consumed by a predator, the mussel must settle on a substrate suitable for its adult life requirements- if it does not, it will die. If successful, the juvenile mussel will burrow into the bottom sediment, becoming interstitial, or attach to a hard substrate with a byssal thread. This thread is quickly lost in the adult stage. At the completion of juvenile metamorphosis the mussel has developed a heart, liver, digestive tract, and muscular foot.
The greatest shell growth occurs in the first few years of life. Rate of shell growth is much lower in adults than juveniles. Once growth begins to slow, the rate of development of the reproductive organs increases; the average age of sexual maturity is greater than six years (McMahon, 1991). In the temperate zone, including the New York metro area, growth slows or stops in winter months. Species burrow into the sediment and enter a period of dormancy (Balfour and Smock, 1995; Amyot and Downing, 1997). During this period a dark band (or, annulus) forms along the shell margin. Annuli are laid down each year during growth interruption, and therefore can be used to determine the age of a shell (Neves and Moyer, 1988).
Competion among and between mussel species has scarcely been investigated, although it has been alluded to for species that occur in dense assemblages such as Elliptio complanata. Possible competition for hosts may also be significant, especially among species in the subfamily Lampsilinae- for example, Lampsilis cariosa, L. radiata radiata, and Leptodea ochracea, which have evolved highly specialized methods of host fish attraction.
Once a mussel reaches maturity its chances for survival increase dramatically. The life spans of various species range from 8 to 20 years (Nedeau et al, 2000), although many North American species live 30 to 80 years (A. Bogan, pers. comm.) and Margaritifera margaritifera may live 100 years or more (Bauer, 1987).