Margaritiferidae

Diversity

The family Margaritiferidae is commonly referred to as pearl mussels, naiads, or margaritiferids. No more than 15 species in 3 genera have been described, which is nearly 5% of the Unionoida species in the Holarctic (Smith, 2001b).

In general, Margaritiferidae are acephalic (no head), bivalved mollusks usually with the beak (the elevated portion of the dorsal margin) slightly anterior. When present, the pseudocardinal teeth are anterior to the beak and the lateral teeth are posterior. The species in this family have a foot rather than a byssus, fibrous structures found in other mussel families. Along with Unionidae , another family included in the order Unionoida , Margaritiferidae does not have true siphons. Unlike the family Unionidae , the inhalant aperture (opening in the posterior end of the mantle border where water enters the mussel) of Margaritiferidae has branched papillae (bumps). The shells are elongate, thick, black, rhomboidal and often arcuate (arched) and range in size from 80 to over 200 mm in length (Smith, 2001b).

Geographic Range

Members of the family Margaritiferidae are found throughout the Holarctic. There are three genera and up to five species found in North America, two genera and two species in Europe, one species in North Africa, one species in Syria, one genus and three species in northeast Asia, and one species in southeast Asia (Smith, 2001b).

Before the supercontinent of Pangaea divided, Margaritiferidae were dispersed throughout. Today most of the Margaritiferidae are found in high altitude streams due most likely to the expanse of the Unionidae in the higher order rivers (Smith, 2001b).

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

Margaritiferids are found in permanent freshwater sources with moving water such as streams and rivers. They are most abundant in depths less than 2 m, but will populate waters as deep as 7 m (Smith, 2001a). Large rivers tend to contain a wider diversity of mussel species and larger populations than smaller streams (Cummings and Mayer, 1992). Watters (1992) found a relationship between the size of the drainage basin and the fish diversity. He also found a linear correlation between the fish diversity and the mussel diversity. Rivers tend to have a more abundant food supply and higher dissolved oxygen content than bodies of water with little or no current. They may also provide a more preferred substrate and water chemistry.

Margaritiferids tend to thrive in neutral to weakly acidic water, but may be found in slightly alkaline water (Smith, 2001b). Acidic water tends to dissolve the calcium content in the shells. As an adaptation to the soft water, Margaritiferidae shells are thick, containing nearly 30% of the mussel's organic content (Bauer, 2001a).

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• rivers and streams

• Other Habitat Features
• riparian riparian
• intertidal or littoral intertidal or littoral

Physical Description

In general, members of the Margaritiferidae family are acephalic (not having a true head), have two calcium carbonate/organic shells called "valves" (bivalved) attached at the hinge by an elastic ligament . They have an umbo (beak) along the dorsal margin and slightly anterior to the hinge and are bilaterally symmetrical along a plane running between the two valves. Individuals do not have true siphons. Instead, they have two openings in the mantle along the posterior margin that act as the inhalant and exhalant apertures (Smith, 2001a). Unlike the Unionidae , which have two openings in the mantle for the exhalant aperture, Margaritiferidae has only one opening. The mantle margin along the inhalant aperture is lined with branched papillae ( Unionidae have unbranched papillae or bumps) (Smith, 2001b). The exhalant aperture has crenulations (grooves) along the mantle margin (Smith, 2001b). Under each mantle, is a gill made up of two demibranchs. Each demibranch is composed of two lamellae fused at the ventral surface but open at the dorsal surface forming a "W." The ax-shaped foot is found on the anterior end of the organism and between the demibranchs in the two valves. The majority of the median visceral mass in the posterior portion of the organism is primarily dorsal and not as confined in the anterior portion (Smith, 2001a). As acephalic organisms, mussels have a simplistic sensory system. Their nervous system is comprised of three pairs of ganglia: cerebropleural, pedal, and visceral. With one on each side of the esophagus, the cerebropleural ganglia are located on the posterior side of the anterior adductor muscle and are connected by a short commissure. In the foot and fused is the pair of pedal ganglia and anterior to the posterior adductor muscle is the partially fused visceral ganglia. The ganglia are connected by long commissures and each pair is the source of the nerve fibers for the surrounding organs (Smith, 2001a). Near the pedal ganglia is a pair of statocysts, which are ovid or spherical. These statocysts are filled with fluid and lined with sensory cells. They also contain a solid sphere called a statolith (Smith, 2001a). Osphradia are specialized epithelium concentrated in two small regions on the roof of the cloacal chamber (the posterior end of the suprabranchial chamber in the gills where it is fused) (Smith, 2001a).

Adults can range anywhere from 80 to over 200 mm (Bauer, 2001a; Smith, 2001a; Smith, 2001b) in length. Mussel species found in low order streams where the stream flow is more turbulent, tend not to have external shell sculpturing (Bauer, 2001a). Sculptures, such as pustules and ridges, aid in the mussel's ability to burrow into the sediments (Bauer, 2001a). Headwater streams have fewer sediment deposits due to the steeper channel gradient and flow rate of the stream. Margaritiferids tend to be found in these headwater habitats and so do not have external sculptures. Margaritiferids tend to have elongate, compressed, thick shells that are rhomboidal and often arcuate (arched) in shape. The periostracum does not contain rays or any other external color pattern, is light brown to a greenish brown in young mussels and black in adults (Smith, 2001b).

Aside from the exterior surface of the shell, researchers involved in identifying mussel species examine various aspects of the interior of the shell, as well. In fact, because of the high individual variability of the exterior, the interior characteristics are relied more heavily upon in identification. Probably the most important interior features are the size, shape, number, and orientation of the hinge teeth . Pseudocardinal teeth are situated slightly anterior to the beak and are generally short and triangular in shape. Lateral teeth are the long, slender, raised ridges posterior to the beak. In young individuals, the teeth are well-developed, while in some older individuals, the teeth are reduced (Smith, 2001b). Another internal shell characteristic of Margaritiferidae is the presence of mantle attachment scars extending from the beak cavity posterior-dorsally to the pallial line (Smith, 2001b). Most species cannot be identified by merely one characteristic. In reality, it is a combination of several characteristics which distinguish one species from another.

Glochidia are the parasitic stage of the larvae and are generally dependent on a host to survive. Mature glochidia are an average of 0.08 mm in diameter (Wachtler et al, 2001). They are circular to ovid bivalves, which are typically attached by a single adductor muscle (Smith, 2001a). Most glochidia have sensory hairs lining their mantle and have reduced or absent hooks (Wachtler et al, 2001). Those species without hooks usually attach to the gills (Smith, 2001a; Wachtler et al, 2001).

Measurements are generally taken of the length, height, and width. The length is the distance from the anterior to the posterior margin. The height is the distance from dorsal to ventral margin, usually at the beak. Width is the widest point when the mussel valves are together, which is usually below the beaks. In addition, some identification keys will use the length to height ratio as a way to distinguish some species.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

Embryonic mussels develop within the marsupia, or specialized portions of the gills, of the female. Once fully developed, they are released from the female and must attach to a host within a few days or they will die. Because margaritiferids lack developed hooks, they must attach to the gills of the host. Generally species that must attach to the gills rather than the fins tend to be more host-specific (Wachtler et al, 2001). Margaritiferids tend to be specific to salmon and trout (Watters, 1994b). Attachment to a wrong species will cause the death of the glochidia from an immune system attack (Watters, 1998). Within a couple of days, the hosts' dermal tissue will encapsulate each glochidium forming a nodular cyst. While encysted, the glochidia will metamorphose, allowing the organs to develop more like an adult's organs (Meglitsch and Schram, 1991). There is a mortality rate of over 99.99% (1 in 100,000,000 survive on average) from the time the glochidia are released from the mother to the time the metamorphosed juveniles reach the sediments (Jansen et al, 2001; McMahon, 1991).

The period of encystment can range from 3 to 10 months (Wachtler et al, 2001). Unlike Unionidae , Margaritiferidae glochidia tend to increase in size during metamorphosis (Wachtler et al, 2001). After this period, metamorphosis will be complete and the glochidia will break from the cysts and drop from the host. The third and final stage of development occurs in the sediments of the stream or lake and may last up to twelve years before the juvenile is sexually mature (McMahon, 1991). In this juvenile stage, the young mussel will complete its internal development, create the adult shell, and begin to live independently on the bottom of the stream or lake (Smith, 2001a).

As in most bivalves, the shell is composed of three layers: the periostracum, the prismatic layer, and the nacre. The periostracum is the outermost layer and is composed of an organic material. The prismatic layer is the middle layer and is composed of thin blocks of a prism-like calcium carbonate, which are oriented perpendicular to the mantle and the other two layers. The nacre, or mother of pearl, is the innermost layer, which is composed of thin, alternating, laminae (flakes or sheets) of calcium carbonate and an organic material (Smith, 2001a). The mantle is responsible for generating new shell as the mussel ages. A mantle flap is pressed against the interior of each valve and ends in three folds. The periostracum forms at the outer margin and the prismatic layer forms at the outer border. The nacre forms along the entire surface of the mantle. Muscle scars form where the muscle attaches to the shell, disrupting the formation of the nacre. Instead of the shell forming along the dorsal edge where the hinge is located, an elastic hinge ligament composed of conchiolin (a protein-rich substance) forms, binding the two valves together (Meglitsch and Schram, 1991).

Growth of the mussel begins at the elevated portion called the umbo or beak . Because new shell is added along the entire edge of the mantle, concentric rings form around the beak. In some species, these rings may be grouped closer together in some areas than others, forming ridges. These ridges indicate the period of diapause during the winter or unfavorable environmental conditions, such as lower water level or lack of food. The period of growth in northern populations is typically from April to September. The growth rate depends mostly on environmental conditions such as water temperature, food supply, and the chemical composition of the water (Smith, 2001a).

• Development - Life Cycle
• metamorphosis metamorphosis
• diapause diapause

Reproduction

Some margaritiferids are occasional or permanent simultaneous hermaphrodites (self-fertilizing); while others are dioecious (sexes are separate) (Bauer, 1987; Smith, 2001b). Bauer (1987) suggested that hermaphroditism occurs when the population density is low or gene flow is limited. In these cases, the female is the only one of the two sexes that can become hermaphroditic. Despite the dioecious nature of most mussels, males and females do not make contact with each other. The male's sperm leaves the suprabranchial chamber of each demibranch and exits the organism through the exhalant aperture to be carried by the water current to a nearby female. Because sperm cannot swim against the current, the receiving female must be downstream (Watters, 1994a). The sperm enters the female through the inhalant aperture and fertilizes the eggs stored in the demibranch (Smith, 2001a).

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Margaritiferids can take up to 12 years to reach full sexual maturity (McMahon, 1991). They are tachytictic (short term) breeders, which means they will release the glochidia in the same year, usually by July or August (Watters 1998), and may have multiple reproductive events each year (Smith, 2001b). Matteson (1948) was convinced that the membrane surrounding the developing embryos provides all of the necessary nutrients, rather than the female transferring food to the developing young. His conclusion was based on a lack of connective structure from the gills to the young and that the fertilization membrane surrounding each embryo, which prevents the passing of any materials, remains until development is complete.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sexual sexual
• fertilization fertilization
  • internal internal
• ovoviviparous ovoviviparous

Margaritiferid embryos spend the first stage of development in the marsupial portion of the female unionid's gills, where they develop into glochidia, the parasitic stage. Once the first stage is complete, usually in the spring, the female will release the glochidia into the water to begin the second stage as a parasite. The number of glochidia in one brood typically depends upon the size of the glochidia and the size of the female. Since Margaritiferidae glochidia are generally small (0.08 mm) and the average female is relatively large (80 to 200 mm), the individual female can incubate an average of 3 to 4 million and up to 17 million glochidia at a time (Wachtler et al, 2001; Smith, 2001b; McMahon, 1991). The glochidia are incubated in both pairs of gills and remain there for only a few weeks before being released (Wachtler et al, 2001; Bauer, 2001a).

• Parental Investment
• precocial precocial
• female parental care female parental care

Lifespan/Longevity

For small organisms, mussels are long-lived (Cummings and Mayer, 1992). Margaritiferids have a much longer lifespan than unionids , with a range of 40 to well over 100 years (Bauer, 2001a). The record for a Margaritifera margaritifera was over 200 years (Bauer, 2001a). Bauer (2001b) suggested life span is dependent upon metabolic rate. Mussels with a higher metabolic rate tend to have a shorter life span. Those in larger rivers or streams would have a higher metabolic rate due to the abundance of food, and would be expected to have a short life. Margaritiferids tend to thrive in low order streams (closer to the headwaters than the mouth), and so tend to live longer than the unionids in the higher order streams. This is possibly because mussels that thrive further upstream may have adapted to a limited food supply by decreasing their metabolic rate. Although metabolic rate is a key factor affecting the longevity in some species, it is not a universal constant. Some species with similar metabolic rates may have very different lifespans.

Behavior

For the most part, mussels are sedentary, but they are capable of a restricted form of locomotion. They move around by a series of muscular motions of the foot located at the anterior end of each individual. The foot is thrust forward first. It then swells and shortens at the same time, causing the body and shell to pull forward slightly. This process is repeated until the mussel has reached its destination. Some species have been recorded to move up to several feet within an hour. Researchers are still unsure what causes this migration, but they suspect the movement is caused by a drop in water level or some other unfavorable change in the surrounding environment (Smith, 2001a).

Like unionids, margaritiferids are solitary organisms. The only intra- or inter-species interactions occur during reproduction. Once it drops from the host, the mussel becomes a solitary individual and lives partially buried in the sediments. As juveniles, mussels burrow into the sediments along the bottom of the stream, which protects them from predators. Once mature, more of the organism must protrude from the substrate in order for the inhalant and exhalant apertures to bring in and expel water. Because more of the shell is visible, they are more susceptible to predation (Smith, 2001a).

During winter months and aestivation periods (Matteson, 1955; van der Shalie, 1940), mussels will burrow into the substrate until only the apertures are protruding. They then go into a state of dormancy where the apertures only open on occasion. Some genera are able to survive in this dormant state among the dry to moist sediments for months at a time (Smith, 2001a).

• Key Behaviors
• fossorial fossorial
• parasite parasite
• motile motile
• nomadic nomadic
• sedentary sedentary
• hibernation hibernation
• aestivation
• solitary solitary

Communication and Perception

Mussels use specialized structures to visually attract potential fish hosts. The combination of the statocysts and the statolith aids the mussel in maintaining equilibrium by sensing gravity. They may also be able to detect vibrations (Meglitsch and Schram, 1991). Although the function of the osphradia is uncertain, some researchers believe that they detect foreign particles brought in through the inhalant aperture (Smith, 2001a). Drastic changes in the intensity of the light in the environment can be detected by the mantle border (Smith, 2001a). Glochidia can usually detect light changes with ocelli, but the eyes are generally lost after metamorphosis (Meglitsch and Schram, 1991). Many mussel species also have tactile cells lining the exposed portion of the mantle, which aid in the organism's sense of touch (Meglitsch and Schram, 1991). The glochidia are especially sensitive to touch, which helps in the attachment to a host as it comes close to them (Arey, 1921).

• Perception Channels
• infrared/heat infrared/heat
• polarized light polarized light
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Adult freshwater mussels are filter feeders; they continuously filter food particles out of the water (Watters, 1998; Allen, 1921). Water is constantly pumped into the inhalant aperture, through the gills , and out the exhalant aperture by cilia. The cilia lining the inner surface of the mantle, demibranchs, and visceral mass create a current by beating in a coordinated manner. Organic and inorganic particles suspended in the water surrounding the inhalant aperture are brought in by the current and caught in the mucus lining the demibranchs. The constant current created by the cilia moves the mucus with any trapped particles to the cilia lining the labial palps . The labial palps remove the inorganic particles and push them toward the ventral margin where they drop off. There they are moved by the cilia backward, and released between the valves just below the inhalant aperture (Smith, 2001a). The organic particles are separated by size in sorting areas on the labial palps and are then directed into the mouth. From the mouth, particles are moved through a short esophagus to the digestive gland surrounding the stomach. Food particles enter the stomach through the subdivided pores of the large digestive gland (Meglitsch and Schram, 1991). Small particles are digested intracellularly as they enter the stomach. The intestinal glands are responsible for phagocytosis, intracellular digestion, food absorption, secretion of enzymes and excretion (Meglitsch and Schram, 1991). The intestine coils behind and below the stomach before it extends dorsally and empties into the mantle cavity through the anus located just above the exhalant aperture.

The exact type of food consumed by adult freshwater mussels has been debated for some time now. Some researchers have suggested mussels eat algae and diatoms (Allen, 1914), while others suggest bacteria, protozoans and other organic particles were ingested (Watters, 1998). A few studies have even suggested ingesting silt somehow enhances the survival of the organism (Watters, 1998). Current views suggest mussels feed on the bacteria and microphytoplankton but nothing larger (Smith, 2001a; Cummings and Mayer, 1992).

The phagocytic mantle cells of the glochidia feed off of the host's tissue (Meglitsch and Schram, 1991). Before attachment, glochidia must locate a proper host. In most cases, they end up in the stream or lake sediments with the open end of the valves up awaiting a fish to brush up against the mud allowing the larvae to attach themselves to the fins. The glochidia of other species swim around in the water by clapping the valves together.

• Primary Diet
• planktivore planktivore
• detritivore detritivore

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Muskrats are probably the most important mammals which prey on freshwater mussels (Cummings and Mayer, 1992; Smith, 2001a). These animals drag the mussels on the shore and either break the shells open with their teeth or leave them on the banks until the mussel dies and the shell opens (Smith, 2001a). In active muskrat foraging areas, there are often middens of a variety of shells which have been cleaned by the muskrats. Other common predators include minks , otters , raccoons , turtles , hellbenders , fish, some species of birds, and humans (Cummings and Mayer, 1992; Smith, 2001a; Watters, 1998). Some of the common fish species include the freshwater drum , sheepshead , lake sturgeon , spotted suckers , common red-horse , and pumpkinseed (Smith, 2001a). In Europe, hooded crows have been known to prey upon mussels. They are able to reach the soft tissue by dropping the mussels to crack the shell open (Watters, 1998).

To avoid these predators, mussels will bury themselves into the lake or stream sediments. Because adults do not have true siphons, only openings in the mantle, they must leave the posterior margin out of the sediments to allow for sufficient respiration. This exposure leaves the organism vulnerable to predation, desiccation, and temperature extremes (Watters, 1998).

Ecosystem Roles

Like all other organisms, freshwater mussels play an important role within their ecosystem. Not only do they provide a food source for muskrats and other predators, but they also aid in the decomposition of detritus and keep the bacterial and planktonic populations under control (Pusch et al, 2001; Jorgensen, 1990). They are important to the second trophic level by feeding heavily upon the phytoplankton (McMahon, 1991). Dense mussel populations rely on rapid current for survival. During periods of little or no current, these dense mussel beds can cause a depletion of the dissolved oxygen and food supply, causing a rise in the mortality rate of the mussel and other faunal populations along the basin (Jorgensen, 1990). In addition, freshwater mussels are important water filters and act as organic nutrient sinks by filtering the suspended seston (McMahon, 1991).

Researchers have found that the glochidia generally do not cause sufficient enough damage to the host to cause problems. Cases of over 3000 glochidia infecting a fish without apparent harm have been reported. However, there have also been cases where 30 mm fingerling trout have died of secondary bacterial infections caused by a little more than 100 glochidia (Smith, 2001a). Some fish species are able to develop an immune response to resist the glochidia causing them to pre-maturely drop off the fish.

• Ecosystem Impact
• parasite parasite

Economic Importance for Humans: Positive

Long before Europeans ever arrived in North America, Native Americans were utilizing freshwater mussels and their shells for food, jewelry, tools, utensils, and pottery temper (Cummings and Mayer, 1992). Native Americans have been carving shells for implements and ornamentation for at least 3000 years. Around 1000 years ago, people in North America discovered that by tempering their pottery with crushed shells rather than sand or gravel allowed them to create a smoother, thinner vessel. During this same period, people were creating beads, hoes and spoons with the freshwater mussels (Wiant, 2000).

Before 1890, freshwater mussels were utilized for only a few decorative items such as pistol grips, brush handles, and jewelry. Both the U.S. tariffs on imported goods (including buttons) and the rise of the new ready-to-wear clothing industry created high demand for buttons. The pearl button industry began in 1891 with the start of a new fashion trend to use shell buttons to fasten clothes. With Muscatine, Iowa as the center of the industry, pearl buttons became the major economy for hundreds of river towns along the Mississippi and other Midwestern rivers. The demand was so high that by 1900 the Illinois and Wabash rivers were depleted of mussels. The peak of the industry occurred in 1909 with a record of 2600 boats on the Mississippi River alone. By the 1940s and 1950s, the invention of and widespread use of plastics replaced the shell buttons with plastic ones, causing the collapse of this industry and the recovery of many impacted mussel populations. (Huitt and Warren, 2003)

In the 1950s, the Japanese developed another use for freshwater mussel shells (Cummings and Mayer, 1992). They discovered that small beads could be carved out of the shells of freshwater mussels and inserted into oysters to artificially form pearls. This discovery was the beginning of the cultured pearl industry. Today, thousands of tons of freshwater mussel shells from North America are exported to Japan to support the pearl industry (Cummings and Mayer, 1992).

In addition to the many products, freshwater mussels act as water quality indicators. Because they are filter-feeders, pollutants in the water will accumulate in the tissue of mussels until they reach a toxic level killing the organism. A drastic drop in the mussel population is an indication of poor water quality.

• Positive Impacts
• food food
• body parts are source of valuable material
• research and education

Economic Importance for Humans: Negative

There are no reported negative effects on humans or the economy due to unionids. A hindrance on the fishing industry by the parasitic glochidia is plausible.

Conservation Status

Worldwide, freshwater mussels are one of the most endangered groups with significant population declines documented in recent surveys. In the United States, nearly 70 species of Unionoida are either endangered or threatened, one of which is in the Margaritiferidae family ( Margaritifera hembeli , the Louisiana Pearlshell, is federally threatened) (USFWS, 2003). In Europe, Margaritifera auricularia ( Pseudunio auricularia ) was once thought to be extinct until a fertile population was found. Now it is viewed as one of the most threatened invertebrates in the world and extensive conservation efforts have been developed to protect it (Araujo and Ramos, 2001). Reasons for the past decline include the effects of the pearl button industry of the late 19th and early 20th centuries and the cultured pearl industry of the past 50 years. Today, siltation from agriculture, forestry, and construction smothers the organisms inhibiting feeding and respiration. Impoundments alter the habitat, killing first the mussels that thrive in rapid currents. Dams cause an increase in silt as well as a constant cold water temperature. Since many mussel species are temperature sensitive, the cold will slow the growth and may inhibit the reproduction of the mussels that survived the initial shock of the construction. In-stream sand and gravel mining often buries, crushes, or removes the mussels in the substrate and releases silt, which affects the species downstream. Agricultural runoff is another threat to mussel populations. Many species cannot tolerate pollutants introduced in the water from pesticides, herbicides, and fertilizers. At sub-lethal concentrations these chemicals inhibit respiration and accumulate in the tissues of the organism. Mussels are also sensitive to heavy metals which accumulate in the tissues. Mine runoff creates an acidic pH in the water, which many mussel species cannot tolerate for long periods of time. Salinity from road salt runoff is lethal to glochidia (Watters, 1998).

In addition to industrial wastes and depletion, mussels now compete for resources with introduced species. The Asian clam and the zebra mussel are probably the two most common exotic species, which have been introduced to North American freshwaters.

Additional Links

Encyclopedia of Life

Contributors

Renee Sherman Mulcrone (editor).

Lisa Winhold (author), Animal Diversity Web.

References

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