Margaritifera margaritifera, also known as the freshwater pearl mussel or the eastern pearlshell, is native to European rivers and streams. The range stretches from Norway to Spain and populations are found in Great Britain and Scotland. This species has also been introduced to North America, where sizable populations are found in the northeastern United States and eastern Canadian coast. ("NatureServe Explorer: An online encyclopedia of life", 2009; Skinner, et al., 2003)
Freshwater pearl mussels live buried or partly buried in stream bed substrates that have sandy patches surrounded by boulders and large rocks. They favor riffles, common sites for heavy sedimentation. Areas of fast-flowing water low in nitrates and phosphates and shaded by riparian growth are preferred due to the lower likelihood of algal blooms. (Moorkens, 2000; Skinner, et al., 2003)
Juvenile freshwater pearl mussels are more vulnerable to fluctuation in environmental conditions than adults and have stricter requirements for survival as a result. While adults can temporarily tolerate muddy or silty conditions, juveniles cannot survive in those habitats. They must settle in sandy habitats interspersed with rocks and boulders. They require clear water not saturated with small sediment particles that interfere with their ability to feed. Water that is 0.3 to 0.4 meters deep and flowing at a rate of 0.25 to 0.75 meters per second is also necessary for these mussels to feed properly. The pH of the water must be under 7.5 in order to support juvenile populations. (Hastie, et al., 2000; Skinner, et al., 2003)
As bivalve mollusks, freshwater pearl mussels have hard shells consisting of two plates attached to a hinge. The color and thickness of the shell changes over time. Juveniles have thin shells that are yellowish-brown, whereas adults have thicker shells that are dark, glossy black. All freshwater pearl mussels, regardless of age, are roughly kidney-shaped and have an appendage on the base of their shell called a foot. The foot allows them to burrow into yielding ground, move slowly about on sandy soil, and anchor themselves in place. They also have a siphon to draw in water, gills to filter out edible material, and a second siphon to expel filtered water. (Degerman, et al., 2006; Skinner, et al., 2003)
Male, female, and hermaphroditic freshwater pearl mussels are identical in appearance. Different sexes have the same coloration and, given equal amounts of food, grow at the same rate. It is therefore impossible to discover the sex of organisms without dissecting them.
Freshwater pearl mussels typically grow to 10 to 13 cm in length, although there will be a large amount of variability in size in a healthy population. A particularly large specimen was over 17 cm long, newborn freshwater pearl mussels may be smaller than 1 cm long. (Bauer, 1987; Degerman, et al., 2006)
The zygote that forms when sperm and egg meet inside of the female (or hermaphrodite) mussel is called a glochidium (plural glochidia). Glochidia spend several weeks developing inside of the mussel, eventually growing 0.6 to 0.7 mm in size and resembling tiny mussels with their shells held open. The glochidia are then released into the open water, where they float with the current. The next stage of development occurs on a fish host. If a glochidium makes physical contact with a fish's gills, it will clamp onto the gills and begin developing into a juvenile freshwater pearl mussel. The presence of glochidia in the gills of a fish has no observable negative effects on the host, but helps the glochidia disperse over a wider range than could be possible by freely floating in a stream. Host attachments typically occur within several hours of glochidia release. Most glochidia never find a host and die after six days of floating in the current. (Degerman, et al., 2006; Skinner, et al., 2003)
Many different fish can act as hosts to glochidia. In Europe, these fish include huchen (Hucho hucho), Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and Eurasian dace (Phoxinus phoxinus). Hosts in the United States are primarily salmon and trout species, including coho salmon (Oncorynchus kisutch), rainbow trout (Oncorhynchus mykiss), cuttthroat trout (Oncorhynchus clarkii clarkii), chinook salmon (Oncorhynchus tshawytscha), and sockeye salmon (Oncorhynchus nerka). Brook trout (Salvelinus fontinalis) can also serve as hosts. ("NatureServe Explorer: An online encyclopedia of life", 2009)
After eight to nine months developing in a fish's gills, the glochidia detach themselves. This is when young freshwater pearl mussels are the most vulnerable, as there are many factors that could result in death. Should glochidia land in clean gravel or sand substrate, they will likely survive and start to grow. But if glochidia land in unfavorable substrates, such as mud or silt, they die. Glochidia are also likely to perish if they have not developed to a sufficient size in the host fish. High levels of ammonia, nitrate, phosphate, sodium, potassium, calcium, and magnesium - all of which are chemicals commonly introduced to rivers by humans - also have a negative impact on glochidia survival. For reasons not fully understood, juvenile mussels growing in locations with sizable aquatic insect (Chironomidae) populations are more likely to survive. If the juvenile mussel survives, its development will be slower than an adult's - about one to two millimeters per year. It will take about twenty years for the juvenile mussel to become an adult, at which time it will continually grow about three millimeters per year until its death. (Buddensiek, 1995; Degerman, et al., 2006; Skinner, et al., 2003)
Freshwater pearl mussels are dioecious, meaning that females and males exist in the species. Both sexes can also change into hermaphrodites, although males rarely do. Hermaphrodites frequently can be found in low-density populations, where there are not enough males to fertilize all of the females. (Bauer, 1987)
In early summer months, cued by rising temperatures, males release sperm into the water column, where it is ingested by females carrying eggs. There is a drawback to this system; since sperm is swept away by the current, not all females may become fertilized in a freshwater pearl mussel colony. In addition, populations with low numbers of males may not be able to produce enough sperm to fertilize most of the females. Because of this, mussels can undergo asexual reproduction and self-fertilize as hermaphrodites. For example, females who are positioned far upstream of males will not become fertilized, and thus may become temporary hermaphrodites. Since hermaphrodites form due to the inability of a male to fertilize a female, it is very unlikely for any males to become hermaphrodites. (Bauer, 1987)
Sexual maturity varies with growth rate of the mussel, but is typically reached in twenty years. The faster the mussel can develop, the sooner it will become sexually mature. Once they are sexually mature, freshwater pearl mussels release sperm or produce eggs every year until they die, although only two out of every three females in a typical population will be fertile each year. (Bauer, 1987; Bauer, 1987)
Interestingly, fecundity does not appear to be affected by pollution, population density of mussels, or water discharge, although those factors will certainly affect the survival of the offspring. (Bauer, 1987)
Fertilized eggs spend roughly four weeks developing inside the female mussel before they are released to the open water. (Degerman, et al., 2006)
Freshwater pearl mussels are a long-lived species. The average lifespan ranges from 86 to 102 years, although lifespan varies greatly with environmental factors such as water quality. The oldest freshwater pearl mussel was found to be 280 years old. However, few freshwater pearl mussels survive their first year; most are either washed away by the current as glochidia or die due to unsuitable living conditions as juveniles. (Bauer, 1987; Degerman, et al., 2006; Skinner, et al., 2003)
Being sedentary animals, freshwater pearl mussels tend to stay in one place and filter food from the water column. Most partially bury themselves in the sand and leave about a third of their shell exposed, although some completely bury themselves. If they need to relocate, freshwater pearl mussels can use their foot organ to slowly move across sandy sediments. This foot organ is also the primary means by which adult mussels anchor themselves to stream beds. Juveniles, which are considerably smaller than adults and therefore at greater risk of being swept away by the current, can also anchor themselves to rocks using a threadlike organ called a byssus thread. (Degerman, et al., 2006; Skinner, et al., 2003)
The home range size of freshwater pearl mussels is unknown, although they typically move little once established at a site.
Freshwater pearl mussels filter small organic particles from the water column. It is not known exactly what they eat, but the organism's diet likely consists of fungal spores, bacteria, tiny phyloplankton and zooplankton, and other very small filterable particles. (Degerman, et al., 2006; Skinner, et al., 2003)
Algal blooms can interfere with a freshwater pearl mussel's ability to filter food from the water. Populations of zebra mussels and other bivalves can also lead to competition for food, as both groups filter for similar food material. (Degerman, et al., 2006)
Juvenile freshwater pearl mussels are eaten by crayfish, eels, and North American muskrats (Ondatra zibethicus). Adult freshwater pearl mussels do not appear to have any predators. Their thicker shells and larger size protect them from organisms that typically eat juveniles. (Degerman, et al., 2006)
It has been suggested that freshwater pearl mussels help to keep water clean, although their environmental impact on water quality is not fully understood. However, an average sized adult freshwater pearl mussel can filter 50 L of water each day, and they also excrete waste products that are broken down by detritus-eating organisms. Those waste products contain important nutrients for plant life. Fish in general, particularly fish in the genera Oncorhynchus and Salmo are important hosts for these mussels. This commensal relationship does not harm the fish hosts. (Degerman, et al., 2006; Skinner, et al., 2003)
As the name might suggest, freshwater pearl mussels were commercially important in the pearl industry before they achieved endangered status. Illegal harvest of pearls from populations of freshwater pearl mussels continue today, making it necessary for wildlife agencies to keep secret the exact locations of freshwater pearl mussel populations. (Skinner, et al., 2003)
There are no adverse effects of freshwater pearl mussels on humans.
Freshwater pearl mussels are listed as an endangered species and conservation efforts have primarily been directed towards increasing juvenile populations. Many populations consist of only adults because environmental conditions are too harsh for juveniles. Eutrophication, where chemicals added into the river cause algal blooms, are particularly devastating as they interfere with the freshwater pearl mussel's ability to obtain food, reproduce, and obtain oxygen from the water. (Moorkens, 2000; Skinner, et al., 2003)
An attempt to conserve freshwater pearl mussels must include an effort to restore populations of host fish. In some mussel populations, a lack of host fish for glochidia to attach to has led to decreased numbers of juveniles. Causes for the lack of host fishes vary depending on the needs of the fish species; common reasons include acidification of lakes and streams and the introduction of invasive host species that out-compete native populations.
Humans have also impacted populations of freshwater pearl mussels. Over-harvesting of mussels for pearls has diminished many populations. This practice has been outlawed in an effort to protect surviving populations, but it continues illegally today. (Skinner, et al., 2003)
Freshwater pearl mussels have seasonal growth bands on their shells. Differences in size between bands can be used as an indicator of the availability of food and the quality of the water in past years.
Daniel Mitchell (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Tanya Dewey (editor), University of Michigan-Ann Arbor.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
animals that grow in groups of the same species, often refers to animals which are not mobile, such as corals.
particles of organic material from dead and decomposing organisms. Detritus is the result of the activity of decomposers (organisms that decompose organic material).
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
mainly lives in water that is not salty.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
fertilization takes place within the female's body
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)
an animal that mainly eats plankton
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)
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Hastie, L., P. Boon, M. Young. 2000. Physical microhabitat requirements of freshwater pearl mussels, Margaritifera margaritifera (L.). Hydrobiologia, 429: 59-71. Accessed February 09, 2010 at http://www.springerlink.com/content/h3242rxv00778524/fulltext.pdf?page=1.
Moorkens, E. 2000. Conservation management of the freshwater pearl mussel Margaritifera margaritifera. Irish Wildlife Manuals, 9: 1-44. Accessed February 03, 2010 at http://www.npws.ie/en/media/Media,4583,en.pdf.
Skinner, A., M. Young, L. Hastie. 2003. Ecology of the Freshwater Pearl Mussel. Conserving Natura 2000 Rivers, 2: 1-20. Accessed February 03, 2010 at http://www.english-nature.org.uk/lifeinukrivers/publications/mussel.pdf.