Margaritifera margaritifera

Geographic Range

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.

• Biogeographic Regions
• nearctic nearctic
  • introduced introduced
• palearctic palearctic
  • native native

Habitat

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.

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.

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• rivers and streams

Physical Description

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.

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.

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

• Sexual Dimorphism
• sexes alike

Development

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.

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 phoxinus ). Hosts in the United States are primarily salmon and trout species, including coho salmon ( Oncorynchus kisutch ), rainbow trout ( Oncorhynchus mykiss ), cuttthroat trout ( Oncorhynchus clarki ), chinook salmon ( Oncorhynchus tshawytscha ), and sockeye salmon ( Oncorhynchus nerka ). Brook trout ( Salvelinus fontinalis ) can also serve as hosts.

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.

• Development - Life Cycle
• colonial growth colonial growth
• indeterminate growth indeterminate growth

Reproduction

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.

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

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.

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.

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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sexual sexual
• asexual asexual
• fertilization fertilization
  • internal internal
• broadcast (group) spawning
• oviparous oviparous

Fertilized eggs spend roughly four weeks developing inside the female mussel before they are released to the open water.

• Parental Investment
• no parental involvement
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• motile motile
• sedentary sedentary
• solitary solitary

Home Range

The home range size of freshwater pearl mussels is unknown, although they typically move little once established at a site.

Food Habits

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.

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.

• Primary Diet
• planktivore planktivore

• Animal Foods
• zooplankton zooplankton

• Plant Foods
• phytoplankton phytoplankton

• Other Foods
• fungus
• detritus detritus
• microbes

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

• Positive Impacts
• body parts are source of valuable material

Economic Importance for Humans: Negative

There are no adverse effects of freshwater pearl mussels on humans.

Conservation Status

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.

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.

Other Comments

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.

The freshwater pearl mussel family, Margaritiferidae is believed, at 200 million years old, to be the oldest large freshwater mussel family in the world.

Additional Links

Encyclopedia of Life

Contributors

Daniel Mitchell (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Bauer, G. 1987. Reproductive Strategy of the Freshwater Pearl Mussel Margaritifera margaritifera . Journal of Animal Ecology , 56/2: 691-704. Accessed February 05, 2010 at http://www.jstor.org/stable/5077?seq=1 .

Buddensiek, V. 1995. The culture of juvenile freshwater pearl mussels Margaritifera margaritifera L. in cages. Biological Conservation , 74: 33-40. Accessed February 09, 2010 at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5X-4002DR0-R&_user=1086025&_coverDate=12%2F31%2F1995&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1200849860&_rerunOrigin=google&_acct=C000051441&_version=1&_urlVersion=0&_userid=1086025&md5=1495d4316e2528bd4b106756d08c6038 .

Degerman, E., S. Alexanderson, J. Bergengren, L. Henrikson, B. Johansson, B. Larsen, H. Soderberg. 2006. "Restoration of Freshwater Pearl Mussel Streams" (On-line pdf). Accessed February 09, 2010 at http://www.wwf.se/source.php/1257735/Restoration%20of%20FPM%20streams.pdf .

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 .

NatureServe. 2009. "NatureServe Explorer: An online encyclopedia of life" (On-line). Accessed February 07, 2010 at http://www.natureserve.org/explorer/servlet/NatureServe?searchName=Margaritifera+margaritifera .