Animal Diversity Web

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. ("Na­ture­Serve Ex­plorer: An on­line en­cy­clo­pe­dia of life", 2009; Skin­ner, et al., 2003)

• Biogeographic Regions
• nearctic
  • introduced
• palearctic
  • 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. (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)

• Habitat Regions
• temperate
• freshwater

• Aquatic Biomes
• benthic
• rivers and streams

• Range depth

  0.3 to 0.4 m

  0.98 to 1.31 ft

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. (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)

• Other Physical Features
• ectothermic
• heterothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes alike

• Range length

  17.6 (high) cm

  6.93 (high) in

• Average length

  11.5 cm

  4.53 in

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. (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)

• Development - Life Cycle
• colonial 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. (Bauer, 1987)

• Mating System
• 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. (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)

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

• Breeding interval

  Sexually mature freshwater pearl mussels breed once every year until their death.

• Breeding season

  Breeding occurs from June to July.

• Range number of offspring

  1 to 4000000

• Range gestation period

  1 to 3 months

• Range age at sexual or reproductive maturity (female)

  10 to 20 years

• Average age at sexual or reproductive maturity (female)

  20 years

• Range age at sexual or reproductive maturity (male)

  10 to 20 years

• Average age at sexual or reproductive maturity (male)

  20 years

Fer­til­ized eggs spend roughly four weeks de­vel­op­ing in­side the fe­male mus­sel be­fore they are re­leased to the open water. (Degerman, et al., 2006)

• 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. (Bauer, 1987; Degerman, et al., 2006; Skin­ner, et al., 2003)

• Range lifespan
  Status: wild

  280 (high) years

• Average lifespan
  Status: wild

  93 years

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. (Degerman, et al., 2006; Skin­ner, et al., 2003)

• Key Behaviors
• motile
• sedentary
• 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. (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)

• Primary Diet
• planktivore

• Animal Foods
• zooplankton

• Plant Foods
• phytoplankton

• Other Foods
• fungus
• detritus
• microbes

• Foraging Behavior
• 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. (Degerman, et al., 2006)

• Known Predators

  • North American muskrats (Ondatra zibethicus)
  • freshwater eels (Anguillidae)
  • crayfish (Astacoidea)

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. (Degerman, et al., 2006; Skin­ner, et al., 2003)

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. (Skin­ner, et al., 2003)

• 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. (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)

• IUCN Red List

  Endangered
  More information

• IUCN Red List

  Endangered
  More information

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  No special status

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. (Degerman, et al., 2006)

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

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.

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.