Freshwater Pearl Mussel

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.

Known Predators:

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

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.

Range length: 17.6 (high) cm.

Average length: 11.5 cm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: sexes alike

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.

Range lifespan
Status: wild: 280 (high) years.

Average lifespan
Status: wild: 93 years.

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.

Range depth: 0.3 to 0.4 m.

Habitat Regions: temperate ; freshwater

Aquatic Biomes: benthic ; rivers and streams

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.

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.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: endangered

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). 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.

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 ; indeterminate growth

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

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

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.

Species Used as Host:

• trout and salmon (Oncorhynchus)
• trout and salmon (Salmo)

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.

Animal Foods: zooplankton

Plant Foods: phytoplankton

Other Foods: fungus; detritus ; microbes

Foraging Behavior: filter-feeding

Primary Diet: planktivore

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 (Introduced ); palearctic (Native )

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)

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.

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.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); simultaneous hermaphrodite; sexual ; asexual ; fertilization (Internal ); broadcast (group) spawning; 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)

Capable of living for up to an incredible 200 years old (7), the freshwater pearl mussel begins life as a tiny larva, measuring just 0.6 to 0.7 millimetres long, which is ejected into the water from an adult mussel in a mass of one to four million other larvae. This remarkable event takes place over just one to two days, sometime between July and September (2). The larvae, known as glochidia, resemble tiny mussels, but their minute shells are held open until they snap shut on a suitable host. The host of freshwater pearl mussel larvae are fish from the salmonid family, which includes the Atlantic salmon and sea trout (2). The chances of a larva encountering a suitable fish is very low (6), and thus nearly all are swept away and die; only a few are inhaled by an Atlantic salmon or sea trout, where they snap shut onto the fish's gills (2). Attached to the gills of a fish, the glochidia live and grow in this oxygen-rich environment until the following May or June, when they drop off. The juvenile must land on clean gravely or sandy substrates if it is to successfully grow (2). Attached to the substrate, juvenile freshwater pearl mussels typically burrow themselves completely into the sand or gravel, while adults are generally found with a third of their shell exposed (2). Should they become dislodged, freshwater pearl mussels can rebury themselves, and are also capable of moving slowly across sandy sediments, using their large, muscular foot (2). The freshwater pearl mussel grows extremely slowly (6), inhaling water through exposed siphons, and filtering out tiny organic particles on which it feeds (2). It is thought that in areas where this species was once abundant, this filter feeding acted to clarify the water, benefiting other species which inhabited the rivers and streams (2). Maturity is reached at an age of 10 to 15 years (2), followed by a reproductive period of over 75 years in which about 200 million larvae can be produced (6). In early summer each year, around June and July, male freshwater pearl mussels release sperm into the water, where it is inhaled by female mussels. Inside the female, the fertilised eggs develop in a pouch on the gills for several weeks, until temperature or other environmental cue triggers the female to release the larvae into the surrounding water (2).

The freshwater pearl mussel, which is completely protected in most European countries (9), has been the focus of a significant amount of conservation efforts (8). Measures have included the transfer of adult mussels to areas where it had gone extinct (8) (10), the culture of juvenile mussels, and the release of juvenile trout, which have been infected with glochidia, into small rivers (10). The freshwater pearl mussel has also benefited from habitat restoration projects in some areas (10). Due to the essential role salmonid fish play in the life of the freshwater pearl mussel, the conservation of salmon and trout is also central in the survival of this endangered freshwater mussel (4).

One of the longest-living invertebrates in existence (2), the freshwater pearl mussel has, like all bivalve molluscs, a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within (3). The shell is large, heavy and elongated (4) (5), typically yellowish-brown in colour when young and becoming darker with age (2). Older parts of the shell often appear corroded, an identifying feature of this mussel species (6). The inner surface of the shell is pearl white, sometimes tinged with attractive iridescent colours (4). Like all molluscs, the freshwater pearl mussel has a muscular 'foot' (3); this very large, white foot enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat (4) (5).

Clean, fast-flowing streams and rivers are required for the freshwater pearl mussel (4) (2), where it lives buried or partly buried in fine gravel and coarse sand (2), generally at depths between 0.5 and 2 metres, but sometimes at greater depths (4). Clean gravel and sand is essential, particularly for juvenile freshwater pearl mussels, as if the stream or river bottom becomes clogged with silt, they can not obtain oxygen and will die (4). Also essential is the presence of a healthy population of salmonids, a group of fish including salmon and trout, on which the freshwater pearl mussel relies for part of its life cycle (4).

The freshwater pearl mussel can be found on both sides of the Atlantic (4), from the Arctic and temperate regions of western Russia, through Europe to north-eastern North America (2).

Classified as Endangered (EN) on the IUCN Red List. Subspecies: Margaritifera margaritifera durrovensis is classified as Critically Endangered (CR) on the IUCN Red List (1).

Once the most abundant bivalve mollusc in ancient rivers around the world, numbers of the freshwater pearl mussel are now declining in all countries and this species is nearly extinct in many areas (4). The causes of this decline are not fully understood, but alteration and degradation of its freshwater habitat undoubtedly plays a central role (4). The negative impacts humans have on rivers and streams come from a wide range of activities, including, but not restricted to, river regulation, drainage, sewage disposal, dredging, and water pollution (4). Anything that affects the abundance of the fish hosts will also affect the freshwater pearl mussel; for example, the introduction of exotic fish species, such as the rainbow trout, reduces the number of native fish hosts (4). Introduced species are also directly affecting the freshwater pearl mussel; the invasion of the zebra mussel (Dreissena polymorpha), which has been spread to new locations by being transported on the bottom of boats or in ballast waters, has impacted freshwater pearl mussel populations in all countries it has invaded (4).

The Nore pearl mussel (Margaritifera durrovensis) is a critically endangered species of freshwater pearl mussel, an aquatic bivalve mollusc in the family Margaritiferidae.

The species is endemic to Ireland and was first identified by R.A. Phillips in 1926, who later declared it a new species in Volume 18 of the Proceedings of the Malacological Society. This designation was controversial, and the taxonomic status of the Nore pearl mussel remains inconclusive. It is often described as a rare ecophenotype of M. margaritifera. The European Union's Habitats Directive on the conservation of natural habitats and wild fauna placed Margaritifera durrovensis on Annex II and Annex V as a separate taxon.