Freshwater bivalves are

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s of the order

Bivalvia Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

that inhabit freshwater ecosystems. They are one of the two main groups of

freshwater mollusc Freshwater molluscs are those members of the phylum Mollusca which live in freshwater habitats, both lotic (flowing water) such as rivers, streams, canals, springs, and cave streams ( stygobite species) and lentic (still water) such as lakes, ...

s, along with

freshwater snail Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs t ...

s. The majority of bivalve molluscs are saltwater

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

that live in the marine habitats, but a number of families have evolved to live in

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

(and in some cases, also in

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...

). These belong to two different evolutionary lineages, i.e. freshwater

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

s and freshwater

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

s, and the two groups are not closely related. Freshwater bivalves have a simple morphology that varies among

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

, and are distributed around most regions of the world. Freshwater bivalve species vary greatly in size. Some pea clams (

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

'' Pisidium'') have an adult size of only . In contrast, one of the largest species of freshwater bivalves is the swan mussel from the family

Unionidae The Unionidae are a Family (biology), family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is a ...

; it can grow to a length of , and usually lives in lakes or slow-flowing rivers. Freshwater pearl mussels are economically important as a source of

pearl A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle (mollusc), mantle) of a living Exoskeleton, shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pear ...

s and mother of pearl. While some species are short-lived, others can be quite long-lived, with some species registering longevity in the hundreds of years. Freshwater bivalves can thrive in many different types of

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

s, ranging from small

ditch A ditch is a small to moderate trench created to channel water. A ditch can be used for drainage, to drain water from low-lying areas, alongside roadways or fields, or to channel water from a more distant source for plant irrigation. Ditches ...

es and

pond A pond is a small, still, land-based body of water formed by pooling inside a depression (geology), depression, either naturally or artificiality, artificially. A pond is smaller than a lake and there are no official criteria distinguishing ...

s to large

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

canal Canals or artificial waterways are waterways or engineered channels built for drainage management (e.g. flood control and irrigation) or for conveyancing water transport vehicles (e.g. water taxi). They carry free, calm surface ...

s and

wetland A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor ( anoxic) processes taking place, especially ...

s. The ecology of freshwater bivalves varies among species with regards to differences in

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

and

predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

. In spite of their variety of ecosystems, freshwater bivalves are some of the most

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...

species on the planet. In

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, for instance, many freshwater mussel species have gone

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

, and of those remaining, 65 percent are rated as endangered, threatened or vulnerable.

Drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...

s, forest clearing,

farming Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...

, use of dams for water management, and changes in water temperature can all pose threats to freshwater bivalve populations. Restoration efforts focus on rebuilding lost mussel populations in the wild and using those mussels to improve and protect water quality and restore broader ecosystems.

Morphology

External

Freshwater bivalves, as their name implies, have a protective

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

composed of two half-

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

s or "

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

s", connected via a soft ligament along a hinge. These two valves are non-living, composed of both organic and inorganic substances that make up three major valve layers. The first, outermost layer is the thin epidermis ( periostracum), followed by a second prismatic layer, containing

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

. The third and innermost layer is also the thickest and is most commonly referred to as the mother of pearl—a widely harvested source for the production of ornamental buttons. The external appearances of these shells can be extremely variable when comparing members of different families, genus, etc., as well as intraspecies. Valve surface appearances can range from smooth to dramatically sculpted, showcasing ornamental pustules, pimples, grooves, and ridges. The overall shape of the valve can also vary drastically, from laterally compressed and narrow, to wide and globular.

Internal

The mantle is a multifunctional, generally thin and fragile structure that line bivalve interiors and encloses their bodies. This structure secretes the shell, contains respiratory organs and facilitates feeding. The cavity that exists between the mantle and other

soft tissue Soft tissue connective tissue, connects and surrounds or supports internal organs and bones, and includes muscle, tendons, ligaments, Adipose tissue, fat, fibrous tissue, Lymphatic vessel, lymph and blood vessels, fasciae, and synovial membranes.� ...

s is aptly named the mantle cavity. Within the mantle cavity on either side of the foot are the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s. The inner gills are adjacent to the foot and the outer gills nearest to the mantle and shell. As expected, these gills mainly act as a respiratory structure, performing

gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a b ...

but can also facilitate

filter feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a spe ...

. Water enters the mantle cavity via an incurrent

siphon A siphon (; also spelled syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, abo ...

, and passes over the gills where food particles are trapped by secreted

mucus Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both Serous fluid, serous and muc ...

. Anterior and posterior adductor muscles connect the left and right valves, facilitating shell opening and closure. The less major anterior and posterior retractor muscles extend from the shell and attach the body to a structure called the foot. This muscular foot is typical of most bivalves, extending anteriorly between the valves (via an anterior protractor muscle) and aiding in locomotion, burrowing and anchorage ( holdfast).

Distribution

The exact distribution of every freshwater bivalve

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

cannot be ascertained because of the lack of data in certain areas of the world like Africa and South America, but freshwater bivalves have been found in all of Earth's

biogeographic realm A biogeographic realm is the broadest biogeography, biogeographic division of Earth's land surface, based on distributional patterns of terrestrial animal, terrestrial organisms. They are subdivided into bioregions, which are further subdivid ...

s, except for the Antarctic biogeographic realm. There are 40 freshwater bivalve representative genera in the Palearctic biogeographic realm, 59 in the Nearctic, 23 in the Afrotropical, 51 in the Neotropical, 47 in the Oriental, 13 in the Australasian, and two in the Pacific Ocean Islands, for a total of 206 freshwater bivalve genera currently identified in the world. Freshwater bivalves are native to the Oriental biogeographic realm and to the southeastern United States and have been introduced to other regions, specifically the two genera in the Pacific Ocean Islands were introduced from Hawaii. As new methods of identifying and locating freshwater bivalves improve, the distribution of where these freshwater bivalves occur can become more apparent.

Taxonomy

The study of freshwater bivalves predates Aristotle and has since been in a state of constant flux and dispute, regarding their identification and classification. As time has progressed, so too have different techniques and technologies that allow scientists to more comprehensively study the assemblages of organisms in the natural world, freshwater bivalves included. Modern genetic analysis has had major applications in the modern history of taxonomy and has been since utilized in the advancement of freshwater bivalve classification by allowing researchers to identify commonly used genes within these groups. The most commonly used method of identification/classification utilizes an exceptionally diverse set of ever-expanding morphological features, ranging from shell anatomy, variations in internal soft tissue, degrees of mantle fusion, to larval stage development.

Orders and families

* Arcoida (order) ** Arcidae (family) * Mytiloida **

Mytilidae The Mytilidae are a family (biology), family of small to large Marine life, marine and Brackish water, brackish-water bivalve molluscs in the order (biology), order Mytilida. One of the genera, ''Limnoperna fortunei, Limnoperna'', even inhabits f ...

* Unionida ** Etheriidae ** Hyriidae ** Iridinidae **

Margaritiferidae Margaritiferidae is a family of medium-sized freshwater mussels, aquatic bivalve molluscs in the order Unionida. Text may have been copied from this source, which is available under Attribution 4.0 International (CC BY 4.0)licence. It is the ...

** Mycetopodidae **

* Veneroida ** Cardiidae ** Corbiculidae ** Sphaeriidae ** Dreissenidae ** Solenidae ** Donacidae ** Navavulidae * Myoida ** Corbulidae ** Erodonidae ** Teridinidae * Anomalodemata ** Lyonsiidae

Unionida

The Unionida, of worldwide distribution, are the pearly freshwater mussels. All reproduce by means of a larval stage that is parasitic on a fish or salamander. Many species are utilized as sources of mother-of-pearl. Families: *

* Hyriidae * Etheriidae * Mutelidae * Mycetopodidae * Iridinidae File:Margaritifera auricularia shell.jpg, Family

, a shell of '' Margaritifera auricularia'' File:Quadrula metanevra.jpg, Family

, '' Quadrula metanevra'', the monkeyface mussel File:Anodonta cygnea1.jpg, Family

, '' Anodonta cygnea''

Veneroida

The Veneroida is a large group of bivalve "clams", most of which are marine. However, several families occur in fresh and brackish waters. Families: * Corbiculidae * Sphaeriidae * Dreissenidae File:Toegeknepen korfmossel.jpg, Family Corbiculidae, shell of '' Corbicula fluminea'' File:Sphaerium_corneum.jpg, Family Sphaeriidae, '' Sphaerium corneum'', one of the small fingernail clams. File:Dreissena polymorpha1.jpg, Family Dreissenidae, '' Dreissena polymorpha''

Reproduction

Mechanisms

Freshwater Bivalves can utilize either ovoviviparous or viviparous reproduction strategies, ovoviviparous meaning that

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

s develop and grow in eggs inside the female until they are ready to be released. The embryos get nutrition from egg yolk, but are not connected to the mother by a placental connection. Viviparous meaning embryos develop inside the body of a female and usually gain nutrition by a placental connection. Females tend to have a single reproductive spawning period (when the ova moves to the gills) while males tend to have two spawning periods (a release of sperm into the water column). Females have highest ova, or mature female reproductive cells, during September with a gradual decrease until December. Males spawn between September and December with a second spawning period between May and July. Males release their sperm into the water column for females to accept. Females take up the sperm along with water through their circulatory system and have the potential to become fertilized when the sperm meets the ova. In contrast to Marine Bivalves, most female freshwater bivalve species hold the fertilized embryos until they develop into

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e when they are released into the water.

Larvae

Once larvae are released into the water column they become semi-parasitic and attach to the

s of freshwater fish. They attach to a host where they grow into juvenile adults while doing little to no damage to the fish host. The order Unionidae have an obligate parasitic larval stage where the larvae are attached to the gills, fins or the body of a particular host fish. Microbial water composition and sediment composition are important in larval nutrition.

Predation

Invasive species An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...

pose a risk to freshwater bivalve populations. Specifically two invasive species of crayfish, ''Procambarus clarkii'' and ''Pacifastacus leniusculus,'' are predators of the freshwater bivalve species ''Anodonta anatina''. In general, freshwater bivalves have predators such as

raccoon The raccoon ( or , ''Procyon lotor''), sometimes called the North American, northern or common raccoon (also spelled racoon) to distinguish it from Procyonina, other species of raccoon, is a mammal native to North America. It is the largest ...

otter Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine. Lutrinae is a branch of the Mustelidae family, which includes weasels, badgers, mink, and wolverines, among ...

s, some species of fish, and some species of turtles.

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

s, and

heat wave A heat wave or heatwave, sometimes described as extreme heat, is a period of abnormally hot weather generally considered to be at least ''five consecutive days''. A heat wave is usually measured relative to the usual climate in the area and ...

s are a few examples of major climatic events that are happening more frequently because of the global changing climate. This is a huge killer of freshwater bivalve populations.

Ecosystem function

Freshwater bivalves are important bioindicators of freshwater ecosystems because they are the connection between the water column and the benthos, for they can provide information on the quality of the water based on the particles and toxins that bioaccumulate in the tissue of bivalves. Freshwater bivalves are filter feeders and provide an ecological service by improving

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

in the bodies of water they inhabit, such as

s, and

s. Water quality is improved by filtering out fine particles of

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

organic matter Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...

, and heavy metals as well as

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

in the water column to reduce turbidity. Freshwater bivalves are also important in the process of

nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...

because they deposit organic matter in the sediment through biodeposition created from the fine particles they filter in. Organic matter can be deposited in the sediment as

feces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

or dead matter, and depending on if the right environmental conditions are present, such as hypoxia, sediment

denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...

can occur, releasing

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

back into the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

. However, other organisms are unable to utilize this

inorganic An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemistry''. Inor ...

form of nitrogen, so freshwater bivalves can also excrete dissolved nutrients in an accessible form for primary producers and

consumers A consumer is a person or a group who intends to order, or use purchased goods, products, or services primarily for personal, social, family, household and similar needs, who is not directly related to entrepreneurial or business activities. Th ...

to assimilate. Any nutrients that were retained by the freshwater bivalve through its lifespan for building shell tissue can serve as a long-term nutrient storage in the benthos when the organism dies, depending on water chemistry and flow conditions. Considering freshwater bivalves can filter particles and process nutrients in the nutrient cycle, there are other species of freshwater bivalves that have more specialized ecosystem functions as well as different vulnerabilities.

Threats to bivalves

Invasive species

Dreissenidae are a family of freshwater mollusks considered to be an invasive species found across Eurasia and North America. The most common types of dreissenids are ''Dreissena polymorpha'' (Zebra mussel) and ''Dreissena rostriformis'' (Quagga mussel). These mussels damage both ecological systems and human infrastructure. In North America, biofouling caused by dreissenids created 267 million dollars’ worth of damage between 1989 and 2004. When introduced to freshwater ecosystems, dreissenids lead to a decline in indigenous marine animal populations and are also known for causing benthic algae and cyanobacterial blooms. The total impact of dreissenids on freshwater ecosystems is still unknown.

Anthropogenic impacts

Pollution, human disturbance, invasive species, and ecosystem modification are the main threats to freshwater bivalves. In North America freshwater bivalves are extremely threatened, with 202 of 300 species considered critical, possibly extinct, or extinct. Of the dangers facing freshwater bivalves, eighty-five percent of them are considered to be “ongoing threats”. Ecosystem modification and pollution are currently the two biggest threats to molluscs and gastropods in Palearctic and Nearctic ecozones. Pollution is the dominant issue for these animals in the Afrotropic and Indomalayan

s. For the Neotropics and Australasia biogeographic realms, ecosystem modification has the largest impact on freshwater bivalve species. Hydropower plants and dams are two examples of human ecosystem modification which contributes to loss of habitat as well as changes to channel morphology, river and floodplain connectivity and nutrient limitation. Rates of extinction among freshwater bivalves are higher than those of terrestrial groups which share the same ecosystem. Among those bivalves, freshwater gastropods are the most highly threatened due to smaller species distribution. Freshwater bivalves are at a heightened risk for endangerment and extinction because of the connectivity of river systems. Anthropogenic impacts on rivers spread throughout the whole ecosystem.

References

{{reflist

External links

Mollusk Collection
at the

University of Illinois The University of Illinois Urbana-Champaign (UIUC, U of I, Illinois, or University of Illinois) is a public university, public land-grant university, land-grant research university in the Champaign–Urbana metropolitan area, Illinois, United ...

Malacology
at the Academy of Natural Sciences of Drexel University
The Freshwater Mussels (Unionoida) of the World (and other less consequential bivalves)
from The Mussel Project
Rhode Island freshwater clams and mussels
by Michael Rice, Rhode Island Sea Grant Fact Sheet