Freshwater

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

s are one kind of freshwater mollusc, along with

freshwater snail Freshwater snails are gastropod mollusks which 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 ...

s. They are bivalves that live in fresh water as opposed to salt water, which is the main habitat type for bivalves. The majority of

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

of bivalve molluscs live in the sea, but in addition, a number of different families live in fresh water (and in some cases, also in brackish water). These families belong to two different evolutionary lineages (freshwater mussels and freshwater clams), and the two groups are not closely related. Freshwater bivalves have a simple morphology that varies among taxa, and are distributed around most regions of the world. Species in the two groups vary greatly in size. Some pea clams (''

Pisidium ''Pisidium'' is a genus of very small or minute freshwater clams known as pill clams or pea clams, aquatic bivalve molluscs in the family Sphaeriidae, the pea clams and fingernail clams. In some bivalve classification systems, the family ...

'' species) have an adult size of only 3 mm. In contrast, one of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae; it can grow to a length of 20 cm, and usually lives in lakes or slow rivers. Freshwater pearl mussels are economically important as a source of freshwater

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

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 live in many types of habitat, ranging from small ditches and ponds to lakes, canals, rivers, and swamps. The ecology of freshwater bivalves varies among species with regards to differences in reproduction and predation. In spite of their variety of

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

s, freshwater bivalves are some of the most endangered species on the planet. In North America, many freshwater mussel species have gone extinct, and of those remaining, 65 percent are rated as endangered, threatened or vulnerable. Droughts, forest clearing, farming, some uses of dams for water management, and changes in water temperature can all pose threats. 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 Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

, as their name implies, are composed of two halves, or a left and right valve, 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 The periostracum ( ) is a thin, organic coating (or "skin") that is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs, it is primarily seen in snails and clams, i.e. in gastropods ...

), followed by a second prismatic layer, containing calcium carbonate. 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 (facilitates respiration), and facilitates feeding. The cavity that exists between the mantle and other soft tissues is aptly named the mantle cavity. Within the mantle cavity on either side of the foot are the gills. 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, facilitating gas exchange but can also facilitate in feeding. Water can enter the mantle cavity via an incurrent

siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically 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 a ...

, and pass over the gills where food and other particles are trapped by secreted mucus. 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, digging and anchoring (holdfast).

Distribution

The exact distribution of every freshwater bivalve

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

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 or ecozone is the broadest biogeographic division of Earth's land surface, based on distributional patterns of terrestrial organisms. They are subdivided into bioregions, which are further subdivided into ecoregions. ...

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 * Unionida ** Etheriidae **

Hyriidae Hyriidae is a taxonomic family of pearly freshwater mussels, aquatic bivalve molluscs in the order Unionida. This family is native to South America, Australia, New Zealand and New Guinea. Like all members of that order, they go through a larv ...

** 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 ** Unionidae * Veneroida ** Cardiidae ** Corbiculidae **

Sphaeriidae Sphaeriidae is a family of small to minute freshwater bivalve molluscs in the order Sphaeriida. In the US, they are commonly known as pea clams or fingernail clams. Heard, William H. 1977. Reproduction of fingernail clams (Sphaeriidae: ''Sphaer ...

** Dreissenidae ** Solenidae ** Donacidae ** Navavulidae * Myoida ** Corbulidae ** Erodonidae ** Teridinidae * Anomalodemata **

Lyonsiidae Lyonsiidae is a family of small saltwater clams, marine bivalve molluscs in the order Anomalodesmata. Description Members of this family have inequivalve, oblong valves that are nearly equilateral. The right valve is more convex than the left, ...

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: *

* Unionidae *

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

, a shell of '' Margaritifera auricularia'' File:Quadrula metanevra.jpg, Family Unionidae, '' Quadrula metanevra'', the monkeyface mussel File:Anodonta cygnea1.jpg, Family Unionidae, ''

Anodonta cygnea The swan mussel, ''Anodonta cygnea'', is a large species of freshwater mussel, an aquatic bivalve mollusc in the family Unionidae, the river mussels. Because of its morphological variability and its wide range of distribution, there are over ...

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 *

* Dreissenidae File:Toegeknepen korfmossel.jpg, Family Corbiculidae, shell of ''

Corbicula fluminea ''Corbicula fluminea'' is a species of freshwater clam native to eastern Asia which has become a successful invasive species throughout North America, South America, and Europe. ''Corbicula fluminea'' is commonly known in the west as the Asian cl ...

'' File:Sphaerium_corneum.jpg, Family

, '' 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 Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

reproduction strategies, ovoviviparous meaning that

embryo An embryo is an initial stage of development of 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 spe ...

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 A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

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 (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e when they are released into the water.

Larvae

Once larvae are released into the

they become semi-parasitic and attach to the

gill A gill () is a respiratory organ that many 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 respiration on land provided they ar ...

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 otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adv ...

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 raccoons,

otter Otters are carnivorous mammals in the subfamily Lutrinae. The 13 extant otter species are all semiaquatic, aquatic, or marine, with diets based on fish and invertebrates. Lutrinae is a branch of the Mustelidae family, which also includes we ...

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

Drought A drought is defined as 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, an ...

flood A flood is an overflow of water ( 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 an area of study of the discipline hydrol ...

s, and

heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual climate in the ...

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 Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.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

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

s, and

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...

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 with water. Silt usually has a floury feel ...

organic matter Organic matter, organic material, or natural organic matter refers to 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 c ...

, and heavy metals as well as

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

and

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

in the water column to reduce

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

. 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 cycli ...

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 ( or faeces), known colloquially and in slang as poo and poop, 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 large intestine. Feces contain a rela ...

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 denit ...

can occur, releasing

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

back into the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

. However, other organisms are unable to utilize this inorganic form of nitrogen, so freshwater bivalves can also excrete dissolved nutrients in an accessible form for primary producers and consumers 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 (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Uni ...

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