The Unionidae are a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

freshwater mussel Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. 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 of bivalve molluscs ...

s, the largest in the order

Unionida Unionida is a monophyletic order of freshwater mussels, aquatic bivalve molluscs.Bieler R., Carter J.G. & Coan E.V. (2010). ''Classification of Bivalve families''. pp. 113–133, in: Bouchet P. & Rocroi J.-P. (2010), ''Nomenclator of Bivalve Fa ...

, the

bivalve 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 ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

s sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is at its most diverse in North America, with about 297 recognised taxa, but China and Southeast Asia also support very diverse faunas. Freshwater mussels occupy a wide range of habitats, but most often occupy

lotic River ecosystems are flowing waters that drain the landscape, and include the Biotic component, biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its man ...

waters, i.e. flowing water such as rivers, streams and creeks.

Origin and early diversification

The recent phylogenetic study reveals that the Unionidae most likely originated in Southeast and East Asia in the Jurassic, with the earliest expansions into North America and Africa (since the mid-Cretaceous) followed by the colonization of Europe and India (since the Paleocene).

Life history

Unionidae burrow into the substrate, with their posterior margins exposed. They pump water through the incurrent

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

, obtaining

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

and food. They remove phytoplankton and zooplankton, as well as suspended bacteria, fungal spores, and dissolved organic matter.Silverman, H., Nichols S.J, Cherry J.S., Archberger E., Lynn J.S., Dietz T.H. (1997). "Clearance of laboratory-cultured bacteria by freshwater bivalves: differences between lentic and lotic unionids." Canadian Journal of Zoology 75: 1857-1866. Despite extensive laboratory studies, which of these filtrates unionoids actually process remains uncertain. In high densities, they have the ability to influence water clarity but filtration rates are dependent on water temperature, current velocity, and particle size and concentration. In addition, gill morphology can determine particle size filtered, as well as the rate.

Reproduction

Unionidae are distinguished by a unique and complex lifecycle. Most unionids are of separate sex, although some species, such as ''

Elliptio complanata The eastern elliptio (''Elliptio complanata'') is a freshwater mussel in the Unionidae family, native to Canada and the United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or Amer ...

'', are known to be

hermaphroditic In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have s ...

. The sperm is ejected from the mantle cavity through the male's excurrent aperture and taken into the female's mantle cavity through the incurrent aperture. Fertilised eggs move from the gonads to the gills ( marsupia) where they further ripen and metamorph into

glochidia The glochidium (plural glochidia) is a microscopic larval stage of some freshwater mussels, aquatic bivalve mollusks in the families Unionidae and Margaritiferidae, the river mussels and European freshwater pearl mussels. These larvae are t ...

, the first larval stage. Mature glochidia are released by the female and then 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 are ...

s, fins, or skin of a host

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

. A cyst is quickly formed around the glochidia, and they stay on the fish for several weeks or months before they fall off as juvenile

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

, which then bury themselves in the sediment. Some of the species in the Unionidae, commonly known as

pocketbook mussel ''Lampsilis ovata'', or pocketbook mussel, is a species of freshwater mussel, a bivalve mollusk in the family Unionidae, the river mussels. This species is endemic to eastern North America. This species uses aggressive mimicry to lure large pre ...

s, have evolved a remarkable reproductive strategy. The edge of the female's body that protrudes from the valves of the shell develops into an imitation of a small fish complete with markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a

conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...

, i.e. a member of their own species. Whatever they see, they approach for a closer look and the mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic young. These glochidial larvae are drawn into the fish's gills, where they attach and trigger a tissue response that forms a small

cyst A cyst is a closed sac, having a distinct envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac (like the manner in which ...

in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst. Sex is determined by a region located on the mitochondrial DNA, the male open-reading frame (M-ORF) and female open-reading frame (F-ORF). Hermaphroditic mussels lack these regions and contain a female-like open-reading frame dubbed hermaphroditic open-reading frame (H-ORF). In many mussels, the hermaphroditic state is ancestral and the male sex evolved later. This region of the mitochondria also may be responsible for the evolution of doubly uniparental inheritance seen in freshwater mussels.

Taxonomy

Genera by taxonomic order

The following classification is based on MolluscaBase and the MUSSEL Project database: * Subfamily

Ambleminae Ambleminae is a subfamily of freshwater mussel in the family Unionidae. They are found throughout much of eastern North America south to Central America, although fossils are also known from Siberia. Some species have also been introduced to East ...

** Tribe

Lampsilini Ambleminae is a subfamily of freshwater mussel in the family Unionidae. They are found throughout much of eastern North America south to Central America, although fossils are also known from Siberia. Some species have also been introduced to East ...

*** Genus ''

Epioblasma ''Epioblasma'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. Most of the species in this genus have been lost in modern times, and the entire genus is threatened with the possibility of ex ...

'' *** Genus ''

Lampsilis ''Lampsilis'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. There are over 100 species in the genus. Some species, notably '' Lampsilis ovata'' (pocketbook mussel) use aggressive mimicry ...

'' *** Genus ''

Potamilus ''Potamilus'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or s ...

'' *** Genus ''

Actinonaias ''Actinonaias'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or ...

'' *** Genus '' Cambarunio'' *** Genus '' Obovaria'' *** Genus ''

Toxolasma ''Toxolasma'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species within the genus ''Toxolasma''

* ''Toxolasma corvuncvulus'' (southern purple lilliput) * ''Toxolasma ...

'' *** Genus '' Medionidus'' *** Genus ''

Ptychobranchus ''Ptychobranchus'' is a genus of freshwater bivalve, freshwater mussels in the family Unionidae. Species of this genus package their larvae, known as Glochidium, glochidia, in conglutinates that resemble prey items such as Juvenile fish, fish fr ...

'' *** Genus ''

Disconaias ''Disconaias'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or ...

'' *** Genus '' Leaunio'' *** Genus '' Venustaconcha'' *** Genus ''

Cyrtonaias ''Cyrtonaias'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or ...

'' *** Genus '' Hamiota'' *** Genus '' Sagittunio'' *** Genus '' Truncilla'' *** Genus '' Villosa'' *** Genus '' Arotonaias'' *** Genus '' Cyprogenia'' *** Genus ''

Delphinonaias ''Delphinonaias'' is a genus of freshwater mussels, aquatic bivalve mollusks in the subfamily Ambleminae of the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs some ...

'' *** Genus '' Ortmanniana'' *** Genus '' Pachynaias'' *** Genus '' Atlanticoncha'' *** Genus '' Dromus'' *** Genus ''

Ellipsaria ''Ellipsaria lineolata'' is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. This is the sole species in the monotypic genus ''Ellipsaria ''. This species is native to the drainage systems o ...

'' *** Genus '' Friersonia'' *** Genus '' Glebula'' *** Genus ''

Lemiox The birdwing pearlymussel (''Lemiox rimosus'') is a rare species of freshwater mussel in the family Unionidae, the river mussels. This aquatic bivalve is native to Tennessee and Virginia in the United States. Its range has declined over 90%.

'' *** Genus ''

Ligumia ''Ligumia'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae. MolluscaBase eds. (2021). MolluscaBase. Ligumia Swainson, 1840. Accessed through: World Register of Marine Species at: http://www.mari ...

'' *** Genus ''

Obliquaria ''Obliquaria reflexa'', common name the threehorn wartyback or three-horn wartyback, is a species of freshwater mussel, an aquatic animal, aquatic bivalve mollusk in the family Unionidae, the river mussels. References

Unionidae Bivalves ...

'' *** Genus '' Paetulunio'' ** Tribe

Pleurobemini Ambleminae is a subfamily of Freshwater bivalve, freshwater mussel in the family Unionidae. They are found throughout much of eastern North America south to Central America, although fossils are also known from Siberia. Some species have also been ...

*** Genus ''

Elliptio ''Elliptio'' is a genus of medium- to large-sized freshwater mussels, aquatic bivalve mollusks in the family Unionidae, commonly known as the unionids, freshwater mussels or naiads. In contrast with many other groups of American Unionidae, th ...

'' *** Genus ''

Pleurobema ''Pleurobema'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species Species within the genus ''Pleurobema'' include: * '' Pleurobema altum'' (Highnut) * '' Pleurobema avellanum'' (Hazel ...

'' *** Genus ''

Fusconaia ''Fusconaia'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae. They are native to North America. Species within the genus ''Fusconaia'' Species include: * '' Fusconaia askewi'' * '' Fusconaia burkei'' (Tapere ...

'' *** Genus '' Plethobasus'' *** Genus '' Pleuronaia'' *** Genus '' Parvaspina'' *** Genus ''

Elliptoideus The purple bankclimber (''Elliptoideus sloatianus'') is a rare and endangered species of freshwater mussel, an aquatic animal, aquatic bivalve mollusk in the family Unionidae, the river mussels. This species is Endemism, endemic to the United St ...

'' *** Genus '' Eurynia'' *** Genus '' Hemistena'' ** Tribe

Popenaiadini Ambleminae is a subfamily of Freshwater bivalve, freshwater mussel in the family Unionidae. They are found throughout much of eastern North America south to Central America, although fossils are also known from Siberia. Some species have also been ...

*** Genus '' Nephronaias'' *** Genus ''

Psoronaias ''Psoronaias'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the subfamily Ambleminae of the family Unionidae, the river mussels.MolluscaBase eds. (2022). MolluscaBase. Psoronaias Crosse & P. Fischer, 1894. Accesse ...

'' *** Genus '' Barynaias'' *** Genus ''

Popenaias ''Popenaias'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve molluscs in the family Unionidae, the river mussels. Species Species within the genus ''Popenaias'' include: * ''Popenaias buckleyi'' * ''Popenaias metallica'' * ''P ...

'' *** Genus '' Sphenonaias'' *** Genus '' Micronaias'' *** Genus '' Nephritica'' *** Genus '' Reticulatus'' *** Genus '' Martensnaias'' ** Tribe Quadrulini *** Genus '' Cyclonaias'' *** Genus ''

Theliderma ''Theliderma'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae. They are native to North America. This genus is sometimes classified in an expanded ''Quadrula''. Species within the genus ''Theliderma'' Note: ...

'' *** Genus '' Uniomerus'' *** Genus ''

Quadrula ''Quadrula'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae native to rivers of the American Midwest and mid-south. All have thick nacreous shells with well-developed hinge teeth, many also with external sh ...

'' *** Genus '' Megalonaias'' *** Genus '' Tritogonia'' ** Tribe Amblemini *** Genus '' Amblema'' *** Genus '' Regianaia'' *** Genus '' Plectomerus'' * Subfamily Unioninae ** Tribe Anodontini *** Subtribe Alasmidontina **** Genus '' Alasmidonta'' **** Genus ''

Lasmigona ''Lasmigona'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or s ...

'' **** Genus '' Pyganodon'' **** Genus '' Utterbackiana'' **** Genus '' Strophitus'' **** Genus '' Utterbackia'' **** Genus '' Anodontoides'' **** Genus '' Arcidens'' **** Genus '' Pseudodontoideus'' **** Genus ''

Pegias ''Pegias'' is a monotypic genusUSFWS''Pegias fabula'' Recovery Plan.September 1989. of freshwater mussels in the family Unionidae. This genus contains the single species ''Pegias fabula'', known commonly as the littlewing pearlymussel. Distribut ...

'' **** Genus '' Simpsonaias'' *** Subtribe Cristariina **** Genus '' Sinanodonta'' **** Genus '' Buldowskia'' **** Genus '' Cristaria'' **** Genus '' Anemina'' **** Genus '' Beringiana'' **** Genus '' Pletholophus'' **** Genus '' Simpsonella'' **** Genus '' Amuranodonta'' *** Subtribe Anodontina **** Genus '' Anodonta'' **** Genus ''

Pseudanodonta ''Pseudanodonta'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. MolluscaBase eds. (2021). MolluscaBase. Pseudanodonta Bourguignat, 1877. Accessed through: World Register of Marine Speci ...

'' ** Tribe Unionini *** Genus '' Unio'' *** Genus '' Nodularia'' *** Genus '' Aculamprotula'' *** Genus '' Acuticosta'' *** Genus '' Cuneopsis'' *** Genus '' Inversiunio'' *** Genus '' Pseudobaphia'' *** Genus '' Rhombuniopsis'' *** Genus '' Lepidodesma'' *** Genus '' Pseudocuneopsis'' *** Genus '' Schistodesmus'' *** Genus '' Arcuneopsis'' *** Genus '' Diaurora'' *** Genus '' Middendorffinaia'' *** Genus '' Protunio'' ** Tribe Lanceolariini *** Genus '' Lanceolaria'' * Subfamily Gonideinae ** Tribe Pseudodontini *** Subtribe Pilsbryoconchina **** Genus '' Sundadontina'' **** Genus '' Monodontina'' **** Genus ''

Pilsbryoconcha ''Pilsbryoconcha'' is a genus of bivalves belonging to the family Unionidae. The species of this genus are found in Southeastern Asia. Species: *''Pilsbryoconcha carinifera'' *''Pilsbryoconcha compressa'' *''Pilsbryoconcha exilis'' *''Pilsbry ...

'' **** Genus '' Bineurus'' **** Genus '' Thaiconcha'' **** Genus '' Namkongnaia'' **** Genus '' Nyeinchanconcha'' *** Subtribe Pseudodontina **** Genus '' Pseudodon'' ** Tribe Contradentini *** Genus ''

Lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

'' *** Genus '' Yaukthwa'' *** Genus '' Physunio'' *** Genus '' Trapezoideus'' *** Genus '' Pressidens'' *** Genus ''

Solenaia ''Solenaia'' is a genus (biology), genus of freshwater mussels, aquatic animal, aquatic bivalve mollusks in the family Unionidae. Distribution Most of the species in this genus are endemic to the Yangtze River Basin of China. Additional species ...

'' ** Tribe Lamprotulini *** Genus '' Lamprotula'' *** Genus ''

Potomida ''Potomida'' is a genus of bivalves belonging to the subfamily Gonideinae of the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or s ...

'' *** Genus '' Schepmania'' *** Genus '' Discomya'' *** Genus '' Pronodularia'' ** Tribe Rectidentini *** Genus ''

Hyriopsis ''Hyriopsis'' is a genus of bivalves belonging to the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The rang ...

'' *** Genus '' Ensidens'' *** Genus '' Ctenodesma'' *** Genus '' Elongaria'' *** Genus '' Khairuloconcha'' *** Genus '' Prohyriopsis'' *** Genus '' Rectidens'' ** Tribe Gonideni *** Genus '' Ptychorhynchus'' *** Genus '' Sinosolenaia'' *** Genus '' Inversidens'' *** Genus '' Leguminaia'' *** Genus '' Parvasolenaia'' *** Genus ''

Gonidea ''Gonidea angulata'', the western ridged mussel or Rocky Mountain ridged mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. It is the only species in the genus ''Gonidea''. Descri ...

'' *** Genus '' Koreosolenaia'' *** Genus '' Microcondylaea'' *** Genus '' Obovalis'' *** Genus '' Pseudodontopsis'' ** Tribe Chamberlainini *** Genus '' Sinohyriopsis'' *** Genus '' Chamberlainia'' *** Genus '' Caudiculatus'' * Subfamily Parreysiinae ** Tribe Coelaturini *** Genus ''

Coelatura ''Coelatura'' is a genus of bivalve in the Unionidae family. Species Species: *''Coelatura aegyptiaca'' *''Coelatura alluaudi'' *''Coelatura bakeri'' *''Coelatura briarti'' *''Coelatura choziensis'' *''Coelatura cridlandi'' *''Coelatura d ...

'' *** Genus '' Nitia'' *** Genus '' Nyassunio'' *** Genus '' Prisodontopsis'' *** Genus '' Brazzaea'' *** Genus '' Grandidieria'' *** Genus '' Moncetia'' *** Genus '' Pseudospatha'' ** Tribe Indochinellini *** Genus '' Indonaia'' *** Genus '' Scabies'' *** Genus '' Radiatula'' *** Genus '' Harmandia'' *** Genus '' Indochinella'' *** Genus '' Scabiellus'' *** Genus '' Unionetta'' ** Tribe Lamellidentini *** Genus '' Lamellidens'' *** Genus '' Trapezidens'' *** Genus '' Arcidopsis'' ** Tribe Leoparreysiini *** Genus '' Leoparreysia'' ** Tribe Parreysiini *** Genus '' Parreysia'' *** Genus '' Balwantia'' * Genus '' Haasodonta'' (subfamily ''incertae sedis'') * Genus '' Germainaia'' (subfamily ''incertae sedis'') * Subfamily Modellnaiinae ** Genus '' Modellnaia''

Genera by alphabetic order and region

Widespread * '' Anodonta'' * ''

'' * '' Unio'' Africa * '' Brazzaea'' * ''

'' * '' Germainaia'' * '' Grandidieria'' * '' Mweruella'' * '' Nitia'' * '' Nyassunio'' * '' Prisodontopsis'' * '' Pseudospatha'' Central America and Mexico * '' Arotonaias'' * '' Barynaias'' * ''

'' * ''

'' * '' Friersonia'' * '' Martensnaias'' * '' Micronaias'' * '' Nephritica'' * '' Nephronaias'' * '' Pachynaias'' * ''

'' * ''

Psorula ''Psorula'' is a fungal genus in the family Psoraceae. It is a monotypic genus, containing the single lichen species ''Psorula rufonigra''. The genus was circumscribed In geometry, the circumscribed circle or circumcircle of a polygon is a c ...

'' * '' Reticulatus'' * '' Sphenonaias'' Eastern Asia * '' Aculamprotula'' * '' Acuticosta'' * '' Anemina'' * '' Arconaia'' * '' Bineurus''C. T. Simpson, 1900 * '' Caudiculatus'' * '' Chamberlainia'' * '' Contradens'' * '' Cristaria'' * '' Ctenodesma'' * '' Cuneopsis'' * '' Discomya'' * '' Elongaria'' * '' Ensidens'' * '' Harmandia'' * ''

'' * '' Inversidens'' * '' Inversiunio'' * '' Lamprotula'' * '' Lanceolaria'' * '' Lepidodesma'' * '' Modellnaia'' * '' Monodontina'' Conrad, 1853 * '' Nodularia'' * '' Oxynaia'' * '' Physunio'' * ''

'' * '' Pressidens'' * '' Prohyriopsis'' * '' Pronodularia'' * '' Protunio'' * '' Pseudobaphia'' * '' Pseudodon'' * '' Ptychorhynchus'' * '' Rectidens'' * '' Rhombuniopsis'' * '' Scabies'' * '' Schepmania'' * '' Schistodesmus'' * '' Simpsonella'' * '' Sinanodonta'' * ''

'' * '' Sundadontina'' Bolotov et al., 2020 * '' Thaiconcha'' Bolotov et al., 2020 * '' Unionetta'' Europe * '' Microcondylaea'' * ''

'' India * '' Arcidopsis'' * '' Lamellidens'' * '' Parreysia'' * '' Radiatula'' * '' Trapezoideus'' * '' Yaukthwa'' Middle East * '' Leguminaia'' * '' Pseudodontopsis'' New Guinea * '' Haasodonta'' North America * ''

'' * '' Alasmidonta'' * '' Amblema'' * '' Anodontoides'' * '' Arcidens'' * '' Cyprogenia'' * '' Dromus'' * ''

'' * ''

'' * '' Glebula'' * ''

'' * '' Hamiota'' * '' Hemistena'' * ''

'' * ''

Leptodea ''Potamilus'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species Species within the genus ''Potamilus'' include: * ''Potamilus alatus'' (pink heelsplitter) * ''Potamilus ...

'' * ''

'' * '' Medionidus'' * '' Megalonaias'' * ''

Unionidae Bivalves ...

'' * '' Obovaria'' * ''

'' * '' Plectomerus'' * '' Plethobasus'' * ''

'' * '' Pleuronaia'' * ''

'' * ''

'' Crosse & P. Fischer, 1894 * ''

'' * '' Pyganodon'' * ''

'' * ''

Reginaia ''Reginaia '' is a genus of freshwater mussels, aquatic bivalve mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 ...

'' * ''

Rotundaria ''Rotundaria'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or ...

'' * '' Simpsonaias'' * '' Strophitus'' * ''

'' * ''

Toxolasma ''Toxolasma'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species within the genus ''Toxolasma''

* ''Toxolasma corvuncvulus'' (southern purple lilliput) * ''Toxolasma ...

'' * '' Truncilla'' * '' Uniomerus'' * '' Utterbackia'' * '' Venustaconcha'' * '' Villosa''

Fossilization and taphonomic implications

In large enough quantities, unionid shells can have enough of an impact on environmental conditions to affect the ability of organic remains in the local environment to fossilize. For example, in the

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...

, fossil hadrosaur eggshell is rare because the breakdown of

tannins Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'', f ...

from local coniferous vegetation would have caused the ancient waters to become acidic. Eggshell fragments are present in only two

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

sites, both of which are dominated by the preserved shells of invertebrate life, including unionids. The slow dissolution of these shells releasing calcium carbonate into the water raised the water's pH high enough to prevent the eggshell fragments from dissolving before they could be fossilized.Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206-218. In: Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.

References

External links

Missouri State Unio Gallery

Freshwater Mollusk Conservation Society

* ttps://web.archive.org/web/20090304195016/http://bama.ua.edu/~musselp/proj/grano_salis/unionidae.html Unionidae at The MUSSEL Project Web Site {{Taxonbar, from=Q1434366 Bivalve families Extant Middle Jurassic first appearances