The Unionidae are a

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

of freshwater mussels, 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 ...

, 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, biva ...

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 est ...

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 (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts.Angelier ...

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 op ...

, 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 ...

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 Am ...

'', are known to be hermaphroditic. 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 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 ar ...

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 liv ...

. 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 mussels, 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 organis ...

, 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 division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...

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 ** Tribe Lampsilini *** Genus ''

Epioblasma ''Epioblasma'' 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, o ...

'' *** Genus '' Lampsilis'' *** Genus '' Potamilus'' *** Genus '' Actinonaias'' *** Genus '' Cambarunio'' *** Genus '' Obovaria'' *** Genus '' Toxolasma'' *** Genus '' Medionidus'' *** Genus '' Ptychobranchus'' *** Genus '' Disconaias'' *** Genus '' Leaunio'' *** Genus '' Venustaconcha'' *** Genus '' Cyrtonaias'' *** Genus '' Hamiota'' *** Genus '' Sagittunio'' *** Genus '' Truncilla'' *** Genus '' Villosa'' *** Genus '' Arotonaias'' *** Genus '' Cyprogenia'' *** Genus '' Delphinonaias'' *** Genus '' Ortmanniana'' *** Genus '' Pachynaias'' *** Genus '' Atlanticoncha'' *** Genus '' Dromus'' *** Genus '' Ellipsaria'' *** Genus '' Friersonia'' *** Genus ''

Glebula ''Glebula rotundata'', the round pearlshell, is a freshwater mussel, 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 ...

'' *** Genus '' Lemiox'' *** Genus '' Ligumia'' *** Genus '' Obliquaria'' *** Genus '' Paetulunio'' ** Tribe Pleurobemini *** 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, t ...

'' *** Genus '' Pleurobema'' *** Genus '' Fusconaia'' *** Genus '' Plethobasus'' *** Genus '' Pleuronaia'' *** Genus '' Parvaspina'' *** Genus '' Elliptoideus'' *** Genus '' Eurynia'' *** Genus '' Hemistena'' ** Tribe Popenaiadini *** Genus '' Nephronaias'' *** Genus '' Psoronaias'' *** Genus '' Barynaias'' *** Genus '' Popenaias'' *** Genus '' Sphenonaias'' *** Genus '' Micronaias'' *** Genus '' Nephritica'' *** Genus '' Reticulatus'' *** Genus '' Martensnaias'' ** Tribe Quadrulini *** Genus '' Cyclonaias'' *** Genus '' Theliderma'' *** Genus '' Uniomerus'' *** Genus '' Quadrula'' *** 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 ...

'' **** Genus ''

Pyganodon ''Pyganodon'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the subfamily Unioninae of the family Unionidae, the river mussels.MolluscaBase eds. (2022). MolluscaBase. Pyganodon Crosse & P. Fischer, 1894. Accessed t ...

'' **** Genus '' Utterbackiana'' **** Genus '' Strophitus'' **** Genus '' Utterbackia'' **** Genus '' Anodontoides'' **** Genus '' Arcidens'' **** Genus '' Pseudodontoideus'' **** Genus '' Pegias'' **** 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'' ** 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'' **** 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'' ** Tribe Lamprotulini *** Genus '' Lamprotula'' *** Genus ''

Potomida ''Potomida'' is a genus of bivalves belonging to the subfamily Gonideinae of the family Unionidae. MolluscaBase eds. (2022). MolluscaBase. Potomida. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=ta ...

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

Hyriopsis ''Hyriopsis'' is a genus of bivalves belonging to the family Unionidae. The species of this genus are found in Southeastern Asia. Species: *'' Hyriopsis altealata'' *'' Hyriopsis bialata'' *'' Hyriopsis bogatchevi'' *''Hyriopsis cumingii'' ...

'' *** 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'' *** Genus '' Koreosolenaia'' *** Genus '' Microcondylaea'' *** Genus '' Obovalis'' *** Genus '' Pseudodontopsis'' ** Tribe Chamberlainini *** Genus '' Sinohyriopsis'' *** Genus '' Chamberlainia'' *** Genus '' Caudiculatus'' * Subfamily Parreysiinae ** Tribe Coelaturini *** Genus '' Coelatura'' *** Genus '' Nitia'' *** Genus '' Nyassunio'' *** Genus '' Prisodontopsis'' *** Genus '' Brazzaea'' *** Genus '' Grandidieria'' *** Genus '' Moncetia'' *** Genus ''

Pseudospatha ''Pseudospatha tanganyicensis'' a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. It is the only species in the genus ''Pseudospatha''. This species is endemic to Lake Tanganyika where found ...

'' ** Tribe Indochinellini *** Genus '' Indonaia'' *** Genus ''

Scabies Scabies (; also sometimes known as the seven-year itch) is a contagious skin infestation by the mite '' Sarcoptes scabiei''. The most common symptoms are severe itchiness and a pimple-like rash. Occasionally, tiny burrows may appear on the skin ...

'' *** 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'' * '' Coelatura'' * '' Germainaia'' * '' Grandidieria'' * '' Mweruella'' * '' Nitia'' * '' Nyassunio'' * '' Prisodontopsis'' * ''

'' Central America and Mexico * '' Arotonaias'' * '' Barynaias'' * '' Cyrtonaias'' * '' Delphinonaias'' * '' Disconaias'' * '' Friersonia'' * '' Martensnaias'' * '' Micronaias'' * '' Nephritica'' * '' Nephronaias'' * '' Pachynaias'' * '' Popenaias'' * '' Psoronaias'' * '' Psorula'' * '' 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'' * '' Pilsbryoconcha'' * '' Pressidens'' * '' Prohyriopsis'' * '' Pronodularia'' * '' Protunio'' * '' Pseudobaphia'' * '' Pseudodon'' * '' Ptychorhynchus'' * '' Rectidens'' * '' Rhombuniopsis'' * ''

'' * '' Schepmania'' * '' Schistodesmus'' * '' Simpsonella'' * '' Sinanodonta'' * '' Solenaia'' * '' Sundadontina'' Bolotov et al., 2020 * '' Thaiconcha'' Bolotov et al., 2020 * '' Unionetta'' Europe * '' Microcondylaea'' * '' Pseudanodonta'' India * '' Arcidopsis'' * '' Lamellidens'' * '' Parreysia'' * '' Radiatula'' * '' Trapezoideus'' * '' Yaukthwa'' Middle East * '' Leguminaia'' * '' Pseudodontopsis'' New Guinea * '' Haasodonta'' North America * '' Actinonaias'' * '' Alasmidonta'' * '' Amblema'' * '' Anodontoides'' * '' Arcidens'' * '' Cyprogenia'' * '' Dromus'' * '' Ellipsaria'' * ''

'' * '' Elliptoideus'' * ''

'' * '' Fusconaia'' * ''

'' * '' Gonidea'' * '' Hamiota'' * '' Hemistena'' * '' Lampsilis'' * ''

'' * '' Lemiox'' * '' Leptodea'' * '' Ligumia'' * '' Medionidus'' * '' Megalonaias'' * '' Obliquaria'' * '' Obovaria'' * '' Pegias'' * '' Plectomerus'' * '' Plethobasus'' * '' Pleurobema'' * '' Pleuronaia'' * '' Potamilus'' * '' Psoronaias'' Crosse & P. Fischer, 1894 * '' Ptychobranchus'' * ''

'' * '' Quadrula'' * '' Reginaia'' * '' Rotundaria'' * '' Simpsonaias'' * '' Strophitus'' * '' Theliderma'' * '' 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 7 ...

, 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' ...

from local

coniferous Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All exta ...

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