The Unionidae are a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

freshwater mussel Freshwater bivalves are molluscs of the order Bivalvia that inhabit freshwater ecosystems. They are one of the two main groups of freshwater molluscs, along with freshwater snails. The majority of bivalve molluscs are saltwater species that l ...

s, the largest in the order

Unionida Unionida is a monophyletic order (biology), order of freshwater mussels, aquatic animal, 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. (20 ...

, the

bivalve 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 ...

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 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, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

, obtaining

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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'', are known to be

hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

. 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 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, fins, or skin of a host

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

. 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, whic ...

, which then bury themselves in the sediment. Some of the species in the Unionidae, notably in the Lampsilini tribe, 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 organism ...

, 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 bubb ...

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 In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...

(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 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 bee ...

** Tribe Lampsilini *** Genus ''

Epioblasma ''Epioblasma'' is a North American genus of freshwater mussels, Aquatic animal, 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 threate ...

'' *** Genus '' Lampsilis'' *** Genus '' Potamilus'' *** Genus '' Actinonaias'' *** Genus '' Cambarunio'' *** Genus ''

Obovaria ''Obovaria'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae, the river mussels. There are at least six described species in ''Obovaria'' at present, all of which are found in the United States. ...

'' *** Genus '' Toxolasma'' *** Genus ''

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

* ''Medionidus acutissimus'' (Alabama moccasinshell) * ''Medionidus conradicus'' * ''M ...

'' *** Genus '' Ptychobranchus'' *** Genus '' Disconaias'' *** Genus '' Leaunio'' *** Genus ''

Venustaconcha ''Venustaconcha'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae The Unionidae are a Family (biology), family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known ...

'' *** Genus '' Cyrtonaias'' *** Genus '' Hamiota'' *** Genus '' Sagittunio'' *** Genus '' Truncilla'' *** Genus ''

Villosa ''Villosa'' is a genus of freshwater mussels, aquatic bivalve molluscs in the family Unionidae. Species Species within the genus ''Villosa'': * '' Villosa amygdala'' * '' Villosa arkansasensis'' - Ouachita creekshell * '' Villosa choctawensi ...

'' *** Genus '' Arotonaias'' *** Genus '' Cyprogenia'' *** Genus ''

Delphinonaias ''Delphinonaias'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the subfamily Ambleminae of the family Unionidae.MolluscaBase eds. (2020). MolluscaBase. Delphinonaias Crosse & P. Fischer, 1894. Accessed through: W ...

'' *** Genus '' Ortmanniana'' *** Genus '' Pachynaias'' *** Genus '' Atlanticoncha'' *** Genus '' Dromus'' *** Genus '' Ellipsaria'' *** Genus '' Friersonia'' *** Genus '' Glebula'' *** 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, the ...

'' *** Genus '' Pleurobema'' *** Genus ''

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

'' *** 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 ''Quadrula'' is a genus of freshwater mussels, Aquatic animal, 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 wit ...

'' *** Genus ''

Megalonaias ''Megalonaias'' is a genus of bivalves belonging to 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 ...

'' *** Genus '' Tritogonia'' ** Tribe Amblemini *** Genus ''

Amblema ''Amblema'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species within the genus ''Amblema'' * ''Amblema elliottii'' (Coosa fiveridge) * ''Amblema neislerii'' (Fat threer ...

'' *** Genus '' Reginaia'' *** Genus '' Plectomerus'' * Subfamily Unioninae ** Tribe Anodontini *** Subtribe Alasmidontina **** Genus ''

Alasmidonta ''Alasmidonta'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. Species within the genus ''Alasmodonta'' * Altamaha arcmussel, ''Alasmidonta arcula'' (I. Lea, 1838) * Cumberland elktoe ...

'' **** Genus '' Lasmigona'' **** Genus '' Pyganodon'' **** Genus '' Utterbackiana'' **** Genus '' Strophitus'' **** Genus ''

Utterbackia ''Utterbackia'' is a genus of freshwater mussels, Aquatic animal, aquatic bivalve mollusks in the family Unionidae. Species Species within the genus ''Utterbackia'' include: * ''Utterbackia imbecillis'' * ''Utterbackia peggyae'' * ''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 ''Anodonta'' is a genus of freshwater mussels in the family Unionidae, the river mussels. Species (and previous species) Species in this genus include: * '' Anodonta anatina'' Linné, 1758 – duck mussel * '' Anodonta beringiana'' Middendo ...

'' **** 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 ''Middendorffinaia mongolica'', formerly ''Middendorffinaia shadini'', is a species of Unionidae, unionid mussel endemic to South Primorye, Russia. Its Specific name (zoology), specific name honours Russian zoologist Vladimir Zhadin. ''Middendor ...

'' *** 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 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 ...

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

Lens A lens is a transmissive optical device that 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'' *** 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 (biology), family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as u ...

'' *** 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''. Descrip ...

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

Scabies Scabies (; also sometimes known as the seven-year itch) is a contagious human skin infestation by the tiny (0.2–0.45 mm) mite ''Sarcoptes scabiei'', variety ''hominis''. The word is from . The most common symptoms are severe itchiness a ...

'' *** 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 * ''

'' * '' Potomida'' * '' Unio'' Africa * '' Brazzaea'' * '' Coelatura'' * '' Germainaia'' * '' Grandidieria'' * '' Mweruella'' * '' Nitia'' * '' Nyassunio'' * '' Prisodontopsis'' * '' Pseudospatha'' Central America and Mexico * '' Arotonaias'' * '' Barynaias'' * '' Cyrtonaias'' * ''

'' * '' 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'' * ''

'' * '' 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'' * ''

'' * ''

'' * '' Anodontoides'' * '' Arcidens'' * '' Cyprogenia'' * '' Dromus'' * '' Ellipsaria'' * ''

'' * '' Elliptoideus'' * ''

'' * ''

'' * '' Glebula'' * ''

'' * '' Hamiota'' * '' Hemistena'' * '' Lampsilis'' * '' Lasmigona'' * '' Lemiox'' * '' Leptodea'' * '' Ligumia'' * ''

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

* ''Medionidus acutissimus'' (Alabama moccasinshell) * ''Medionidus conradicus'' * ''M ...

'' * ''

'' * '' Obliquaria'' * ''

'' * '' Pegias'' * '' Plectomerus'' * '' Plethobasus'' * '' Pleurobema'' * '' Pleuronaia'' * '' Potamilus'' * '' Psoronaias'' Crosse & P. Fischer, 1894 * '' Ptychobranchus'' * '' Pyganodon'' * ''

'' * '' Reginaia'' * '' Rotundaria'' * '' Simpsonaias'' * '' Strophitus'' * '' Theliderma'' * '' Toxolasma'' * '' Truncilla'' * '' Uniomerus'' * ''

'' * ''

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'' is widely applied to any large po ...

from local

coniferous Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...

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 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 ...

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 {{Authority control Bivalve families Extant Middle Jurassic first appearances