Unionidae
Encyclopedia
Unionidae is a family
of freshwater mussels, the largest in the order Unionoida
, the bivalve mollusks sometimes known as river mussels, naiad
s, 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 waters.
, obtaining oxygen
and food.
Unionidae are distinguished by a unique and complex life cycle. Most Unionids are of separate sex (although some species, such as Elliptio complanata
, 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
s, fins or skin of a host fish
. 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 which then bury themselves in the sediment.
, 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. 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 glochidia larvae are drawn into the fish's gills where they attach and trigger a tissue response that forms a small cyst
in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.
Africa
Central America and Mexico
Eastern Asia
Europe
India
Middle East
New Guinea
North America
, fossil hadrosaur eggshell is rare. This is because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic. Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including unionids. It was the slow dissolution of these shells releasing calcium carbonate
into the water that raised the water's pH
high enough to prevent the eggshell fragments from dissolving before they could be fossilized.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of freshwater mussels, the largest in the order Unionoida
Unionoida
Unionoida is an order of freshwater mussels, aquatic bivalve mollusks. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels...
, the bivalve mollusks sometimes known as river mussels, naiad
Naiad
In Greek mythology, the Naiads or Naiades were a type of nymph who presided over fountains, wells, springs, streams, and brooks....
s, 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 waters.
Life habits
Unionidae burrow into the substrate, with their posterior margins exposed. They pump water through the incurrent apertureAperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
, obtaining oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and food.
Unionidae are distinguished by a unique and complex life cycle. Most Unionids are of separate sex (although some species, such as Elliptio complanata
Elliptio
Elliptio is a genus of medium-sized 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 Elliptio reach their greatest diversity in the...
, are known to be hermaphroditic
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
). The sperm is ejected from the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
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
Glochidium
The glochidium is a special microscopic larval stage of larger freshwater mussels, aquatic bivalve mollusks in the families Unionidae and Margaritiferidae, the river mussels and European freshwater pearl mussels....
, the first larval stage. Mature glochidia are released by the female and then attach to the gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s, fins or skin of a host fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
. 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
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
mussels which then bury themselves in the sediment.
Reproduction
Some of the species in the freshwater mussel family, UnionidaeUnionidae
Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.The range of distribution for this family is world-wide...
, 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. 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 glochidia larvae are drawn into the fish's gills where they attach and trigger a tissue response that forms a small cyst
Cyst
A cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...
in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.
Genera
Widespread- Genus AnodontaAnodontaAnodonta is a genus of medium-sized to large freshwater mussels, aquatic bivalve molluscs in the family Unionidae.-Species:Species in this genus include:* Duck mussel, Anodonta anatina Linné, 1758* Anodonta beringiana Middendorff, 1851...
- Genus Potomida
- Genus UnioUnio (genus)Unio is a genus of medium-sized freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.Unio is the type genus of the family Unionidae.-Species:Species in the genus Unio include:* Unio cariei - extinct...
Africa
- Genus Brazzaea
- Genus CoelaturaCoelaturaCoelatura is a genus of bivalve in the Unionidae family.-Species within the genus Coelatura:* Coelatura alluaudi* Coelatura cridlandi* Coelatura horei* Coelatura ratidota...
- Genus Germainaia
- Genus Grandidieria
- Genus Mweruella
- Genus Nitia
- Genus Nyassunio
- Genus Prisodontopsis
- Genus PseudospathaPseudospathaPseudospatha is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Pseudospatha:* Pseudospatha tanganyicensis...
Central America and Mexico
- Genus Arotonaias
- Genus Barynaias
- Genus Cyrtonaias
- Genus Delphinonaias
- Genus DisconaiasDisconaiasDisconaias is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels....
- Genus Friersonia
- Genus Martensnaias
- Genus Micronaias
- Genus Nephritica
- Genus Nephronaias
- Genus Pachynaias
- Genus PopenaiasPopenaiasPopenaias is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels....
- Genus Psoronaias
- Genus Psorula
- Genus Reticulatus
- Genus Sphenonaias
Eastern Asia
- Genus Aculamprotula
- Genus Acuticosta
- Genus Anemina
- Genus Arconaia
- Genus Caudiculatus
- Genus Chamberlainia
- Genus Contradens
- Genus Cristaria
- Genus Ctenodesma
- Genus Cuneopsis
- Genus Discomya
- Genus Elongaria
- Genus Ensidens
- Genus Harmandia
- Genus Hyriopsis
- Genus Inversidens
- Genus Inversiunio
- Genus Lamprotula
- Genus Lanceolaria
- Genus Lepidodesma
- Genus Modellnaia
- Genus Nodularia
- Genus Oxynaia
- Genus Physunio
- Genus Pilsbryoconcha
- Genus Pressidens
- Genus Prohyriopsis
- Genus Pronodularia
- Genus Protunio
- Genus Pseudobaphia
- Genus Pseudodon
- Genus Ptychorhynchus
- Genus Rectidens
- Genus Rhombuniopsis
- Genus Scabies
- Genus Schepmania
- Genus Schistodesmus
- Genus Simpsonella
- Genus Sinanodonta
- Genus Solenaia
- Genus Unionetta
Europe
- Genus Microcondylaea
- Genus PseudanodontaPseudanodontaPseudanodonta is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species:Species within this genus include:* Pseudanodonta complanata* Pseudanodonta middendorffi...
India
- Genus Arcidopsis
- Genus Lamellidens
- Genus Parreysia
- Genus Radiatula
- Genus Trapezoideus
Middle East
- Genus Leguminaia
- Genus Pseudodontopsis
New Guinea
- Genus Haasodonta
North America
- Genus ActinonaiasActinonaiasActinonaias is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Actinonais:* Actinonaias ligamentina* Actinonaias pectorosa...
- Genus AlasmidontaAlasmidontaAlasmidonta 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...
- Genus AmblemaAmblemaAmblema is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Amblema:* Coosa threeridge, Amblema elliottii* Fat threeridge, Amblema neislerii...
- Genus AnodontoidesAnodontoidesAnodontoides is a genus of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.-Species:Species within the genus Anodontoides:* Anodontoides denigratus* Anodontoides ferussacianus...
- Genus Arcidens
- Genus Cyclonaias
- Genus CyprogeniaCyprogeniaCyprogenia is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Cyprogenia:* edible naiad * fanshell...
- Genus Dromus
- Genus Ellipsaria
- Genus ElliptioElliptioElliptio is a genus of medium-sized 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 Elliptio reach their greatest diversity in the...
- Genus Elliptoideus
- Genus EpioblasmaEpioblasmaEpioblasma 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 extinction....
- Genus FusconaiaFusconaiaFusconaia is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Fusconaia:* Apalachicola Ebonyshell, Fusconaia apalachicola* Fusconaia askewi* Fusconaia barnesiana...
- Genus Glebula
- Genus Gonidea
- Genus HamiotaHamiota (genus)Hamiota is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.- Species within the genus Hamiota:* Fine-lined pocketbook * Southern sandshell...
- Genus Hemistena
- Genus LampsilisLampsilisLampsilis is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae,the river mussels.- Species within the genus Lampsilis:* pink mucket * lined pocketbook...
- Genus LasmigonaLasmigonaLasmigona is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Lasmigona:* Alabama heelsplitter * white heelsplitter...
- Genus Lemiox
- Genus LeptodeaLeptodeaLeptodea is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Leptodea:* fragile papershell * scale shell...
- Genus LexingtoniaLexingtoniaLexingtonia is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Lexingtonia:* slab-sided naiad * Lexingtonia subplana...
- Genus LigumiaLigumiaLigumia is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species:Species within the genus Liguma include:* Ligumia nasuta* Ligumia recta* Ligumia subrostrata...
- Genus MedionidusMedionidusMedionidus is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Medionidus:* Alabama moccasinshell * Medionidus conradicus...
- Genus Megalonaias
- Genus Obliquaria
- Genus ObovariaObovariaObovaria is a genus of freshwater mussels, 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 PegiasPegiasPegias is a monotypic genus of freshwater mussels in the family Unionidae containing the single species Pegias fabula, known commonly as the littlewing pearlymussel. It is native to the Cumberland and Tennessee River systems in the United States, where it is present in streams in North Carolina,...
- Genus Plectomerus
- Genus PlethobasusPlethobasusPlethobasus is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Plethobasus:* white warty-back pearly mussel...
- Genus PleurobemaPleurobemaPleurobema is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae,the river mussels.-Species within the genus Pleurobema:* highnut * hazel pigtoe...
- Genus Pleuronaia
- Genus PotamilusPotamilusPotamilus is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Potamilus:* pink heelsplitter * Texas heelsplitter...
- Genus PtychobranchusPtychobranchusPtychobranchus is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Ptychobranchus:* Ptychobranchus fasciolaris* Ptychobranchus foremanianus ...
- Genus PyganodonPyganodonPyganodon is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Pyganodon:* Pyganodon cataracta* Pyganodon fragilis* Pyganodon gibbosa* Pyganodon grandis...
- Genus QuadrulaQuadrulaQuadrula is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae native to rivers of the American Midwest and mid-south...
- Genus QuincuncinaQuincuncinaQuincuncina is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species:Species in the genus Quincuncina include:* Quincuncina burkei Walker, 1922* Quincuncina infucata...
- Genus Simpsonaias
- Genus StrophitusStrophitusStrophitus is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.- Species within the genus Strophites:* Strophitus connasaugaensis* Strophitus subvexus* Strophitus undulatus...
- Genus ToxolasmaToxolasmaToxolasma is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.-Species within the genus Toxolasma:* southern purple lilliput * pale lilliput...
- Genus TruncillaTruncillaTruncilla is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels.- Species within the genus Truncilla :* Truncilla donaciformis* Truncilla truncata...
- Genus Uniomerus
- Genus Utterbackia
- Genus Venustaconcha
- Genus VillosaVillosaVillosa is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.-Species within the genus Villosa:* Villosa amygdala* Villosa arkansasensis* Villosa choctawensis* Villosa constricta...
Taphonomic implications
In large enough quantities, unionid shells can have enough of an impact on environmental conditions to effect the ability of organic remains in the local environment to fossilize. For example, in the Dinosaur Park FormationDinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
, fossil hadrosaur eggshell is rare. This is because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic. Eggshell fragments are present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life, including unionids. It was the slow dissolution of these shells releasing calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
into the water that raised the water's pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
high enough to prevent the eggshell fragments from dissolving before they could be fossilized.