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Endangered freshwater mussels (8003788857).jpg
Six endangered species of Unionidae
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Unionida
Superfamily: Unionoidea
Family: Unionidae
Fleming, 1828

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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. [1] [2]


The range of distribution for this family is world-wide. It is at its most diverse in North America, with about 297 recognised taxa, [3] [4] [5] but China and Southeast Asia also support very diverse faunas.

Freshwater mussels occupy a wide range of habitats, but most often occupy lotic 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). [6]

Life history

Unionidae burrow into the substrate, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and food. They remove phytoplankton and zooplankton, as well as suspended bacteria, fungal spores, and dissolved organic matter. [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Despite extensive laboratory studies, which of these filtrates unionoids actually process remains uncertain. In high densities, they have the ability to influence water clarity [17] [18] 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. [11]


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. [19]

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 gills, 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.

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, 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 in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst. [20]

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. [21]


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. [22] For example, in the Dinosaur Park Formation, fossil hadrosaur eggshell is rare [22] because the breakdown of tannins from local coniferous vegetation would have caused the ancient waters to become acidic. [22] Eggshell fragments are present in only two microfossil sites, both of which are dominated by the preserved shells of invertebrate life, including unionids. [22] 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. [22]

Related Research Articles

Mussel Common name for members of several families of bivalve molluscs

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 are often more or less rounded or oval.

Bivalvia Class of molluscs

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. Bivalves as a group have no head and they lack some usual molluscan organs like the radula and the odontophore. They include the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops and file shells, can swim. The shipworms bore into wood, clay, or stone and live inside these substances.

<i>Corbicula fluminea</i> Species of mollusc

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 clam or Asiatic clam. In Southeast Asia, C. fluminea is known as the golden clam, prosperity clam, pygmy clam, or good luck clam.

Alasmidonta atropurpurea, common name Cumberland elktoe, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

<i>Sinanodonta woodiana</i> Species of bivalve

Sinanodonta woodiana, the Chinese pond mussel, Eastern Asiatic freshwater clam or swan-mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

Unionida Order of bivalves

Unionida is a monophyletic order of freshwater mussels, aquatic bivalve molluscs. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels. The most common families are the Unionidae and the Margaritiferidae. All have in common a larval stage that is temporarily parasitic on fish, nacreous shells, high in organic matter, that may crack upon drying out, and siphons too short to permit the animal to live deeply buried in sediment.

<i>Cyprogenia</i> Genus of bivalves

Cyprogenia is a genus of freshwater mussels, aquatic bivalve mollusks in the family Unionidae.

Fanshell Species of bivalve

The fanshell is a species of aquatic bivalve mollusk in the family Unionidae. This clam is native to the United States, where breeding populations remain in only three rivers. It is a federally listed endangered species of the United States.

<i>Elliptio</i> Genus of bivalves

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.

Oyster mussel Species of bivalve

The oyster mussel is a rare species of freshwater mussel in the family Unionidae. This aquatic bivalve mollusk is native to the Cumberland and Tennessee River systems of Kentucky, Tennessee, Alabama, and Virginia in the United States. It has been extirpated from the states of Georgia and North Carolina. It is a federally listed endangered species of the United States.

Lampsilis rafinesqueana, the Neosho mucket or Neosho pearly mussel, is a species of North American freshwater mussel endemic to Arkansas, Oklahoma, Illinois, Missouri and Kansas.

<i>Popenaias popeii</i> Species of bivalve

Popenaias popeii, common name the Texas hornshell, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

The triangular kidneyshell is a species of freshwater mussel, in the family Unionidae, the river mussels. It is endemic to Alabama in the United States, where it is known from several rivers and streams in the Mobile River Basin. It is a federally listed endangered species of the United States.

Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves.


The glochidium 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.

<i>Epioblasma triquetra</i> Species of bivalve

Epioblasma triquetra, common name the snuffbox mussel, is a species of freshwater mussel, a mollusk in the family Unionidae. It is native to eastern North America, where it is a listed as an endangered species in both Canada and the United States.

<i>Villosa iris</i> Species of bivalve

Villosa iris, the rainbow mussel or rainbow-shell, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

Obovaria arkansasensis, the Ouachita creekshell, is a species of freshwater mussel, an aquatic bivalve mollusc in the family Unionidae, the river mussels. It is endemic to certain rivers and streams in the Ouachita Mountains, Arkansas, and has a complex life history including its larvae being parasitic on a fish host.

<i>Fusconaia flava</i> Species of bivalve

Fusconaia flava, the Wabash pigtoe, is a freshwater mussel in the family Unionidae.

<i>Megalonaias nervosa</i> Species of bivalve

Megalonaias nervosa is a freshwater mussel species in the family Unionidae. Washboard is the common name used for Megalonaias nervosa.


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