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.
|A live individual of Anodonta anatina, the duck mussel|
Families, genera, and species in the order Unionida are found on six continents, where they are restricted exclusively to freshwater rivers, streams, creeks and some lakes. There are approximately 900 species worldwide. Around 300 species of these freshwater mussels are endemic to North America. Unlike other bivalve orders, Unionida has no marine species, although one species (Glebula rotundata) tolerates brackish water. This widespread trait and its global distribution suggests the group has inhabited freshwater throughout its geologic history.
Unionida burrow into the substrate in clean, fast flowing freshwater water rivers, streams and creeks, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and filtering food from the water column. Freshwater mussels are some of the longest-living invertebrates in existence. These clams have, like all bivalve mollusks, a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within. Like all mollusks, the freshwater mussels have a muscular "foot", which enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat.
Unionida have a unique and complex life cycle involving parasitic larvae. This larval form used to be described as "parasitic worms" on the fish host, however, the larvae are not "worms" and do not harm fish under normal circumstances. Most of these freshwater mussel species have separate sexes (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 gills, fins or skin of a host fish. Typically, the freshwater mussel larvae (glochidia) have hooks, which enable the individual to attach itself to fish. Some freshwater mussels release their glochidia in mucilaginous packets called conglutinates. The conglutinate has a sticky filament that allows it to adhere to the substrate so it is not washed away. There is also an even more specialized way of dispersal known as a super-conglutinate. The super-conglutinate resembles an aquatic fly larva or a fish egg, complete with a dark area that looks like an eyespot, and it is appetizing to fish. When a fish consumes it, it breaks up, releasing the glochidia. Mussels that produce conglutinates and super-conglutinates are often gill parasites, the glochidia attaching to the fish gills to continue their development into juveniles. 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 freshwater mussels which then bury themselves in the sediment. This unique life cycle allows Unionida freshwater mussels to move upstream with the fish host species.
Many of these freshwater mussel species face conservation issues due to habitat degradation and in some cases due to over-exploitation for the freshwater pearl industry, and for the nacre of their shells, which was used in button manufacturing. Of the North American Unionida about 70% are either extinct (21 species), endangered (77 species), threatened (43 species) or are listed as species of special concern (72 species).
These bivalve mollusks were heavily exploited for freshwater pearls, and for their nacre which was used in the button manufacturing industry in the late 19th and early 20th centuries. The effects of heavy fishing for freshwater mussels in North America in for use in manufacturing buttons put many of these species close to extinction.
The "pearl rush" in North America occurred in the mid to late 1800s as people could easily find freshwater mussels in rivers and streams by "pollywogging" for mussels, some of which had freshwater pearls which they could sell for a significant price. The art of "pollywogging" involves shuffling one's feet in the mud feeling around for freshwater mussels. Because this was relatively easy to do, and an easy way to make money from freshwater selling pearls, this period has been euphemistically called the "pearl rush", and some historians have compared it to the gold rush in California. A formal freshwater mussel fishing industry was established in the mid-1850s to take advantage of this natural resource. The "pearl rush" to find freshwater pearls became so intense in some rivers that literally millions of freshwater mussels were killed in a few years. In some rivers and streams entire freshwater mussel beds were completely eliminated. Although the negative impact of the "pearl rush" on freshwater mussel populations was significant, in the cold light of history it was relatively minor compared to the over fishing that took place just a few years later with the "pearl" button industry.
Freshwater pearls from North America come from freshwater mussels primarily in the family Unionidae. About 20 different species of Unionidae are commercially harvested for pearls. The common names of the most prolific pearl-bearing species include: the butterfly, ebony, elephant ear, heelsplitter, mapleleaf, three-ridge pigtoe, pimple back, pistol grip, and washboard. While white is the most common color, freshwater pearls are valued in part for their wide range of lustrous colors, including: blue, bronze, brown, copper, cream, green, lavender, pink, purple, red, salmon, silvery white, white, and yellow. The different colors of freshwater pearls are primarily a function of which species of freshwater mussel they were formed in, although various factors including position of the pearl nucleus in the shell, water quality, and species type all affect the color of the freshwater pearl.
With the decline in the numbers of native freshwater mussels in North America people began to culture freshwater pearls; this became a big industry in Japan. Natural freshwater pearls are rarely perfectly round, more often than not freshwater pearls are naturally shaped as baroque, slug, or wing shaped. Round pearls are sought after as more desirable for use in jewelry. The shape of the "seed" or nucleus of the freshwater pearl, and the position of the "seed" in the mussel determines the ultimate shape the cultured pearl will take, hence with careful advanced planning cultured pearls can be made round. Cultured pearls have a similar color to natural pearls as the nacre is laid down by the mantle of the freshwater mussel, and thus the color of the pearl may be species specific. Exportation of freshwater mussels for the use in the Japanese cultured pearl industry has supported the North American freshwater mussel fisheries since the late 1950s. The mother of pearl (or nacre) from exported freshwater mussels are used to make a bead nucleus which is placed in a living animal to form a pearl. In the 1990s, the United States exported $50 million worth of freshwater mussel shells to Japan. Exports of freshwater mussel shells declined so that by 2002 the annual revenue of freshwater mussel exportation to Japan had dropped to $35 million. By 1993 in the United States 31 different states were still reporting production of freshwater pearls and export of freshwater mussel shells, including: Alabama, Arkansas, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Minnesota, Missouri, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, and Wisconsin. To this date the bulk of the freshwater mussel shell and freshwater pearl production comes from Alabama, Arkansas, Louisiana, and Tennessee.
In 1963 the first experimental United States freshwater mussel cultured pearl farm was established in Tennessee by John Latendresse, who is widely proclaimed as "the father of the U.S. cultured freshwater pearl industry." Over the course of nearly 30 years, John Latendresse devoted his money, time and effort to research and develop the cultured freshwater pearl industry in the United States. There are currently six freshwater cultured pearl farms in Tennessee and one in California to support the increasing popularity and demand of freshwater pearl jewelry with consumers in the United States.
The North American button industry began with a German craftsman named John Boepple, who had made buttons from seashells, horns and antlers in his native country. John Boepple immigrated to the United States in 1887 and found that there were vast beds of thick freshwater mussel shells in Muscatine, Iowa, which he determined were perfect for making "pearl buttons". By 1891, Boepple had set up a shop and was in business as a craftsman making buttons. John Boepple's buttons became popular locally and to ward off competition he was very protective of the secrets of his trade. Since freshwater mussels were so common and the profit potential in making "pearl" buttons was so high, some of Boepple's staff who knew his techniques were "recruited" by other businessmen to start competing businesses. Within a few years there were button factories along the length of the Mississippi River.
The need to "catch" freshwater mussels for the "pearl" button industry spurred the invention of tools to make the job easier than "pollywogging" with bare feet. In 1897 inventive mussel fishermen bent steel bars into wide open hooks which they called "brail hooks" or "crow foots" and used them to try to catch or trap the freshwater mussels. The process was simple: several of these "brail hooks" were attached to a long wooden bar with lengths of rope and the entire assembly was lowered into the river and dragged behind a boat along the river bottom. When the tips of these hooks came in contact with an "open" freshwater mussel, the mussel clamped its valves shut around the hooks and could be lifted from the bottom. Within a short period of time millions of freshwater mussels were collected in this manner.
By 1899, there were sixty button factories in the river states of Illinois, Iowa, Missouri, and Wisconsin, employing 1,917 people. Millions of "pearl" buttons were made annually. This new button industry quickly placed a huge ecological demand on the freshwater mussels of the Midwestern United States. In 1899, clammers harvested over sixteen million pounds of freshwater mussel shells in Wisconsin alone, and the harvest of freshwater mussels in the late 1890s numbered in the tens of millions of pounds per year. Freshwater mussel beds which had previously been so dense as to virtually "carpet" miles of river bottom were almost completely harvested, leaving just a few living freshwater mussels per mile. In 1908, in what was deemed a drastic response to the rapidly declining freshwater mussel population, the U.S. Bureau of Fisheries established a mussel propagation program at the Fairport Biological Station. The purpose of this program was to regulate the overharvesting of freshwater mussels. Freshwater mussels are slow growing sessile organisms, and their reproduction is complex. The button industry in North America was in trouble because years of overharvesting the freshwater mussels had caused a shortage of freshwater mussels and pushed many of the species close to extinction. The invention of plastic and its use in manufacturing buttons during World War II replaced shell "pearl" buttons as the most popular product, and foreshadowed the end of the pearl button manufacturing business.
The superfamilies and families in the order Unionida, as listed by Bieler et al (2010). The use of † indicate families and superfamilies that are extinct.
Anodontites is a genus of freshwater mussels, aquatic bivalve mollusks in the family Mycetopodidae. Anodontites are present in South and Middle America, as far north as Mexico.Aspatharia
Aspatharia is a genus of freshwater mussel, an aquatic bivalve mollusk in the family Iridinidae.Aspatharia divaricata
Aspatharia divaricata is a species of freshwater mussel in the family Iridinidae. It is endemic to Lake Victoria in Tanzania.Aspatharia subreniformis
Aspatharia subreniformis is a species of freshwater mussel in the family Iridinidae. It is endemic to Lake Malawi in Malawi.Bryant Walker
Bryant Walker (1856–1936) was an American malacologist who specialized in the non-marine Mollusca. He mainly studied the freshwater mollusks of the USA, in particular those of Michigan. He published many papers on the Unionida, an order of freshwater mussels.Etheria elliptica
Etheria is a genus of freshwater mussels in the Etheriidae family of mollusk bivalves, and a part of the Unionida order. The genus includes a single species, Etheria elliptica, that is found throughout Africa and Madagascar.Etheria elliptica was first described by Lamarck in 1807, and lives in river basins along the Nile, Lake Tanganyika and Lake Victoria, and in Chad, Zaire, Niger, Senegal, and Angola.Etheria are found as fossils at paleontological sites in Africa, including at Lake Turkana 3-5 million years ago. It first appears in the Miocene in northeast Zaire.Etheriidae
Etheriidae is a small family of medium-sized freshwater mussels, aquatic bivalve molluscs in the order Unionida.
It contains two monotypic genera.Freshwater bivalve
Freshwater bivalves are one kind of freshwater molluscs, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves.
The majority of species of bivalve molluscs live in the sea, but in addition, a number of different families live in freshwater (and in some cases also in brackish water). These families belong to two different evolutionary lineages (freshwater mussels and freshwater clams), and the two groups are not closely related.
Freshwater bivalves live in many types of habitat, ranging from small ditches and ponds, to lakes, canals, rivers, and swamps.
Species in the two groups vary greatly in size. Some of the pea clams (Pisidium species) have an adult size of only 3 mm. In contrast, one of the largest species of freshwater bivalves is the swan mussel, in the family Unionidae; it can grow to a length of 20 cm, and usually lives in lakes or slow rivers.
Freshwater pearl mussels are economically important as a source of freshwater pearls and mother of pearl.Hyriidae
Hyriidae is a taxonomic family of pearly freshwater mussels, aquatic bivalve molluscs in the order Unionida. This family is native to South America, Australia, New Zealand and New Guinea. Like all members of that order, they go through a larval stage that is parasitic on fish (see glochidium).
The classification recognized by Banarescu (1995) uses three subfamilies. This family contains eighteen genera.IUCN Red List of extinct species
On 29 January 2010, the IUCN Red List of Threatened Species identified 842 (746 animals, 96 plants) extinct species, subspecies and varieties, stocks and sub-populations.Iridinidae
Iridinidae is a family of medium-sized freshwater mussels, aquatic bivalve mollusks in the order Unionida.List of endangered molluscs
As of September 2016, the International Union for Conservation of Nature (IUCN) lists 507 endangered mollusc species. 7.0% of all evaluated mollusc species are listed as endangered.
The IUCN also lists nine mollusc subspecies as endangered.
No subpopulations of molluscs have been evaluated by the IUCN.
For a species to be considered endangered by the IUCN it must meet certain quantitative criteria which are designed to classify taxa facing "a very high risk of exintction". An even higher risk is faced by critically endangered species, which meet the quantitative criteria for endangered species. Critically endangered molluscs are listed separately. There are 1088 mollusc species which are endangered or critically endangered.
Additionally 1988 mollusc species (27% of those evaluated) are listed as data deficient, meaning there is insufficient information for a full assessment of conservation status. As these species typically have small distributions and/or populations, they are intrinsically likely to be threatened, according to the IUCN. While the category of data deficient indicates that no assessment of extinction risk has been made for the taxa, the IUCN notes that it may be appropriate to give them "the same degree of attention as threatened taxa, at least until their status can be assessed."This is a complete list of endangered mollusc species and subspecies evaluated by the IUCN.List of near threatened molluscs
As of September 2016, the International Union for Conservation of Nature (IUCN) lists 535 near threatened mollusc species. 7.4% of all evaluated mollusc species are listed as near threatened.
The IUCN also lists nine mollusc subspecies as near threatened.
No subpopulations of molluscs have been evaluated by the IUCN.
This is a complete list of near threatened mollusc species and subspecies evaluated by the IUCN.Margaritiferidae
Margaritiferidae is a family of medium-sized freshwater mussels, aquatic bivalve molluscs in the order Unionida. They are known as freshwater pearl mussels, because the interior of the shell of these species has a thick layer of nacre or mother of pearl, and the mussels are thus capable of producing pearls.Mycetopodidae
The Mycetopodidae are a family of freshwater pearly mussels in the order Unionida restricted to South America. They are named for the mushroom-like shape of their foot. Like all members of the Unionida they reproduce via a larval stage that temporarily parasitizes fish. Banarescu lists four subfamilies with ten genera in total.Palaeoheterodonta
Palaeoheterodonta is a subclass of bivalve molluscs. It contains the extant orders Unionida (freshwater mussels) and Trigoniida. They are distinguished by having the two halves of the shell be of equal size and shape, but by having the hinge teeth be in a single row, rather than separated into two groups, as they are in the clams and cockles.Spathopsis
Spathopsis is a genus of fresh-water mussel; it is a bivalve or clam which is in the family Iridinidae.It contains the following species:
Spathopsis rubensSpathopsis wissmanni
Spathopsis wissmanni is a species of bivalve in the family Iridinidae. It is endemic to Burundi. Its natural habitat is rivers.Unionidae
The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve mollusks 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 waters, i.e. flowing water such as rivers, streams and creeks.