| Thick shelled river mussel | |
|---|---|
 | |
| Unio crassus from Lužnice River, Czech Republic | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Order: | Unionida |
| Family: | Unionidae |
| Genus: | Unio |
| Species: | U. crassus |
| Binomial name | |
| Unio crassus | |
 | |
Unio crassus, the thick shelled river mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. [3]
The species was split into two species, Unio crassus and Unio tumidiformis , in 2009. Various subspecies have been noted, but their validity is not always recognized. Recently, two clades and possible subspecies, Unio crassus crassus and Unio crassus courtillieri, have been identified based on genetic sampling. [4]
Its range occurs across much of Europe, excluding Britain, Italy, and the Iberian Peninsula. It extends from France in the west to the Ural River basin in Russia and Kazakhstan in the east, and historically occupied most major river systems. It is considered extirpated from the Netherlands, where it was last seen in 1967. [5]
Unio crassus is known for its thick-walled shell. They are generally 3–7 cm in length, though some individuals reach 9–11 cm. The size and shape of the shell may vary by population or by environmental factors. [4]
The species lives in large streams and rivers with clear water, though it is more tolerant of silt than other threatened species. [6] Due to its sensitivity to pollution, it is considered a bioindicator for water quality. [4] It prefers slower-flowing waters on the banks or near roots, boulders, or other features. [1]
It is especially vulnerable to changes in water chemistry that affect local fish populations, such as nitrate and phosphate concentrations. [1]
Its lifespan can be up to 80 years, depending on the water temperature, with an average of 20–30 years. It often forms colonies in large streams and rivers with clear, free-flowing water. They bury themselves in the stream bed, leaving only their siphons exposed through which they inhale oxygen and food (algae and microorganisms) and expel waste. It can move several meters, sometimes farther, to find suitable habitat conditions. [4]
They are tachytictic (short-term) breeders, meaning the cycle of fertilization to juvenile takes place in one season. Males release their spermatozoa into the water in the spring, which females take in through siphoning. Females can have several broods of eggs per year. The eggs develop for a few weeks until the larvae are ready for release. Like most Unionidae mussels, the larvae have a parasitic stage where they are required to attach and feed off a host fish. In spring and summer, the female releases around 100,000 glochidia into the water current. [4]
The female has a unique method of dispersing the larvae. She crawls up to the edge of the water, exposing her excurrent aperture, and then lets loose a stream of water containing glochidia (larvae). It is suggested that this spurting behavior may lure host fish that are attracted to the water disturbance. The larvae can then attach to the fish's gills. [7] (video of spurting mussel) [8]
The glochidia attach to the gills of a fish and remain for 20–50 days while they metamorphose into juvenile mussels, after which they drop off and bury themselves in the stream bed for 1–3 years. [9]
The most frequent host fishes have been identified as bullhead (Cottus gobio), minnow (Phoxinus phoxinus), chub (Leuciscus cephalus and Squalius cephalus), rudd (Scardinius erythrophthalmus), bleak (Alburnus alburnus), nase (Chondrostoma nasus), stickleback (Gasterosteus aculeatus), ide (Leuciscus idus) and perch (Perca fluviatilis). It is not able to metamorphose on roach (Rutilus rutilus) and sterlet (Acipenser ruthenus). The most suitable host fish can vary by locality and population. For example, in the Danube drainage, European chub (Squalius cephalus) is considered the most suitable host for U. crassus. Invasive species such as round goby (Neogobius melanostomus) and rainbow trout (Oncorhynchus mykiss) were also unable to keep larvae alive. [4]
Freshwater mussels were consumed by prehistoric people in Central Europe. Archaeological evidence from shell collections in Hungary as far back as the Neolithic period indicate that several mussel species were gathered from a nearby river and consumed raw as an important supplementary food source. Unio crassus appeared to be a preferred species, likely because of its relatively high meat content. [10] Unio shells used as tools, including U. crassus, have been found at archaeological sites dating to the 5th millennium BC. [11]
Unio crassus was also one of several mussels used to make ornamental buttons between 4200-3800 BC in central Europe. Widespread occurrence of these ornaments, from Denmark to Romania, indicates trade or cross-cultural exchange of knowledge. The white hue of the nacre was highly sought after. Freshwater mother-of-pearl was used in ornaments through the Middle Ages. [12]
The species was used as livestock fodder in Central Europe in the 19th century. [10] Today, it is still used as fodder for poultry in Romania, and for human consumption in some areas. [1]
In addition to being a parasite in its larval stage, the thick-shelled river mussel also acts as a host for the endangered European bitterling (Rhodeus amarus). The bitterling's eggs must develop in the gills of freshwater mussels. Bitterling egg-infested mussels may have reduced growth rates. [13]
During the 20th century, the thick shelled river mussel declined in Europe by as much as 50% due to deteriorating water quality, [14] habitat fragmentation, host fish limitation, [15] canalisation of rivers, and introduction of the muskrat (Ondatra zibethicus) to European rivers. [4] A 2020 study identified habitat destruction, predation by muskrat, and lack of host fish as the major factors for decline. U. crassus was noted to be more tolerant of silt in the water than other threatened species. [6]
The most stable populations are found in Russia and northern Europe. [6]
A project in Poland successfully used artificial breeding methods including release of gravid females, release of glochidia-infested fish, and in vitro-raised juveniles, to double the population of the mussel in the Biała Tarnowska river. [4]
Long-term conservation of the species will require changes in river management practices, controlling pollution and invasive species, restoration of river basins, and further outreach and education. More research is needed to identify priority populations for monitoring. [1]
Its native distribution is Europe and Western Asia. [1] [16] It is mentioned in annexes II and IV of the European Union Habitats Directive. Globally, it is ranked endangered on the IUCN Red List. Local statuses are as follows: [4]
The freshwater pearl mussel is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae.
Unio is a genus of medium-sized freshwater mussels, aquatic bivalve mollusks in the family Unionidae, the river mussels. They are found throughout Europe, Africa, and the Middle East, with some species introduced to East Asia. Fossil species are also known from the Jurassic of North America.
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.
Epioblasma is a North American 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.
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Lampsilis abrupta, the pink mucket or pink mucket pearly mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels. This species is endemic to the United States.
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Potamilus leptodon, the scaleshell mussel or scale shell, is a species of freshwater mussel in the family Unionidae, the river mussels. This aquatic bivalve mollusk has disappeared from much of its historical range. It is endemic to the United States, where it is now present in four or fewer states; it is only found with any regularity in Missouri. It is a federally listed endangered species of the United States.
Pleurobema oviforme, the Tennessee clubshell, is a species of freshwater mussel in the family Unionidae, the river mussels. It is native to the eastern United States, where it occurs in Alabama, Kentucky, North Carolina, Tennessee, and Virginia. It also previously occurred in Mississippi.
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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 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.
Paetulunio fabalis, the rayed bean, 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 Paetulunio, and was formerly classified in Villosa until a 2018 study.
The shinyrayed pocketbook is an endangered species of freshwater mussel in the family Unionidae, the river mussels. This species is endemic to the United States in the states of Georgia, Florida, and Alabama.
Microcondylaea compressa is a species of freshwater mussel in the family Unionidae, the river mussels.
Velesunio wilsonii is a species of freshwater mussel endemic to Australia and comes from the Hyriidae family. Velesunio wilsonii mussels have a thick, dark shell that are sometimes flaky and mostly closed. Velesunio wilsonii have a fleshy foot that sometimes extends outside of the valves. The shell of the Velesunio wilsonii varies from oblong to a symmetrical circle. Velesunio wilsonii size ranges from 40 mm to 120 mm.
Unio tumidiformis is a species of bivalve belonging to the family Unionidae. It is endemic to the southwestern Iberian Peninsula. Existing populations are known from the Guadiana, Mira, and Sado Rivers in Portugal and Spain.
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