Unio crassus

Thick shelled river mussel
Unio crassus from Lužnice River, Czech Republic
Conservation status
Endangered  (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Unionida
Family:	Unionidae
Genus:	Unio
Species:	U. crassus
Binomial name
Unio crassus Philipsson [de] in Retzius, 1788[2]

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. It is considered extirpated from the Netherlands and Lithuania.

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 prefers large streams and rivers with clear water, though it is more tolerant of silt than other threatened species.^[5] Due to its sensitivity to pollution, it is considered a bioindicator for water quality.^[4]

Its lifespan can be up to 80 years, depending on the water temperature. 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.^[6] (video of spurting mussel)^[7]

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.^[8]

The most frequent host fishes have been identified as bullhead (Cottus gobio), minnow (Phoxinus phoxinus), chub (Leuciscus cephalus and Squalius cephalus), rudd (Scardinius erythrophtalamus), 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]

During the 20th century, the thick shelled river mussel declined in Europe by as much as 50% due to deteriorating water quality,^[9] habitat fragmentation host fish limitation,^[10] 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.^[5]

The most stable populations are found in Russia and northern Europe.^[5]

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]

Its native distribution is Europe and Western Asia.^[1][11] 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]

• Austria - critically endangered
• Albania - vulnerable
• Belarus - vulnerable
• Belgium - strictly protected
• Bulgaria - protected
• Croatia - endangered
• Czech Republic – endangered^[12][13] Its conservation status in 2004-06 was unfavourable (U2) according to a report for the European Commission in accordance with the Habitats Directive.^[14]
• Denmark – believed to have been extirpated until rediscovered in Odense River in 2003. The Odense River population is estimated to number c. 3000 individuals and subsequently it has been rediscovered from Suså River where the population size is unknown (might also survive in a couple of other rivers, but this remains unconfirmed).^[15][16]
• Finland – in southern Finland. Vulnerable.^[17]
• France^[18]
• Germany – critically endangered (vom Aussterben bedroht), strictly protected^[19] In Germany this bivalve has disappeared from 90% of its former range.
• Hungary - protected and rare
• Kazakhstan - least concern
• Latvia - vulnerable and protected
• Lithuania - extirpated
• Netherlands – locally extinct. In the Netherlands it has not been seen alive after 1968 and is most likely extinct in that country.^[20][21]
• Poland – endangered^[22]
• Romania - endangered
• Russia - least concern
• Slovakia – vulnerable and protected^[23]
• Sweden – endangered^[24]
• Switzerland - critically endangered

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