{{Short description|Species of bivalve}} {{Speciesbox | name = Thick shelled river mussel | image = Thick shelled river mussel - Unio crassus.gif | image_caption = ''Unio crassus'' from Lužnice River, Czech Republic | status = EN | status_system = IUCN3.1 | status_ref = <ref name=iucn>{{cite iucn |author=Lopes-Lima, M. |author2=Österling, M |author3=Zając, T.A |date=2024 |title=''Unio crassus'' |volume=2024 |article-number=e.T210291828A215467836|access-date=26 August 2024}}</ref> | genus = Unio | species = crassus | range_map = Unio-crassus-range.png | authority = {{interlanguage link|Lorens Münter Philipson|de|lt=Philipsson}} ''in'' [[Anders Jahan Retzius|Retzius]], 1788<ref name=ITIS/> }}
'''''Unio crassus''''', the '''thick shelled river mussel''', is a [[species]] of [[freshwater mussel]], an [[aquatic animal|aquatic]] [[bivalve]] [[mollusk]] in the [[Family (taxonomy)|family]] [[Unionidae]], the river mussels.<ref name="WoRMS">MolluscaBase eds. (2023). MolluscaBase. Unio crassus Philipsson, 1788. Accessed through: World Register of Marine Species at: https://marinespecies.org/aphia.php?p=taxdetails&id=1044116 on 2023-01-09</ref>
==Taxonomy== 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.<ref name=poland>{{cite journal |author=Marianna Soroka |title=Conservation status and a novel restoration of the endangered freshwater mussel Unio crassus: Poland case |journal=Knowledge and Management of Aquatic Ecosystems |date=2021 |volume=422 |issue=3 |doi=10.1051/kmae/2021003|doi-access=free }}</ref>
==Geography== Its range occurs across much of Europe, excluding [[British Isles|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.<ref name=neth>{{cite journal|title=Two subspecies of Unio crassus Philipsson, 1788 (Bivalvia, Unionoidea, Unionidae) in The Netherlands|last=Nienhuis|first=Jozef A J H|journal=Basteria|date=January 2012 |volume=76|issue=4/6 |pages=107–116 |url=https://natuurtijdschriften.nl/pub/597419|access-date=28 August 2024}}</ref>
==Description== ''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.<ref name=poland />
==Habitat== The species lives in large streams and rivers with clear water, though it is more tolerant of silt than other threatened species.<ref name=bavaria>{{cite journal |doi=10.1002/aqc.3310 |doi-access=free |title=Conservation status of two endangered freshwater mussel species in Bavaria, Germany: Habitat quality, threats, and implications for conservation management |date=2020 |last1=Stoeckl |first1=Katharina |last2=Denic |first2=Marco |last3=Geist |first3=Juergen |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |volume=30 |issue=4 |pages=647–661 |bibcode=2020ACMFE..30..647S }}</ref> Due to its sensitivity to pollution, it is considered a [[bioindicator]] for water quality.<ref name=poland /> It prefers slower-flowing waters on the banks or near roots, boulders, or other features.<ref name=iucn />
It is especially vulnerable to changes in water chemistry that affect local fish populations, such as nitrate and phosphate concentrations.<ref name=iucn />
==Ecology== 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.<ref name=poland /> [[File:Unio crassus unterwasser.jpg|thumb|left|Thick shelled river mussel in a small river]] 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.<ref name=poland />
The female has a unique method of dispersing the larvae. She crawls up to the edge of the water, exposing her excurrent [[aperture (mollusc)|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.<ref>{{Cite journal|last=Vicentini|first=Heinrich|date=2005-11-01|title=Unusual spurting behaviour of the freshwater mussel Unio crassus|journal=Journal of Molluscan Studies|language=en|volume=71|issue=4|pages=409–410|doi=10.1093/mollus/eyi045|issn=0260-1230|doi-access=free}}</ref> ([https://www.youtube.com/watch?v=d2Tzskuoqg video of spurting mussel])<ref>{{Citation|last=Karel Douda|title=Spurting Mussel (Unio crassus)|date=2017-10-26|url=https://www.youtube.com/watch?v=d2TzskuoqgE |archive-url=https://ghostarchive.org/varchive/youtube/20211219/d2TzskuoqgE |archive-date=2021-12-19 |url-status=live|access-date=2017-10-26}}{{cbignore}}</ref>
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.<ref>{{cite journal |author=Jens-Eike Taeubert |title=The relationship between endangered thick-shelled river mussel (Unio crassus) and its host fishes |journal=Biological Conservation |date=2012 |volume=155 |pages=94–103 |doi=10.1016/j.biocon.2012.06.005|bibcode=2012BCons.155...94T }}</ref>
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 River|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.<ref name=poland />
==Use by humans== 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.<ref name=zooarch>{{cite journal|title="The zooarchaeological analysis of freshwater bivalve shells and their relevance regarding the life of a Neolithic community"|last1=Gulyás|first1=Sándor|journal=VarArchHung|date=2007|volume=21}}</ref> ''[[Unio (bivalve)|Unio]]'' shells used as tools, including ''U. crassus'', have been found at archaeological sites dating to the [[5th millennium BC]].<ref>{{cite journal|title=Learning from Experiment: Unio Freshwater Mussel Shells in Fifth-millennium BC Romania|last1=Margarit|first1=Monica|journal=European Journal of Archaeology|date=2020|volume=24 |issue=2 |pages=180–203 |doi=10.1017/eaa.2020.51}}</ref>
''Unio crassus'' was also one of several mussels used to make ornamental buttons between 4200 and 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]].<ref name=palaeoshell>{{cite journal|title='Palaeoshellomics' reveals the use of freshwater mother-of-pearl in prehistory|last1=Sakalauskaite|first1=Jorune|journal=eLife|date=2019|volume=8 |doi=10.7554/eLife.45644|doi-access=free |pmid=31060688 |pmc=6542584 }}</ref>
The species was used as livestock fodder in Central Europe in the 19th century.<ref name=zooarch /> Today, it is still used as fodder for poultry in Romania, and for human consumption in some areas.<ref name=iucn />
==Use as a host== 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.<ref>{{cite journal|title=The endangered thick-shelled river mussel (Unio crassus): a new host species for the European bitterling (Rhodeus amarus)|last1=Krzysztof|first1=Tatoj|last2=Ćmiel|first2=Adam M.|journal=Biodiversity and Conservation|volume=26|date=2017|issue=5 |pages=1217–1224 |doi=10.1007/s10531-017-1295-y|doi-access=free|bibcode=2017BiCon..26.1217T }}</ref>
[[File:Unio_crassus4_A_MRKVICKA.JPG|thumb|Thick shelled river mussel in its habitat]]
==Threats and conservation== During the 20th century, the thick shelled river mussel declined in [[Europe]] by as much as 50% due to deteriorating water quality,<ref name="Douda 2010">{{cite journal |last1=Douda|first1=Karel |title=Effects of nitrate nitrogen pollution on Central European unionid bivalves revealed by distributional data and acute toxicity testing |journal=Aquatic Conservation: Marine and Freshwater Ecosystems |date=2010 |volume=20 |issue=2 |pages=189–197 |doi=10.1002/aqc.1076|bibcode=2010ACMFE..20..189D }}</ref> [[habitat fragmentation]], host fish limitation,<ref name="Douda et al 2012">{{cite journal |last1=Douda|first1=Karel|last2=Horký|first2=P.|last3=Bílý|first3=M.|last4=Gompper|first4=Matthew|last5=Johnson|first5=Jerald |title=Host limitation of the thick-shelled river mussel: identifying the threats to declining affiliate species |journal=Animal Conservation |date=2012 |volume=15 |issue=5 |pages=536–544 |doi=10.1111/j.1469-1795.2012.00546.x|bibcode=2012AnCon..15..536D }}</ref> canalisation of rivers, and introduction of the [[muskrat]] (''Ondatra zibethicus'') to European rivers.<ref name=poland /> 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.<ref name=bavaria />
The most stable populations are found in Russia and northern Europe.<ref name=bavaria />
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 (Dunajec)|Biała Tarnowska]] river.<ref name=poland />
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.<ref name=iucn />
==Status by country== Its native distribution is Europe and [[Western Asia]].<ref name=iucn /><ref name="Fauna Europaea"/> 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:<ref name=poland />
* Austria – critically endangered * Albania – vulnerable * Belarus – vulnerable * Belgium – strictly protected * Bulgaria – protected * Croatia – endangered * [[List of non-marine molluscs of the Czech Republic|Czech Republic]] – endangered<ref>Juřičková L., Horsák M. & Beran L., 2001: Check-list of the molluscs (Mollusca) of the Czech Republic. Acta Soc. Zool. Bohem., 65: 25-40.</ref><ref>{{Cite web |title=Red List of the molluscs (Mollusca) of the Czech Republic |url=http://mollusca.sav.sk/malacology/redlist.htm |url-status=live |archive-url=https://web.archive.org/web/20231209061048/https://mollusca.sav.sk/malacology/redlist.htm |archive-date=Dec 9, 2023 |website=Malacologica Bohemoslovaca}}</ref> Its conservation status in 2004-06 was unfavourable (U2) according to a report for the [[European Commission]] in accordance with the Habitats Directive.<ref>Dušek J., Hošek M. & Kolářová J. (2007) Hodnotící zpráva o stavu z hlediska ochrany evropsky významných druhů a typů přírodních stanovišť v České republice za rok 2004-2006. - Ochrana přírody, 62(5): appendix 5:I-IV. (in Czech)</ref> * Denmark – believed to have been extirpated until rediscovered in [[Odense River]] in 2003. The Odense River population is estimated to number {{circa}} 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).<ref>{{cite web| title=Tykskallet malermusling (Unio crassus) | url=https://naturstyrelsen.dk/naturbeskyttelse/naturprojekter/odense-aa-og-fjord/eu-life-projekt/tykskallet/ | publisher=Naturstyrelsen (Ministry of Environment) | access-date=18 October 2023 }}</ref><ref>{{cite news| title=Sjælden ferskvandsmusling fundet på Sydsjælland | url=https://www.tv2east.dk/region-sjaelland/sjaelden-ferskvandsmusling-fundet-pa-sydsjaelland | date=13 June 2008 | publisher=TV2 Øst | access-date=18 October 2023 }}</ref> * Finland – in southern Finland. Vulnerable.<ref>{{cite web |url=http://www.ymparisto.fi/default.asp?node=8662&lan=en |title=Threatened and Near Threatened molluscs |year=2001 |archive-url=https://web.archive.org/web/20130511195046/http://www.ymparisto.fi/default.asp?node=8662&lan=en |archive-date=2013-05-11 }}</ref> * France – protected<ref>{{cite web |url=http://natura2000.environnement.gouv.fr/especes/1032.html |title=Recherche de sites par espèce: Invertébrés: Unio crassus (Unio crassus) |year=2007 |access-date=2007-05-15 |archive-url=https://web.archive.org/web/20070921144505/http://natura2000.environnement.gouv.fr/especes/1032.html |archive-date=2007-09-21 }}</ref> * Germany – critically endangered (''vom Aussterben bedroht''), strictly protected<ref>Glöer P. & Meier-Brook C. (2003) Süsswassermollusken. DJN, pp. 134, strana 109, {{ISBN|3-923376-02-2}}</ref> In Germany this bivalve has disappeared from 90% of its former range. * Hungary – protected and rare * Kazakhstan – least concern * Latvia – vulnerable and protected * Lithuania – sometimes reported as "extinct in the wild",<ref name=poland /> but populations exist in the [[Neman River]] drainage.<ref name="kilikowska">{{cite journal | last1=Kilikowska | first1=Adrianna | last2=Mioduchowska | first2=Monika | last3=Wysocka | first3=Anna | last4=Kaczmarczyk-Ziemba | first4=Agnieszka | last5=Rychlińska | first5=Joanna | last6=Zając | first6=Katarzyna | last7=Zając | first7=Tadeusz | last8=Ivinskis | first8=Povilas | last9=Sell | first9=Jerzy | title=The Patterns and Puzzles of Genetic Diversity of Endangered Freshwater Mussel Unio crassus Philipsson, 1788 Populations from Vistula and Neman Drainages (Eastern Central Europe) | journal=Life | volume=10 | issue=7 | date=2020-07-21 | issn=2075-1729 | pmid=32708316 | pmc=7400583 | doi=10.3390/life10070119 | doi-access=free | page=119| bibcode=2020Life...10..119K }}</ref> * [[List of extinct animals of the Netherlands|Netherlands – locally extinct]]. In the [[Netherlands]] it has not been seen alive after 1968 and is most likely [[Extinction|extinct]] in that country.<ref>{{cite web |url=https://minez.nederlandsesoorten.nl/content/bataafse-stroommossel-unio-crassus |title=Bataafse stroommossel (''Unio crassus'') |publisher=Ministerie van Landbouw, Natuur en Voedselkwaliteit (Dutch Ministry of Agriculture, Nature and Food Quality) |work=Beschermde natuur in Nederland: soorten en gebieden in wetgeving en beleid |language=nl |access-date=6 June 2024 }}</ref><ref>{{cite web|url=http://www.anemoon.org/anm/voorlopige-kaarten/zoetwatermollusken/wetenschappelijk/unio-crassus/|title=Anemoon > Flora en Fauna > Soorteninformatie|website=www.anemoon.org}}</ref> * Poland – endangered<ref>Polish Red Data Book of Animals http://www.iop.krakow.pl/pckz/opis.asp?id=130&je=en {{Webarchive|url=https://web.archive.org/web/20070310194344/http://www.iop.krakow.pl/pckz/opis.asp?id=130&je=en |date=2007-03-10 }}</ref> * Romania – endangered * Russia – least concern * Slovakia – vulnerable and protected<ref>{{in lang|cs}} [http://www.mollusca.cz/table/table.php Měkkýši (Mollusca) České republiky a Slovenska<!-- Bot generated title -->] {{Webarchive|url=https://archive.today/20120529140645/http://www.mollusca.cz/table/table.php |date=2012-05-29 }}</ref> * Sweden – endangered<ref>{{cite web|url=http://www.nrm.se/researchandcollections/zoology/invertebratezoology/factsoninvertebrates/thelargefreshwatermussels/thickshelledrivermussel.4.5fdc727f10d795b1c6e800012441.html|title=Thick shelled river mussel - Naturhistoriska riksmuseet<!-- Bot generated title -->|access-date=2007-03-28|archive-url=https://web.archive.org/web/20070313101247/http://www.nrm.se/researchandcollections/zoology/invertebratezoology/factsoninvertebrates/thelargefreshwatermussels/thickshelledrivermussel.4.5fdc727f10d795b1c6e800012441.html|archive-date=2007-03-13}}</ref> * Switzerland – critically endangered
==References== {{Reflist|30em|refs= <ref name="Fauna Europaea">{{cite web |url=https://fauna-eu.org/cdm_dataportal/taxon/3c4134cb-db13-40aa-bde4-2849da96a211 |title=''Unio crassus'' Philipson, 1788 |work=Fauna Europaea |publisher=Fauna Europaea Secretariat, Museum für Naturkunde, Berlin |access-date=13 July 2017}}</ref>
<ref name=ITIS>{{ITIS|id=983714 |taxon=''Unio crassus'' Philipsson in Retzius, 1788 |access-date=13 July 2017}}</ref> }}
==External links== {{Commons category|Unio crassus|Thick shelled river mussel}} * [https://web.archive.org/web/20070312053129/http://clade.acnatsci.org/mussel/m/mom/archive/2005/05-11.html The Mussel Project] * [https://web.archive.org/web/20141029040959/http://www.faunaeur.org/full_results.php?id=337983 ''Fauna Europaea'']
{{Taxonbar |from=Q798236}}
[[Category:Unio (bivalve)|crassus]] [[Category:Bivalves of Asia]] [[Category:Bivalves of Europe]] [[Category:Freshwater bivalves]] [[Category:Bivalves described in 1788]] [[Category:Habitats Directive species]]