The thick-shelled river mussel (Unio crassus) is a red listed species classified as Endangered (EN) and is also considered within the Habitats Directive. The distribution in Sweden is fragmented and it is mostly occurring in the south eastern parts. We investigated the presence of Unio crassus in a section of the stream Storån, Östergötland County, from Falerum to the inflow into Lake Åkervristen. The environmental parameters investigated were water depth, bottom substrate, shading, water velocity and the slope over the water surface. In this thesis I have focused mainly on water depth and shading, comparing sites with and without mussels. In addition, I used a multivariate PCA analysis to evaluate all parameters together. The water depth was significantly larger in habitats with mussels than in those without. Shading varied from 5 to 80 %, but there was no significant difference between habitats with and without mussels. There was no correlation between water depth and mussel density and not between shading and mussel density either. The multivariate PCA analysis showed that the habitats with and without mussels were different from with regard to the PC1 axis, which included water depth, bottom substrate and water velocity. Alone, the water depth is not enough to predict the presence of Unio crassus in the stream, and it is likely that more parameters need to be considered.  Previous work indicate that the more parameters and habitats that are investigated, the more confident results can be stated of which habitats Unio crassus prefer.

The thick shelled river mussel belongs to a highly endangered group of organisms worldwide. A parasitic larval stage is included in the mussel's life cycle where its glochidia larvae attach to the gills of a host fish. The glochidia infection can affect the ability of fish populations to form schools, a behavior that is fundamental to the survival of many fish species. As the mussel is involved in conservation projects where the reintroduction of the glochidia larva is included, one should know more about how the parasite affects its host to be able to make well-founded decisions regarding possible reintroduction. In this study, a group of glochidia-infected minnows is compared to a control group where it was analyzed if the infected fish obtained a higher nearest neighbor distance (NND), a larger average school area (ASA) and a lower movement of the schools (MOS), two and four weeks after being infected. The minnows were infected in vitro where their schooling behavior was recorded with a video camera of which the footage material where analyzed. Collected data were analyzed with 2-way ANOVAS, where the results showed that there is a relationship between infection and changed schooling behavior with respect to the density and area of the fishes in general. NND and ASA increased in the infected group, but not in the non-infected group, at two and four weeks. If the goal through conservation measures is to restore the original balance of the parasite-host interactions of the thick shelled river mussel and the minnow, more research on how mussel parasites affect its fish hosts should be done. / Den tjockskaliga målarmusslan tillhör en starkt hotad organismgrupp över hela världen. Ett parasitiskt larvstadium ingår i musslans livscykel där dess glochidielarver fäster vid gälarna hos en värdfisk. Denna glochidieinfektion kan påverka fiskpopulationens förmåga att bilda stim, ett beteende som är grundläggande för många fiskarters överlevnad. Då musslan är med i bevarandeprojekt där återintroduktion av glochidielarven ingår bör kunskapen fördjupas gällande hur parasiten påverkar sin värd för att kunna ta välgrundade beslut inför eventuella åtgärder. I den här studien jämfördes glochidieinfekterade elritsor med en kontrollgrupp där det undersöks om de infekterade fiskarna får en högre nearest neighbour distance (NND), en större average school area (ASA) och en lägre movement of the schools (MOS) två och fyra veckor efter att de infekterats. Elritsorna infekteras in vitro där deras stimbeteende spelades in med en videokamera för att sedan bildanalyseras. Insamlade data analyserades med 2-vägs-ANOVOR, där resultaten visade att det finns samband mellan infektion och förändrat stimbeteende med avseende på fiskarnas täthet och area generellt. NND och ASA ökade hos de infekterade, men inte för de icke-infekterade under två och fyra veckor. Om målet genom bevarandeåtgärder är att återställa den ursprungliga balansen i parasit-värdinteraktionen för den tjockskaliga målarmusslan och elritsan, bör det undersökas vidare hur musselparasiter påverkar sina värdfiskar.