A thick-shelled river mussel, Unio crassus

Between 2012 and 2016, researchers at Karlstad University have worked together with several county boards in southern Sweden in the EU-funded life project “Unio crassus for life” (målarmusslans återkomst). The thick-shelled river mussel (Unio crassus) is one of the most threatened bivalve species in Europe. In Sweden, the distribution of the species has decreased with 50% over the last 100 years.

In twelve streams in southern Sweden, a total of 300 km has been restored as part of the project to improve the habitat for the mussel. The project has also examined what fish species that are suitable hosts for the obligate parasitic larvae of the thick-shelled river mussel. Species like minnow (Phoxinus phoxinus), bleak (Alburnus alburnus) and bullhead (Cottus gobio) seem to be important hosts for the mussel. In addition, juvenile mussels have been reared in captivity with the aim to successfully reintroduce them into the wild.

The project has been very successful, and has now been nominated as one of the best EU life projects. The ceremony where the best project will be awarded is held in Brussels 23 May. Martin Österling, associate professor at Karlstad University, will attend the ceremony. We wish Martin and all other people that have worked on the Unio crassus for life project the best of luck, and we keep our fingers crossed that Unio crassus for life will be awarded the best EU life project.

Read more about the project at the Unio crassus for life official web page, or on the Skåne and Södermanland county board web pages.

The project has also gotten publicity in media, and you can read more about the project on svt or Smålands-tidningen.

Watch a short film about the project here.

Job: Project assistant

Posted by Karl Filipsson | Jobs

A thick-shelled river mussel (Unio crassus).

NRRV is looking for a part-time project assistant to work with the endangered thick-shelled river mussel (Unio crassus). The project concerns rearing of juvenile mussels in the mussel lab in Veberöd, Skåne, and reintroduction of mussels in streams in southern Sweden. Read more and apply for the position here (Swedish). The last day to apply is 2 March 2018.

A thick shelled river mussel (Unio crassus).

Lea SchneiderAnders Nilsson, and Martin Österling from Karlstad University, and Johan Höjesjö from University of Gothenburg, recently published the scientific article “Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish in Aquatic Conservation. In the article the authors present a study on thick shelled river mussels (Unio crassus) and their interaction with potential host fishes originating from the same or a different river than the individual mussels. In the abstract they write:

“1. Parasite–host interactions can involve strong reciprocal selection pressure, and may lead to locally adapted specializations. The highly threatened unionoid mussels are temporary parasites on fish, but local adaptation has not yet been investigated for many species.

2. Patterns of local adaptation of one of Europe’s most threatened unionoids, the thick-shelled river mussel (Unio crassus) were investigated. Eurasian minnows (Phoxinus phoxinus) from two rivers (separate drainage areas) were cross-infested in the laboratory with sympatric and allopatric mussel larvae, while bullheads (Cottus gobio), inhabiting only one of the rivers, were infested with sympatric or allopatric mussel larvae. Larval encystment, juvenile mussel excystment and survival were measured.

3. For one river, but not the other, juvenile excystment from P. phoxinus was highest when infested with sympatric mussels. The opposite pattern was found for C. gobio in this river, where juvenile excystment and post-parasitic juvenile survival from allopatric C. gobio were highest. The results thus cannot confirm local adaptation of U. crassus to P. phoxinus in the study rivers, as excystment was not consistently higher in all sympatric mussel–host combinations, whereas there were potential maladaptive signs of U. crassus in relation to C. gobio. There was no loss of encysted larvae 3 days after infestation until juvenile excystment. Most juveniles were excysted between 17 and 29 days after infestation, and the numbers of excysted juveniles increased with fish size.

4. The results have implications for parasite–host ecology and conservation management with regard to unionoid propagation and re-introduction. This includes the need to (1) test suitability and adaptation patterns between U. crassus and multiple host fish species, (2) evaluate the suitability of certain unionoids and host fish strains after more than 3 days, and (3) determine whether large fish produce more juvenile mussels than smaller fish.”

Access the paper here: “Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish“, or email any of the authors.


Adult thick shelled river mussel (Unio crassus) from the River Tommarpsån, Sweden.

Lea Schneider, Anders Nilsson, and Martin Österling from Karlstad University, recently published the scientific article “Evaluating temperature- and host-dependent reproduction in the parasitic freshwater mussel Unio crassus” in the journal Hydrobiologia. In the article they present a study on the thick shelled river mussel (Unio crassus) and its release of glochidia (mussel larvae) in different temperature regimes.

In the abstract they write: “Adaptation to temperature regimes and host presence may enhance fitness in parasites. In an experimental study, we evaluated the timing of glochidia release by Unio crassus subjected to three spring water temperature regimes in the presence and absence of the host fish Cottus gobio. The timing of glochidia release was delayed at (i) constantly low temperatures (<10°C), in contrast to earlier and pronounced releases at (ii) natural temperature increases that level off at intermediate temperatures (10–15°C), and (iii) higher-than-normal temperatures (10–20°C). Mussels from treatment (i) that had not released glochidia during the experiment did so soon after being moved to the temperature in (ii), indicating a temperature threshold for glochidia release. Neither host fish presence nor the combined effect of temperature and host fish presence significantly affected the timing of glochidia release. The treatment with natural spring water temperatures indicated possible fitness benefits for U. crassus through combined effects of high intensities of glochidia releases and high survival of released glochidia. The furthered understanding of climate change effects on mussel and host phenology in seasonal environments, potentially inducing temporal mismatches of glochidia release to host availability, is key to mussel conservation.”

Acces the paper here: Evaluating temperature- and host-dependent reproduction in the parasitic freshwater mussel Unio crassus

The research was part of the LIFE project UCforLIFE. Read more about the thick shelled river mussel and related conservation work at the projects homepage: www.ucforlife.se


Caryn Vaughn (left) asking questions to Lea Schneider (right) during the PhD-defense.

Today Lea Schneider successfully defended her PhD-thesis ”Conservation ecology of the thick-shelled river mussel Unio crassus – the importance of parasite-host interactions” at Karlstad UniversityCaryn Vaughn (University of Oklahoma, USA) was the opponent, and Leonard Sandin (Swedish University of Agriculture), Niklas Janz (Stockholm University, Sweden), and Annie Jonsson (University of Skövde, Sweden) constituted the grading committee (betygskommitté).

Lea Schneiders supervisors during her PhD were Martin Österling and Anders Nilsson from Karlstad University, and Johan Höjesjö from the University of Gothenburg, Sweden. Her work constituted on part of the LIFE-project UCforLife – Målarmusslans återkomst.

The thesis is available here. Contact Lea Schneider for questions and additional information.


According to tradition, Lea Schneider (center) last week nailed her thesis to the wall, at the entrance to Karlstad University. On the photo are also Martin Österling (supervisor) and Reine Lundin (dean).

On February 24, Lea Schneider, will defend her PhD-thesis “Conservation ecology of the thick-shelled river mussel Unio crassus – the importance of parasite-host interactions”. In the abstract Lea Schneider writes: “Unionoid mussels are globally threatened and their conservation requires species-specific knowledge on their ecology and parasite-host interaction. Unio crassus is one of Europe’s most threatened unionoid species and has a temporary obligate parasitic life stage (glochidia) on fish. A lack of suitable hosts is probably a major limitation for mussel recruitment, but host species composition, suitability and availability in time and space have yet to be fully explored. This thesis examines different aspects of the host fish species, including their composition, suitability and ecological importance, in relation to U. crassus, using both field and laboratory studies. The effects of mussel and host density on mussel reproductive potential were considered, as were aspects of evolutionary adaptations between mussels and fish and how climate change may affect their interaction.

The results show that U. crassus is a host generalist, parasitizing a variety of fish species. Host suitability and density, which varied among fish species and rivers, affected the level of glochidia encapsulation, hence mussel reproductive potential, more so than the density of mussels taking part in reproduction. Ecologically important hosts included both highly suitable primary hosts, and less suitable hosts that were highly abundant. Whether or not U. crassus has specific adaptations to its hosts to enhance juvenile transformation remains unclear. No distinct pattern of local adaptation was found, nor was there an effect of host fish presence on the timing of glochidia release by adult mussels. Instead, temperature played a major role, with results suggesting that changes in spring water temperature regimes can cause temporal and spatial mismatches in the mussel-host interaction. This thesis indicates that investigations of local mussel-host interactions help in identifying mechanisms important for unionoid conservation management and prioritization.”

The defense will take place on February 24 at 10:15 in room 1B309 (Sjöströmsalen) at Karlstad University. The frame of the thesis is available online here.

For the defense, Caryn Vaughn (University of Oklahoma, USA) is the opponent, and Leonard Sandin (Swedish University of Agriculture), Niklas Janz (Stockholm University, Sweden), and Annie Jonsson (University of Skövde, Sweden) constitute the grading committee (betygskommitté).

In the afternoon (from 13:30 onwards) the day before the defense (Feb 23), seminars related to the thesis will be given in Room 5F416 at Karlstad University. Here Caryn Vaughn will present on “Consumer aggregations act as hotspots of ecosystem function and services in rivers”, Niklas Janz on “What is host range?”, and Leonard Sandin on “Evaluation of ecological restoration in Swedish streams – some results from the EKOLIV project”. 

Everyone is welcome to attend both the PhD-defense and the seminars.

The Fyleån creek (photo from uc4lifeconference.se).

The international river restoration conference “Restoring floodplains, habitats and connectivity using mussels and brains” will be held in Lund, Sweden, on 28–30 September, 2016. The organizers write: “Conference objectives are to highlight experiences and results gained from a wide range of river restoration projects focusing on rehabilitation of species of freshwater mussels and fish. Mussels and fish are often used as indicators and flagship species while restoring rivers. But are they any good? If so – why? The first two days will focus on presentations by invited experts and conference participants who want to present their work. The third day includes field visits to Fyleån Creek, Klingavälsån River, both sites re-meandered, and the Hemmestorp Mölla rearing facility. We look forward to seeing you in Lund!”

Registration and more information on www.uc4lifeconference.se.

Earlier this week Lea Schneider, PhD-student at Karlstad University, gave a presentation about her research on parasite-host ecology, local adaptation and conservation of the thick shelled river mussel. The seminar was titled: ”Local adaptation studies and conservation: parasite-host interactions between freshwater mussels and fish” and is now available online upon registration.

To access the seminar, send an e-mail to lea.schneider@kau.se.

Today Lea Schneider, PhD-student at Karlstad University, will give a seminar titled: “Local adaptation studies and conservation: parasite-host interactions between freshwater mussels and fish”. The seminar will be given at 13:30 in room 5F416 on Karlstad University. Everyone is welcome to attend!

Read more about the thick shelled river mussel on NRRV.se or on UCforLife.