RESEARCH ARTICLE Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish Lea D. Schneider 1 | P. Anders Nilsson 1,2 | Johan Höjesjö 3 | E. Martin Österling 1 1 Department of Environmental and Life Sciences – Biology, Karlstad University, Karlstad, Sweden 2 Department of Biology – Aquatic Ecology, Lund University, Sweden 3 Department of Biology and Environmental Sciences, University of Gothenburg, Sweden Correspondence L. D. Schneider, Department of Environmental and Life Sciences – Biology, Karlstad University, Universitetsgatan 2, 651 88 Karlstad, Sweden. Email: lea.d.schneider@gmail.com Funding information European Commission 0 s LIFE program fund, Grant Number: LIFE10 NAT/SE/000046; Fortum Nordic Environmental Fund, and the Swedish Society for Nature Conservation Abstract 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 0 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 allo- patric 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 combina- tions, 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 suitabil- ity 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. KEYWORDS Conservation, host fish, local adaptation, parasite‐host interaction, threatened species, Unio crassus 1 | INTRODUCTION Parasites and hosts live under constant reciprocal selection pressures in their arms‐race dynamics (Gokhale, Papkou, Traulsen, & Schulenburg, 2013; Poulin, 2007; Thompson, 1994). This can lead to changes in life‐history traits and drive local adaptation of fitness‐ related characters (Blanquart, Kaltz, Nuismer, Gandon, & Ebert, 2013; Kaltz & Skyhoff, 1998). Parasites are generally viewed as having a higher evolutionary potential than their hosts, and thereby also a higher potential for local adaptation (Gandon & Michalakis, 2002). This can be particularly true when parasites have shorter generation times, larger population sizes, and higher migration and mutation rates than their hosts (Dybdahl & Lively, 1996; Lively & Dybdahl, 2000; Morgan & Buckling, 2006). Patterns of local adaptation can vary greatly between different parasite–host systems. Parasites with complex life‐cycles, for example, Received: 19 April 2016 Revised: 20 April 2017 Accepted: 22 June 2017 DOI: 10.1002/aqc.2816 Aquatic Conserv: Mar Freshw Ecosyst. 2017;1–9. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/aqc 1