Glochidial infestation by the endangered mollusc Unio crassus in rivers of north‐eastern France: Phoxinus phoxinus and Cottus gobio as primary fish hosts

Abstract The thick‐shelled river mussel Unio crassus is an endangered freshwater bivalve species with a life cycle that includes an obligate glochidial larval stage that attaches to the gills of freshwater fish for several days to months. Fish hosts were identified in nine north‐eastern French rivers in order to improve unionid conservation management. Eurasian minnow Phoxinus phoxinus was the most important host at all study sites, with prevalence ranging from 15–100% (122 of 238 individuals infested), and bullhead Cottus gobio usually the second most important (prevalence 0–73%; 37 of 121). Other host species had a relatively low rate of infestation: chub Squalius cephalus (5 of 41), roach Rutilus rutilus (1 of 23), gudgeon Gobio gobio (2 of 50), stone loach Barbatula barbatula (2 of 58) and dace Leuciscus leuciscus (1 of 19). No glochidia were found on spirlin Alburnoides bipunctatus (0 of 24). Parasitic load usually ranged between one and three per fish (60% of cases), and rarely exceeded 10. First results suggest that U. crassus glochidia influence the weight:length ratio of P. phoxinus but not C. gobio, with infested fish being heavier. To date, the ecological function of P. phoxinus (and C. gobio) in small streams has been largely underestimated or ignored as the species is small, common and of no interest to anglers. There is increasing evidence, however, that P. phoxinus deserves conservation management owing to the essential role it plays in the reproduction, development and distribution of U. crassus. Copyright © 2016 John Wiley & Sons, Ltd.