ABSTRACT1. The complex life cycle of endangered European freshwater pearl mussel Margaritifera margaritifera L. involves an obligatory parasitic phase on a host fish. Knowledge on the host-parasite interaction and on the suitability of different host fish species and strains is required both for the management of wild fish and mussel populations as well as for improving the efficiency of captive breeding methods.2. In this study, the suitability of different salmonid strains for hosting glochidia was tested, including Danube salmon (Hucho hucho L.) and three brown trout (Salmo trutta L.) strains from inside and outside the freshwater pearl mussel distribution range. All brown trout strains as well as Danube salmon were successfully infected with freshwater pearl mussel glochidia and encystment of mussel larvae was detected.3. One brown trout strain originating from the natural pearl mussel distribution range was identified as the most suitable host, revealing the highest fish-weight-normalized infection rates and highest glochidial growth rates, whereas endemic Danube salmon was least suitable. Under natural conditions, the role of Danube salmon may be attributed to the long-distance dispersal of glochidia in the Danube system, whereas sedentary brown trout appear to be the most important hosts at a local scale.4. Successful infection of suitable hosts and the maintenance of these host-parasite systems in calcareous water were demonstrated in this study. These results indicate that neither the infection process nor the encystment phase of freshwater pearl mussels is dependent on low lime concentrations.5. The results of this study suggest that careful selection and management of appropriate host fish strains is mandatory for sustainable conservation management of freshwater pearl mussel populations.

ABSTRACT1. The complex life cycle of the endangered thick-shelled river mussel (Unio crassus, Philipsson 1788) includes an obligatory parasitic phase on a host fish. Consequently, knowledge of the interaction of U. crassus with its host species is crucial for the development of effective conservation strategies.2. The objective of this study was to assess systematically the host suitability of eight different fish species, including six native species which naturally co-occur with U. crassus, as well as two non-native species. All tested fish species were successfully infected with glochidia of U. crassus, which were present on their gills 2 days after exposure.3. Phoxinus phoxinus and Squalius cephalus were the most suitable hosts as indicated by both the highest total glochidial load and the highest fish-weight-normalized glochidial load after 16 days and 48 days. Salmo trutta, Alburnoides bipunctatus and Cottus gobio were less suitable, losing~90% of glochidia within 16 days. Alburnus alburnus, invasive Neogobius melanostomus and introduced Oncorhynchus mykiss lost more than 98% of glochidia within 16 days, indicating they are unsuitable hosts.4. U. crassus larvae did not grow significantly (<15%) during their metamorphosis on suitable hosts, suggesting that the most obvious advantage of the host-dependent phase in the U. crassus life cycle is the dispersal by fish vectors. The observed differences in the developmental speed and the timing of excystment on different suitable host species are likely to increase the chances of successful dispersal and survival in adverse environmental conditions. 5. The sustainable conservation management of U. crassus populations is closely linked to the effective management of their host fish populations. In particular, the currently underestimated ecological functions of low-valued fish species such as S. cephalus and P. phoxinus clearly deserve better consideration in the conservation management of U. crassus habitats and stream ecosystems.

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Unionid freshwater mussels need to attach to a host fish for completion of their life cycle. It remains unclear whether the relationship between these mussels and their host fishes can be considered parasitic, mutualistic, or commensal. Herein, we studied the effects of Margaritifera margaritifera infestation on Salmo trutta, the most important host of this endangered mussel species in Central Europe. Glochidial load of host fish increased with increasing glochidial concentration, but the highest ratios of encysted glochidia to exposed glochidia were found at low concentration (15,000 glochidia L(-1)) during infestation. Host fish mortality occurred at infestation rates of ~350 glochidia per g fish weight and was highest (60%) at the highest infestation rates (~900 glochidia per g fish weight). On a sublethal level, swimming performance of hosts was inversely related to infestation rates, with infestation of ~900 glochidia per g fish weight reducing critical swimming speed of S. trutta significantly by ~20% compared to infestation with 6 glochidia per g fish weight. The high mortality and the impaired swimming capability of highly infested hosts indicate a parasitic interaction between M. margaritifera and its host. For conservation and reintroduction of M. margaritifera via glochidia-infested S. trutta, we recommend glochidial loads of 5-100 glochidia per g fish weight, while for artificial breeding of juvenile M. margaritifera under laboratory conditions, higher infestation rates of up to 300 glochidia per g fish weight are ideal to balance high yields of mussels and welfare of host fishes.

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