Host-specificity of the endangered thick-shelled river mussel (Unio crassus, Philipsson 1788) and implications for conservation

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. 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. 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 16days and 48?days. Salmo trutta, Alburnoides bipunctatus and Cottus gobio were less suitable, losing similar to 90% of glochidia within 16days. Alburnus alburnus, invasive Neogobius melanostomus and introduced Oncorhynchus mykiss lost more than 98% of glochidia within 16?days, indicating they are unsuitable hosts. 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. 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. Copyright (C) 2011 John Wiley & Sons, Ltd.