Host Specialisation in an Unusual Symbiosis: European Bitterlings Spawning in Freshwater Mussels

This study investigated host specialisation in an unusual symbiosis between a species of fish, the European bitterling (Rhodeus sericeus: family Cyprinidae, subfamily Acheilognathinae), and four species of freshwater mussels (family Unionidae) which bitterlings use for spawning. A field study from a single site in England examined four aspects of host use. First, comparisons among mussel species showed significant differences in production of bitterling young: Unio pictorum released the most young fish, followed in order by Anodonta anatina, Unio tumidus, and Anodonta cygnea. These findings pertained to mussels which had received eggs naturally, and were repeated in an experiment in the wild which controlled for mussel density and prior occupancy by bitterling embryos. Second, there were no significant differences within species in host productivity according to mussel size for individuals > 60 mm in length. Third, field manipulations indicated microhabitat differences in host use which depended on the mussel species: U. pictorum individuals were used most heavily when closest to the river bank, whereas the opposite trend occurred for U. tumidus and A. anatina. Finally, the total productivity of mussels which had been clustered in groups of three was three times higher than single individuals. This latter effect suggests a benefit to males from choosing to defend groups of mussels. Benefits from specialisation among mussel species are not clear, but host use may reflect the broader context of overlaps between the fish and mussels in habitats. The ranges of all four mussel species overlap widely with those of bitterlings, but the least-used mussel species, A. cygnea, has the least habitat overlap with bitterlings, suggesting the potential for weaker selection on the fish for host recognition or adaptation to this species.