No effect of recent sympatry with invasive zebra mussel on the oviposition decisions and reproductive success of the bitterling fish, a brood parasite of unionid mussels
The presence of non-native species can affect coevolved relationships. However, rapid reciprocal changes in coevolutionary associations provide the potential to quickly respond to a new situation. We studied a system where bitterling fish (Rhodeus amarus) parasitize unionid mussels by laying their eggs onto their gills. This association is affected by the infestation of unionid shells by the non-native zebra mussel (Dreissena polymorpha). In a series of experiments under experimental, semi-natural and natural conditions, we compared the behavioural response to zebra mussel infestation of unionid shells, its effect on oviposition decisions and their population consequences between bitterling populations naïve to zebra mussels and those recently sympatric with zebra mussels. We found no effect of recent sympatry on bitterling preoviposition behaviour and oviposition decisions and only a weak effect on their reproductive success. Bitterling from both populations inspected infested and non-infested mussels at the same rate but preferred to oviposit into non-infested unionid hosts. However, neither bitterling population completely avoided oviposition into infested unionids and three ovipositions into zebra mussels were observed. Overall, there was a clear negative relationship between the number of zebra mussels on unionid host shells and the number of juvenile bitterling emerging from the mussels. Our study demonstrated a lack of rapid evolutionary response to adaptively modulate oviposition choice after recent zebra mussel invasion.