Energetic costs in the coevolutionary relationship between bitterling fish and freshwater mussels

Bitterling fishes and unionid mussels are involved in a two-sided co-evolutionary association. On the one side, bitterling exploit unionids by ovipositing in their gills. On the other side, unionids develop via a larval stage (glochidium) that attaches to fish gills. Both interactions are parasitic and expected to have negative consequences for the host. Here, we examine the effects of this association on the metabolic rates of mussel and fish hosts by measuring oxygen uptake rates (MO₂). Measurements were performed on two widespread and broadly coexisting species, namely the rose bitterling Rhodeus ocellatus and Chinese pond mussel Sinanodonta woodiana. As predicted, we observed an increase in routine MO₂ in mussels parasitized by bitterling, but only when hosting early stages of bitterling embryos that reside in the interlamellar space of the gills and obstruct water circulation. Hosting later-stage bitterling embryos (that reside in the suprabranchial cavity outside the host gills) was not associated with a higher routine MO₂. We did not observe an acute negative effect of glochidial infestations on maximal oxygen uptake rate (MO₂max), but glochidia-infested bitterling showed consistently lower oxygen consumption rates during recovery from MO₂max. Our results suggest that acute costs of this mutually parasitic relationship might be mitigated, at least in part, by adaptations to limit infestation rates.