Aim: The indirect consequences of biotic homogenization, the process of a gradual increase in the similarity of regional biotas driven by the combined effects of species invasions and extinctions, are still poorly understood. In this study, we aimed to assess the ability of a native affiliate species to maintain its host resources under the condition of biotic homogenization of host communities.Location: Central (Vltava River Basin, Czech Republic) and western (Douro River Basin, Portugal) Europe.Methods: We tested the ability of non‐native species to serve as an alternative partner in local host–affiliate relationships. We used a European freshwater mussel, Anodonta anatina, which is considered to be a host generalist of native fish species, and compared the compatibility of its glochidia with native versus non‐native fishes in two distinct European regions. Subsequently, we projected the obtained host compatibility data into the recent progress of biotic homogenization and estimated the degree of host dilution.Results: We found significant differences in the ability of A. anatina glochidia to parasitize the native and non‐native fish species in both the central and peripheral parts of the mussel's distribution range. As a result, the increasing presence of non‐native species within fish communities across Europe likely significantly decreases the availability of the mussel's host. Biotic homogenization of host communities may interfere with general life history traits (host specificity) of their local affiliate species.Main conclusions: This study demonstrates that the mixing of regional biotas may lead to an excessive loss of host availability even for host generalists, such as the freshwater mussel A. anatina, with potentially broad consequences for their population dynamics. Conservation strategies of endangered affiliate species need to incorporate the biogeographical context of host–affiliate relationships and particularly the consequences of biotic homogenization.

Freshwater mussels are one of the most imperiled faunistic groups in the world and environmental exposure to toxic heavy metals, which result in deregulation of calcium absorption and deposition in the laboratory, may be a contributing factor in their decline. To address potential effects of heavy-metal exposure on calcium transport and metabolism in freshwater bivalves, adult Anodonta cygnea (L., 1758) were exposed to a sublethal concentration (1.0 × 10⁻⁶ mol/L) of essential (Zn²⁺and Cu²⁺) or nonessential (Pb²⁺and Cr³⁺) metal for 30 days in the laboratory. Inorganic composition of extrapallial, haemolymph, heart, and pericardium fluids, and kidney tissue, as well as shell morphology by scanning electron microscopy, were compared in treated and untreated mussels. Calcium levels in fluids varied after exposure to any of the metals investigated, although the magnitude and threshold of effect were metal- and compartment-specific. Ca²⁺levels increased robustly in all fluids following exposure to Zn²⁺, Cu²⁺, or Cr³⁺, whereas levels decreased significantly in heart fluid alone following Pb²⁺exposure (p < 0.05). In constrast to exposure to the other metals, Cu²⁺revealed an interesting reverse-accumulation pattern, decreasing in the fluids but not in the kidney, where it clearly accumulates for excretion. In addition, whereas essential Cu²⁺and Zn²⁺are closely regulated, the nonessential metals Pb²⁺and Cr³⁺increase to very high levels. Drastic alterations in shell morphology, specifically the structure of border and inner pallial regions of the nacreous layer, were observed after Cu²⁺or Cr³⁺exposure. Collectively, data suggest that prolonged exposure to a sublethal concentration of these heavy metals can adversely affect compartmental calcium availability and shell composition in A. cygnea.