Combined effects of salinity and trematode infections on the filtration capacity, growth and condition of mussels

Abstract

The blue mussel (Mytilus species complex) is an important ecosystem engineer, and salinity can be a major abiotic driver of mussel functioning in coastal ecosystems. However, little is known about the interactive effects of abiotic drivers and trematode infection. This study investigated the combined effects of salinity and Himasthla elongata and Renicola roscovita metacercarial infections on the filtration capacity, growth, and condition of M. edulis from the Baltic Sea. In a laboratory experiment, groups of infected and uninfected mussels were exposed to a wide range of salinities (6-30, in steps of 3) for 1 mo. Shell growth was found to be positively correlated with salinity and optimal at 18-24 at the end of the experiment, imposed by constraints in shell calcification under lower salinities. Mussel shell growth was not affected by H. elongata infection. While salinity had only a minor effect on tissue dry weight, infected mussels had a significantly lower tissue dry weight than uninfected mussels. Most interestingly, the combination of salinity and trematode infections negatively affected the mussels’ condition indices at lower salinity levels (6 and 9), suggesting that trematode infections are more detrimental to mussels when combined with freshening. A significant positive effect of salinity on mussel filtration was found, with an initial optimum at salinity 18 shifting to 18-24 by the end of the experiment. These findings indicate that salinity and parasite infections act as synergistic stressors for mussels, and enhance the understanding of potential future ecosystem shifts under climate change-induced freshening in coastal waters.

Non-linear effects of non-host diversity on the removal of free-living infective stages of parasites

“…The different non-host diversity effects observed in the experiments can be expected to result in a differential risk for the downstream mussel hosts to become infected through altering their exposure to infective stages. Given that infection levels in the second intermediate host are dose-dependent for the parasite-host system used in our experiments (Liddell et al 2017 ), any changes in exposure are likely to translate into changes in transmission and infection levels, with fitness consequences for the mussel hosts due to the intensity-dependent negative effects of metacercarial infections (Thieltges 2006 ; Stier et al 2015 ; Bommarito et al 2022 ). Hence, depending on the outcome of the effects of intra- and interspecific interactions between non-host species on the total parasite removal, focal hosts are likely to experience very different parasite exposure and associated disease risks.…”

“…Hence, any alterations of the number of infective cercarial stages by non-hosts in the environment surrounding mussels can, in principle, affect the risk of mussels becoming infected with metacercarial stages. In cases where this differential infection risk leads to altered transmission and metacercarial infection levels, this will have fitness consequences for mussels, e.g., reduced condition (Bommarito et al 2022 ). Such negative effects of metacercarial infections in mussels are generally considered to be density-dependent (Thieltges 2006 ; Stier et al 2015 ).…”