By analyzing data by Ekelund Ugge et al. (Ekelund Ugge et al. 2021a, b), we found that nickel (Ni) concentration (mg/kg) in gills is moderately correlated with shell height (mm) and wet weight (g) in European freshwater duck mussels (Anodonta anatina), with correlation coefficients (r) of 0.698 and 0.558, respectively (Figure 1, 2). We found that other elements (As, Cr, Zn, Fe, Pb, Cd, Ca, Mn, Na, K, Co, Hg and Cu) show low (0.3<r<0.5) or almost no correlation (r<0.3) with shell height and wet weight of duck mussels, with the exception that Arsenic (As) in gills (mg/kg) is moderately correlated with shell height (r = 0.532) (Figure 1).Furthermore, we found no correlation between shell length and the concentration of any of the elements in gills (data not shown). Thus, we proposed that nickel concentration in gills is only correlated with shell height, but not with shell length in duck mussels. Why nickel levels in gills are correlated only with shell height but not with shell length is currently unknown, and further research is needed. Similarly, no correlations were found between shell height, shell length or wet weight and element concentrations in digestive glands (data not shown). Thus, it can be speculated that nickel concentration only in gills, but not in digestive glands of duck mussels, is correlated with shell height. This tissue-specificity might be due to positions of these tissues with respect to outer environment and higher concentrations of Ni in gills (0.093 mg/kg) compared to digestive glands (0.067 mg/kg). Hannan et al. recently showed that mussel shells are efficient agents in immobilizing Ni (Hannan et al. 2021). Thus, it can be hypothesized that increased levels of Ni in gills might contribute to higher Ni immobilization in shells, leading to an increase in shell height. Considering that nickel in ambient water is first exposed to gills than digestive glands, and that gills are in closer contact with shells, nickel levels in gills might be expected to be better correlated with Ni immobilization in shells and thus with shell height.
These results also suggest that nickel levels in aquatic environments might have an impact on mussel shell height or more generally on mussel morphology. How these changes in animal morphology due to Ni concentrations might effect animal health and fitness remains to be identified.