Differential elemental accumulation of the signal crayfish (Pacifastacus leniusculus) along an invasion gradient

The non-native signal crayfish (Pacifastacus leniusculus) can influence elemental cycling in aquatic ecosystems through bioaccumulation and transfer of chemical elements, with potential spatial variation along invasion gradients. In this study, we investigate the differences in elemental exposure in signal crayfish in the Rabaçal River, Portugal. We focus on potential intra-population differences along a well-defined invasion gradient, comparing individuals from the invasion core (upstream) and the downstream expanding front (n = 30 each). We examined 57 elements in the muscle of the signal crayfish, including essential elements (EEs): Mn, Co, Ni, Cu, Zn, Mg, Ca, Fe, Se, V, S; non-essential and potentially toxic elements (PTEs): Cd, Hg, Pb, U, As, Sr, Ba, Cr, Zr, Cs, Tl; and technology-critical elements (TCEs): Ti, Rb, La, Ce, Pr, Gd, Dy, Ho, Er, Yb, Ga, Ge, Hf, Ta, In, Re, Te, Pt. We explored the relationship between element concentrations and signal crayfish trophic ecology, inferred through stable isotope analysis (δ13C and δ15N), behaviour, and epibiotic associate load – factors known to shape invasion success through their effects on resource acquisition, competition, and physiological stress. Significant differences in element concentrations were found between individuals from the core and front. Individuals from the front showed higher levels (mean µg/kg, dry weight) of Co: (476 vs. 297), V: (390 vs. 262), Mn: (2.6 × 104 vs. 1.0 × 104), Hg: (2526 vs.1658), and Ta: (21 vs. 11). These patterns suggest that front individuals, with higher δ15N values and more exploratory behaviour, feed at higher trophic levels (e.g., macroinvertebrates), which may explain the elevated concentrations of biomagnifying elements such as Hg and Ta. On the other hand, individuals from the core had higher levels of Pb: (361 vs. 234), and Sr: (39772 vs. 20018), likely due to a diet based on basal resources, as supported by the strong negative correlation between Pb and δ13C, indicating reliance on benthic sources more prone to lead accumulation. This study increases our understanding of contaminant accumulation along an invasion gradient, offering insights for management practices – such as targeted removal of highly contaminated individuals, improved monitoring of metal levels in invaded areas, and public awareness campaigns – to minimise ecological impacts on higher trophic levels.