Size-dependent biomarker profiles in the zebra mussel Dreissena polymorpha: implications for freshwater biomonitoring

In freshwater biomonitoring, the zebra mussel Dreissena polymorpha is commonly used as a sentinel organism, with individuals usually standardised by shell length as a proxy for age to limit inter-individual variability. However, the relationship between shell length and estimated biological age can vary both within and across populations. This raises questions about the biological relevance of size-based standardisation and the comparability of data across studies using different size ranges. The present study examined how a battery of biochemical biomarkers, covering energy metabolism, antioxidant and detoxification defences, and cellular damage, varied across five defined size classes distributed along juvenile and adult life stages, sampled during a pre-spawning and a post-spawning period. Biochemical biomarker profiles were differentiated across size classes during the pre-spawning period, consistent with distinct physiological strategies between juveniles and adults, whilst discrimination was reduced during the post-spawning. These results were consistent with body mass patterns, which diverged among classes during pre-spawning but were less differentiated following gamete release. Biomarker profiles thus differed across size classes and sampling period, with a significant interaction between size class and sampling period detected for six of the eleven biomarkers analyzed, highlighting the importance of accounting for the size-age relationship in biomonitoring studies to achieve a biologically realistic interpretation of results.