Change in activity in Baltic clam (Macoma balthica) exposed to elevated sulphate concentrations.

Behavioral endpoints have been increasingly used in ecotoxicology for their relevance to individual fitness and ecological effects. However, due to the lack of standard experimental protocols, behavioral endpoints are mostly excluded when deriving environmental quality guidelines. To increase reliability and comparability, behavioral endpoints need to be developed alongside standard endpoints in experiments incorporating elements of standard protocols. In this study, we explored a low-cost methodology to examine the use of activity as a chronic endpoint in a bivalve species in sulphate toxicity tests, following principles of the standard protocol. Adults and juveniles of Baltic clam (Macoma balthica) were exposed to sodium sulphate separately. The adult bivalves were individually inspected for their activity at frequent intervals. Activity was at first inhibited, but the extent of activity inhibition decreased and activity EC10 (10% effective concentration) increased as the exposure continued. In M. balthica juveniles, LC10 based on survival was 9001 mg/L sulphate. In M. balthica adults, activity EC10s (1713 − 3176 mg/L) were consistently lower than wet mass EC10 (5511 mg/L). Siphonal autotomy in M. balthica was observed at similar sulphate levels as the activity-based chronic EC10s. We found that the sensitivity of behavioral endpoints in major ion exposures depended on the duration of exposure and, consequently, they need to be evaluated in a case-by-case scenario in the regulatory ecotoxicological context.