Résumé
Environmental stressors are known to play an important role in determining the distribution and abundance of intertidal species. Marine molluscs are particularly susceptible to changes in water temperature and salinity in intertidal zones. Ruditapes decussatus and Ruditapes philippinarum are marine intertidal clams, constantly exposed to salinity variations in their intertidal habitat. The goal of this study was to investigate whether these species would handle salinity increase as well as salinity decrease, given the general paucity of data on R. decussatus and R. philippinarum exposed to salinity changes. In this context, leucine aminopeptidase (LAP) activity changes following exposure to salinity change were investigated in clams from Tunisia and from Brittany. Samples of R. decussatus from Tunisia were maintained in salinity of 10 parts per thousand, 20 parts per thousand, 30 parts per thousand, 45 parts per thousand and 55 parts per thousand for four weeks. Effects of salinity on LAP activity are also investigated in R. decussatus and in R. philippinarum from Brittany in salinities of 5 parts per thousand, 10 parts per thousand, 15 parts per thousand, 17 parts per thousand, 25 parts per thousand, 30 parts per thousand, 40 parts per thousand, 45 parts per thousand and 55 parts per thousand. Three sets of experiments were conducted: Short (20 h), medium (96 h) and long-term (four weeks). During salinity trials, LAP activity of R. decussatus ranged from 0.07 +/- 0.04 to 0.2 +/- 0.05 mu mol of paranitroanilide mg protein(-1) min(-1) and from 0.15 +/- 0.015 to 0.56 +/- 0.41 mg protein(-1) min(-1) for clams from Tunisia and from Brittany, respectively. In the R. philippinarum species, results showed the occurrence of higher activity levels, values ranged from 0.26 +/- 0.11 to 1.04 +/- 0.38 mu mol of paranitroanilide mg protein(-1) min(-1). None of the salinity regimes gave significant differences in LAP ctivity in R. decussatus from Tunisia. Conclusion has also been deduced from experiments carried out on R. decussatus and R. philippinarum from Brittany. Results reported here suggest that LAP ctivity seems to be not affected by salinity changes in populations investigated. The study provided some useful insights into response of the bivalves R. decussatus and R. philippinarum to salinity stress and offered a number of candidate hypothesis as potential explanations for moderate changes in LAP activity.