## 4 Discussion
The detoxification mechanisms were probably dynamic over time and not constant, being alternately activated and exhausted, explaining the great variations observed in the temporal accumulation rates of _A. anatina_. The cumulative accumulation factor being overall higher during exposure to 100 \(\mu\)g MC/L compared to 10 \(\mu\)g MC/L might be explained by exhaustion of detoxification processes at the highest concentration of exposure. Regarding _D. polymorpha_, its detoxification systems may have been more efficient, as illustrated by its constant and far lower MC accumulation factors than the ones of _A. anatina_. Moreover, our results suggest that the mechanisms of MC detoxification may have been triggered earlier and at a lower exposure concentration (from 1 \(\mu\)g MC/L) in _D. polymorpha_ compared to _A. anatina_. This hypothesis is also supported by the literature. Indeed, _D. polymorpha_ showed elevated sGST levels after 24-h exposure to 10 or 50 \(\mu\)g MC/L of a cyanobacterial crude extract, an increase still visible but not significant after seven days of exposure [66]. Here, _A. anatina_ accumulated a higher percentage of ingested MC compared to _D. polymorpha_. However, the comparison was performed between the digestive gland of _A. anatina_, MC target organ, and whole individuals of _D. polymorpha_. We therefore also compared the total MC concentrations in whole _A. anatina_ versus whole _D. polymorpha_, and in the digestive glands of both species. Results still showed a higher accumulation in _A. anatina_ compared to _D. polymorpha_, which was previously reported and attributed to _D. polymorpha_ selective feeding and its strong MC biotransformation capacities. [39]. Potential Use of the Two Bivalves as Bioindicators of the Presence of MC-Producing Cyanobacteria in Fresh Waters  
The main criteria for a species to be qualified as a bioindicator is to be able to accumulate toxins in proportion with the level of the environmental contamination. The species should also be able to highlight the dynamic of this contamination. In addition, to detect cyanobacterial blooms' onset, bioindicator species must accumulate MC from low densities of their producers in water. Here, both bivalve species were able to reveal low contamination levels of intracellular MC. Indeed, MC were detected in digestive glands of _A. anatina_ from the first day of exposure, when exposed to approximately \(2.0\times 10^{5}\pm 5.0\times 10^{4}\) cells/L (density of filaments producing an equivalent of 1 \(\mu\)g MC/L). This density is lower than the one used in a 24 h-exposure of _Unio elongatulus_ [PERSON], 1825 to _M. aeruginosa_ (\(2.0\times 10^{6}\pm 4.5\times 10^{5}\) cells/L) where MC were detected in five out of six mussels [28]. _D. polymorpha_ was able to reveal MC presence when exposed to approximately \(2.0\times 10^{5}\pm 4.0\times 10^{4}\) cells/L (producing an equivalent of 1 \(\mu\)g MC/L). However, MC content in digestive gland of _A. anatina_ was not time nor concentration dependent under these laboratory conditions. Thus, digestive glands of _A. anatina_ might not be pertinent to evaluate levels of water contamination by cyanobacteria over a short timeframe. _A. anatina_ musselsmay, therefore, be useful to assess the presence or absence of MC-producing cyanobacteria in medium- or long-term studies with monthly animal samplings. _D. polymorpha_ started to accumulate later (from day three) at the lower concentration of exposure (1 \(\upmu\)g MC/L), but was further able to accumulate both total and free MC more steadily (with few variations of accumulation ratios over time and among concentrations) and more proportionally to the concentration present in the medium than _A. anatina_. Thus, _D. polymorpha_ could be more appropriate to estimate the levels of intracellular MC concentrations in fresh waters, pending further investigations to assess the stability of the correlation between both parameters.