Effects of sodium chloride exposure on ion regulation in larvae (glochidia) of the freshwater mussel Lampsilis fasciola

• The mode of action of chloride toxicity in glochidia is ionoregulatory disturbance.
• Ionoregulatory disturbance is mainly associated with an elevated Cl− influx. ​
• Hypo and hyper-concentrations of NaCl can stimulate the uptake of both Na± and Cl-.
• Recovery of Na+ influx to background levels in glochidia occur 3 h after salt exposure.

The salinization of freshwater can have negative effects on ecosystem health, with heightened effects in salt-sensitive biota such as glochidia, the larvae of freshwater mussels. However, the toxicological mechanism underlying this sensitivity is unknown. Therefore, Lampsilis fasciola glochidia were exposed to NaCl (nominally 0.25 and 1.0 g/L) prepared in reconstituted moderately-hard water (control), as well as to a dilution of that water (1:4) with ultrapure reference water (diluted control). Unidirectional Na+ influx (measured with 22Na) was evaluated after 1, 3 and 48 h of exposure. In addition, unidirectional Cl− influx (measured with 36Cl), whole-body ion (Cl− and Na+) concentrations, and glochidia viability (measured as the ability to close valves) were assessed after 48 h of exposure. Significantly reduced glochidia viability (56%) was observed after exposure to 1.0 g/L NaCl. Na+ influx was significantly higher in glochidia exposed to both 0.25 and 1.0 g/L NaCl for 1 h than in those kept under control conditions. After 3 and 48 h of exposure, differences in Na+ influx rate between salt-exposed and control glochidia were generally reduced, indicating that larvae may be able to, at least temporarily, recover their ability to regulate Na+ influx when exposed to elevated NaCl concentration. Compared to the moderately-hard water control, whole-body Na+ and Cl− concentrations were relatively unchanged in glochidia exposed to 0.25 g/L NaCl, but were significantly elevated in glochidia exposed to 1.0 g/L NaCl and the diluted control. While Na+ influx rate had recovered to the control level after 48 h of exposure to 1.0 g/L NaCl, Cl− influx rate remained elevated, being ~7-fold higher than the Na+ influx rate. These findings suggest that the loss of viability observed when glochidia were exposed to a high NaCl concentration (1.0 g/L) could be caused by ionoregulatory disturbances mainly associated with an elevated Cl− influx.