EXPOSURE OF THE SANDY-BEACH BIVALVE DONAX-SERRA RODING TO A HEATED AND CHLORINATED EFFLUENT .2. EFFECTS OF CHLORINE ON BURROWING AND SURVIVAL

The effect of chlorine as free residuals in the range of 0.1 to 1.2 pp m on the survival and burrowing ability of the sandy-beach bivalve Don ax serra was investigated. Median lethal times were calculated from da ily observations over 2 weeks of the number of dead and buried D. serr a. Recovery in nonchlorinated sea water was monitored for 12 days afte r the transference of some individuals every 24 hours. The addition of chlorine resulted in immediate valve closure, a position maintained f or 6 hours at concentrations <0.3 ppm and up to 8 days al >0.6 ppm. No median lethal times were measurable at <0.6 ppm, but above this conce ntration, the median lethal times approximated 10 days. After 14 days' exposure to between 0.6 and 1.2 ppm, 90 to 100% of the bivalves died. Full recovery in fresh sea water occurred after 6 days' exposure to a ll chlorine concentrations. Longer exposure to >0.6 ppm resulted in 50 % mortality during the recovery test, whereas below this concentration , >50% recovered but burrowing was delayed. Chemoreceptors on the siph ons and mantle edge of D. serra probably enable the rapid detection of chlorine and hence immediate valve closure. Possible modes of action of chlorine on physiological fitness are discussed. Long-term (>14 day s) survival and retention of burrowing ability were invariable at chlo rine concentrations <0.6 ppm.