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Effects of chlorination on two larval stages of the oyster Crassostrea virginica (Gmelin, 1790), were studied in flowing estuarine water. Straight-hinge veliger larvae were exposed to concentrations of 0.01, 0.05, 0.1 and 0.2 ppm chlorineproduced oxidants (CPO) for 6, 12,24 and 36 h, and to 0.05, 0.1,0.2 and 0.3 ppm CPO for 8, 24, 48,72 and 96 h. Setting pediveliger larvae were exposed to 0.05, 0.1,0.2 and 0.3 ppm CPO for 6, 24, 48, 72 and 96 h in two experimental trials. Mortality of straighthinge and pediveliger larvae was directly related to increased concentrations of CPO and extended exposure tlme. Pediveliger larvae were generally more resistant to CPO than straight-hinge larvae, especially with longer exposure time. Equations for predicting mortality under different conditions of time and CPO concentration were developed using these data.

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Thermal tolerance was tested in cleavage stages, trochophores and straight hinge larvae of the bay scallop Argopecten irradians. Experiments were des~gned to simulate larval entrainment in power plant coollng systems. An 11 (temperature) X 8 (time) matrix was used with temperatures ranging from 20.6 to 43.0 "C and exposure times from 1 min to 6 h. Pooled mortality data from triplicate experiments for each larval stage were subjected to stepwise regression analysis (darcsine 90 mortality on temperature and time). Equations derived from these analyses, comprising first, second and third order terms for temperature and time, were used to generate response surfaces relating mortality to temperature and exposure time. In general, higher mortalities were associated with higher temperatures and with longer time exposure at any one temperature. However, there was some evidence of cold shock in trochophore and straight hinge larvae, with elevated mortalities occurring at temperatures lower than the spawning temperature. There was a trend towards increased thermal tolerance in older larvae, although in general A. irradians showed the greatest sensitivity to thermal shock of the estuarine bivalves so far tested in our laboratory.