Straight-hinge, umbo and pediveliger larvae of the mactrid bivalves Spisula solidissima, Mulinia lateralis and Rangia cuneata were exposed to intense salinity discontinuities of 0, 5, 10 and 15 parts per thousand in vertically oriented swimming chambers. Larvae concentrated in the region of highest gradient, i.e. at the salinity discontinuity, regardless of species, stage of development or larval brood. S. solidisima larvae, initially swimming at 30 parts per thousand salinity, crossed discontinuities of both 5 and 10 parts per thousand but not of 15 parts per thousand. M. lateralis larvae, initially swimming at 25 parts per thousand salinity, also crossed a discontinuity of 5 parts per thousand but not of 10 or 15 parts per thousand. R. cuneata larvae, initially swimming at 10 parts per thousand salinity, generally preferred to remain at that salinity. Swimming and passive sinking velocities, defined as vertical distance traversed per unit time, were measured in different salinities under constant temperature and light. For all species, swimming rate changed with larval stage, highest velocity occurring at the umbo stage. Upward swimming rate of S. solidissima larvae ranged from 0.18 to 0.49 mm s-1 and increased with increasing salinity. Upward swimming rate of M. lateralis larvae ranged from 0.25 to 0.50 mm s-1, but was not consistently related to salinity. Upward swimming rate of R. cuneata larvae ranged from 0.18 to 0.53 mm s-1; swimming rate of pediveliger larvae increased consistently as salinity decreased. Downward swimming rates were similar to upward rates. No significant differences in downward swimming rate were detected in relation to salinity. Passive sinking was more frequent than active downward swimming in umbo and pediveliger larvae. Sinking rate increased with larval size of S. solidissima and M. lateralis larvae; however, R. cuneata straight-hinge larvae sank faster than umbo and pediveliger larvae. Species-specific differences in larval sinking and swimming are related to the different habitats occupied by adults. Larvae of S. solidissima, a marine stenohaline species, remained in high salinity water. Larvae of M. lateralis, a euryhaline species, use their preference for discontinuities or higher salinity water to assist retention in partially mixed estuaries. High sinking rate and short larval period of R. cuneata may offset the behavioural characteristic and aid in the retention of R. cuneata larvae in the low salinity zone of most partially mixed estuaries
