The movement characteristics of Pacific oyster trochophores have received very little coverage in the scientific literature. Described here are not only changes in the swimming characteristics of Crassostrea gigas trochophores (size: 53-77 mu m) in relation to time after fertilization and to inter-female variation but also the effects of salinity and pH on movement characteristics. The percentage of motile trochophores was measured on images obtained through a dissecting microscope and the Velocity Average Path (VAP) was assessed using a Computer Assisted Sperm Analysis (CASA) system. At 20 degrees C, the first movements of the trochophores were observed at 6.5 h post-fertilization. The mean (+/- SD) percentage of swimming trochophores and the VAP observed in seawater at 11.5 h post-fertilization were 85 +/- 10% and 146 +/- 75 mu m s(-1), respectively (n = 8 females). Significant inter-female variation in the percentage of motile trochophores (range: +/- 16-93 +/- 3%) and in the VAP (55 +/- 47 -180 +/- 90 mu m s(-1)) was detected. Most of the trochophores were motile in a 9 parts per thousand salinity solution. Adjusting the pH of the seawater to values from 5.10 to 9.08 had no effect on swimming characteristics. The present study shows that the movement of oyster trochophores exhibits high plasticity in relation to environmental conditions because the highest percentages of swimming trochophores and optimal velocity values were recorded within large ranges of salinity and pH. Further research is required to determine whether the swimming performances of trochophores can be used to assess embryo quality in commercial hatcheries.