We examined the effect of low temperature treatment (12 degrees C), followed by transfer to higher temperature (25 degrees C), on resting egg formation of the rotifer Brachionus plicatilis, Kamiura strain. This strain has been mass cultured as live feed at Kamiura Station (Japan Sea Farming Association) for 9 years at 20 degrees C without the appearance of sexual reproductive stages. After preculture in 20 1 of 27 parts per thousand sea water at 12 degrees C for 0, 10, 20, and 30 days, rotifers were inoculated into 0.5 1 mass cultures and cultured at 25 degrees C for 7-9 days. The inoculation densities were changed from 20 to 400 ind./ml, depending on mixis rate. Condensed and frozen Nannochloropsis oculata was fed to rotifers at the feeding rate of 0.14 mu g (dry weight)/rotifer/day. The control was cultured at 12 degrees C for the entire 36 day experiment. No mixis appeared and no resting eggs were produced when the low temperature treatment was 0 or 10 days. However, mixis rates reached 50-60% after 20 or 30 days of exposure to 12 degrees C. The number of resting eggs produced in these treatments reached 25,500 about 13 times higher than the control. Our results suggest that low temperature stimulated mictic female production and the transfer to the high temperature accelerated resting egg formation. This method may be useful for producing resting eggs of rotifer strains that lack sexual reproduction in the common culture condition at larval rearing facilities.
(Japan Sea Farming Assoc., Kamiura Stn., Oita 57926, Japan)
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