Low‐temperature preservation (at 4 °C) of marine rotifer Brachionus

Mass production of rotifers is essential as live food during the larval rearing season, but a major problem of rotifer culture is unpredictable culture collapse. If mass-produced rotifers could be kept alive at low temperature for an extended period of time, they could be supplied as live food to cultured marine fish larvae without interruption. Four experiments were performed to test this possibility in six strains of Brachionus plicatilis O. F. Muller and eight strains of Brachionus rotundiformis Tschugunoff. The results showed that: (1) B. rotundiformis strains were less tolerant to 4 ∞C than B. plicatilis strains. Among the B. rotundiformis strains, the strains known as SS type were the most susceptible and showed the lowest survival. (2) Exchange of culture media during the incubation at 4 ∞ Ci nB. plicatilis and B. rotundiformis resulted in higher survival than not changing the culture media, but there were no differences in the regression slope with or without changing the culture media. (3) Acclimation at 15 ∞C for 96 h for B. plicatilis and B. rotundiformis before transfer to 4 ∞C resulted in higher survival rates than acclim- ation at 10 ∞C. (4) The combination of frequent exchange of culture media and acclimation sig- nificantly improved the survival of B. plicatilis and B. rotundiformis compared with controls that were maintained at 4 ∞C without exchange of the culture media. Large-scale trials using B. plicatilis (Kamiura strain) cultured in 30-L tanks were conducted in a hatchery at a density of 2000-20 000 individuals mL -1 . Rotifers were transferred directly from 25 ∞C to 4 ∞C. About 50% of the rotifers at 20 000 individuals mL -1 survived after 14 days at 4 ∞C. These preserved rotifers could be cultured at 20 ∞C, recovering within 4 days.