Effect of water viscosity on the population growth of the rotifer Brachionus plicatilis Muller

We measured the relative viscosity of water taken from a 500 ml rotifer batch culture against natural sea water using a capillary type viscometer (Ostwald type) during 30 days. The relative viscosity ranged from 1.054 to 1.148. We observed a negative correlation between population growth rate and viscosity. We experimentally tested the effect of viscosity on reproduction, swimming activity and ingestion rate of Brachionus plicatilis. By dissolving methyl cellulose, the relative viscosity of natural sea water (salinity 22 ppt) was increased from 1.0 (control group) to 1.022, 1.031, 1.078 and 1.169. At higher viscosity, individually cultured rotifers at 25 degrees C that were fed Nannochloropsis oculata showed slower swimming, lower ingestion rate and lower fecundity than control. By increasing the relative viscosity from 1 to 1.169, the swimming activity index decreased from 28.3 to 4.9 (1 mm(2) square grids/30 s), the ingestion rate decreased from 1.13 to 0.34 (x10(3) cells ind(-1) h(-1)), and the number of offspring per amictic female decreased from 20.8 +/- 3.0 to 8.1 +/- 3.7 (mean +/- SD).