External factors that regulate movement in the marine rotifer Brachionus plicatilis

There are two types of movement pattern in the Brachionidae (rotifers), swimming and attachment, although the factors that induce a shift between them have not been adequately clarified. This study investigated the effects of five external factors—food limitation, temperature, salinity, predator, and un-ionized ammonia—on movement in females of the euryhaline rotifer Brachionus plicatilis sensu stricto. Rotifers under periodic starvation showed increased swimming frequency (percentage of swimming rotifers) to about 50% on feeding compared to controls under continuous feeding (21%). Starved rotifers were further exposed to the following conditions for 2 days: a range of water temperature (15–25 °C), salinity (17–34 practical salinity units), a predator (or other rotifers)-conditioned medium, and un-ionized ammonia (NH3-N; 5–20 mg/l). Neither temperature nor predator conditioning significantly affected rotifer swimming frequency. However, rotifers transferred to a higher salinity or to a rotifer-conditioned medium ceased to exhibit swimming. All the tested ammonia levels caused vigorous swimming of rotifers during the initial experimental period. The results indicate that swimming in female rotifers can be classified as (1) an escape behavior induced by environmental stress, and (2) a response to higher viability under certain favorable environmental conditions.