Phototactic behavior of live food rotifer Brachionus plicatilis species complex and its significance in larviculture: A review

We review a recent progress of photophysiological studies in the rotifer Brachionus plicatilis species complex. The rotifers have a light sensor i.e., eyespot inducing phototactic behavior. For the rotifer B. plicatilis sp. complex, the eyespot efficiently absorbs the light wavelength ranging from 450 to 550 nm. The function of eyespot is affected by diet species from 30-day batch cultures fed by either microalgae Nannochloropsis oculata or baker's yeast Saccharomyces cerevisiae. By feeding baker's yeast, rotifer eyespot gradually lost its function: area (5.5 times) and absorbance (2.2 times) decrease compared to those fed by N. oculata. Phototactic behavior and reproductive characteristics of the rotifer B. plicatilis sp. complex varied with different light wavelengths and intensities. The rotifers show light wavelength dependent phototaxis associated with the reception of an eyespot. For the phototactic behavior in horizontal level, light intensity is also a significant factor to regulate phototaxis. The rotifers show strong positive phototaxis under blue (peaks at 470 nm), green (525 nm), and white (460 and 570 nm) lights at 0.5 W/m2. Rotifer reproduction is also affected by light wavelength and intensities. Asexual reproduction of rotifers is accelerated by green and red lights at 0.5 W/m2. On the other hand, active sexual reproduction is observed with blue light at 1.4 W/m2. Under a certain light condition inducing active phototactic behavior, the rotifers show continuous swimming movement without attaching to substrates. The different behaviors associated with light conditions affect the reproductive characteristics of rotifers. The regulation of live food distribution is significant for feeding efficiency of fish larvae. The efficient feeding promotes larval growth and survival: hence it is a significant factor for successful larviculture. Fish larvae also show different phototactic behavior related to light wavelengths and intensities. Therefore, the distributions of fish larvae under the applied light conditions should be considered.