We investigated the influence of light on phototactic behavior and reproduction in two species of rotifer from the Brachionus plicatilis species complex (B. plicatilis sensu stricto (s. s.) and Brachionus manjavacas). This was done to understand how light effects these species so that we might use this knowledge to establish a more efficient aquaculture protocol. We used four different light wavelengths (white, with peaks at 460 and 570 nm; blue at 470 nm; green at 525 nm; and red at 660 nm) and four intensities (i.e., 0.5-30.0 W/m 2 ). Using microspectrophotometry we determined that eyespots of these two Brachionus species absorbed blue and green light 5.5 times more than red light. B. plicatilis s. s. showed positive phototaxis under white, blue, and green light at lower light intensities, but no phototaxis under red light at all intensities (0.5, 6.2, 15.0, and 30.0 W/m 2 ). Similar patterns of phototaxis were observed in B. manjavacas and did not differ among mictic, amictic females, and male rotifers. Population growth rate of B. plicatilis s. s. under dark condition was 1.1-1.2 times higher than that under white light condition. No significant differences were observed in population growth rate at 3.8 and 6.2 W/m 2 at all light wavelengths. On the other hand, population growth rates at 0.5 and 1.6 W/m 2 were the lowest under blue light. According to these results, both wavelength and intensity of light affect the population growth of rotifers, which in turn may be influenced by the rotifers' wavelength-dependent phototaxis.

Rotifers have a light sensor called "eyespot" which is expected to be composed of rhodopsin. Based on the molecular feature of rhodopsin as regenerated with 11-cis-retinal, we hypothesized that phototactic behavior should be affected by the nutritional level of food; especially vitamin A availability. This study intended to address the following questions on the nutritional effects of using baker's yeast (Saccharomyces cerevisiae) and Nannochloropsis oculata: how does diet affect the pigmented area and absorbance of the eyespot, and how do these changes characterize phototactic behavior and population growth in the monogonont rotifer Brachionus plicatilis sensu stricto. The pigmented area of the eyespot decreased to 14.7 μm 2 with baker's yeast while it was maintained at the initial size of 82.9 μm 2 with N. oculata. Maximum absorbance of the eyespot was observed at a range of 470 to 525 nm in the initial rotifers and it was not significantly changed with diet type and culture day. The value of the maximum absorbance was maintained with N. oculata, while it rapidly decreased on day 10 with baker's yeast. Stronger positive phototaxis with N. oculata was observed under lower light intensity (0.1 and 0.5 W m-2) at 470 nm. On the other hand, phototaxis with baker's yeast became weak and no phototactic reactions were observed under the same lighting condition. From the genomic DNA database of rotifers, 12 putative opsin-relevant genes were identified. These results corroborate the hypothesis that rhodopsin is the visual pigment in the rotifer eyespot. Lack of vitamin A with baker's yeast should induce reduction of the pigmented area and the sensitivity of the rotifer eyespot resulting in weak phototaxis. The population growth of rotifers showed different patterns related to the food type and light intensity. The lowest population growth (0.33-0.37 day-1) was shown with baker's yeast diet at 0.5 W m-2. This phenomenon may be significantly related to malnutrition on baker's yeast which is deficient not only in vitamin A but also fatty 3 acids, vitamin B12 and its derivatives.