Nutritional effects on the visual system of the rotifer Brachionus plicatilis sensu stricto (Rotifera: Monogononta)

Abstract

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 14.7μm2 with baker's yeast while it was maintained at the initial size of 82.9μm2 with N. oculata. Maximum absorbance of the eyespot was observed at a range of 470 to 525nm 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 0.5Wm−2) at 470nm. 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 (0.33–0.37day−1) was shown with baker's yeast diet at 0.5Wm−2. This phenomenon may be significantly related to malnutrition on baker's yeast which is deficient not only in vitamin A but also in fatty acids, vitamin B12 and its derivatives.