ANALYSIS OF PHOTOTAXIS IN NEMATODES USING DIRECTIONAL STATISTICS

The direction of horizontal movement of 2 nematode spp. [Oncholaimus vesicarius and Enoplus anisospiculus] with eyespots was observed both with and without directional sources. The results comprise statistical proof of phototaxis in nematodes and demonstrate the usefulness of directional statistics in evaluating and comparing the responses. Two directional sources were tried, one including all the visible spectrum but red (blue source) and one containing only red. Both species had a significant tendency to move in the direction away from the blue source (0.degree.), whereas the directions taken under control conditions were not significantly different from random. A tendency to move away from the red source was significant for O. vesicarius but not for E. anisospiculus. Bimodal distributions with < 90.degree. between the modes occurred in at least 2 cases and their statistical treatment is illustrated and discussed. The component of the mean direction along the 0.degree. direction, rc, was significantly greater for O. vesicarius than for E. anisospiculus. A measure of the tendency to move horizontally indicated no significant difference between species. The difference in rc is probably not due to a difference in motivation. A difference in steering strategy is suggested by a negative correlation between rc and the angle between modes. The results are interpreted in terms of ocellus morphology and possible steering strategies.