Abstract
Taxis is the orientation toward or away from a stimulus. Planaria, in their natural habitat, are known to exhibit positive geotaxis and negative phototaxis. This experiment was conducted with nine different planaria which were observed in the amount of time spent in each condition. In the first set of trials, their preference for gravity (geotaxis) was measured by calculating the mean time spent in each half. The mean turned out to be 112.7 seconds 112.1 (mean SE) in the top and 185.7 seconds 112.9 (mean SE) in the bottom. In the second set of trials, their preference for light (phototaxis) was measured the same way. The mean was found to be 192.7 seconds 88.5 (mean SE) in the illuminated half and 107.3 seconds 88.5 (mean SE) in the dark.
Introduction
Dugesia is a freshwater flatworm of the phylum Platyelminthes. They are found primarily in lakes and streams (Pongratz, et al., 2002). At the anterior end of the planarian are a set of eyespots that are photosensitive and used for negative phototaxis (Wisenden and Millard, 2001). Taxis is the orientation towards (positively) or away from (negatively) a stimulus. This experiment tested two different types of taxis: phototaxis, or the orientation in response to light, and geotaxis, or the orientation in response to gravity. Planaria exhibit negative phototaxis (Agata et al., 2004). Planaria also exhibit positive geotaxis (Beauchamp, 1932). Based on this knowledge, the experimental hypothesis that was tested in this experiment was the planarian will show a preference to dark half of the test tube over light half and the bottom of the test tube over the top. With that as the experimental hypothesis, the null was stated that the animal will show no preference. These hypotheses were tested in this experiment.
Methods and Materials
On February 14, 2008, one large test tube was obtained and marked by a grease pencil at the middle. One planarian was obtained from a habitat bowl by a pipette and added into the test tube. The test tube was then filled the rest of the way with pond water and stoppered by a rubber or cork stopper. Then, by manipulating the test tube, the planarian was lined up at the middle, placed horizontally in a test tube rack, and a timer was set for 300 seconds. The planarian was then observed, and the amount of time it spent in the upper half and the lower half was then recorded and statistically analyzed by use of mean and a Two Tailed T-Test.
Two-thirds of the water was then removed and replenished with the pond water, to restore the control environment and then stoppered. A sleeve of aluminum foil was fitted to the test tube, to create a dark half. The planarian was then lined back up at the midline of the test tube. The sleeve was then applied and the test tube was place horizontally under a lamp. The planarian was then observed for another 300 seconds, and the amount of time it spent in the dark half and the light half was then recorded and statistically analyzed by use of mean and a Two Tailed T Test.
Results
After conducting the geotaxis experiment, the means and standard error were found. In the upper half, the mean was found to be 112.7 seconds 112.1 (mean SE). In the lower half the mean was found to be 185.7 seconds 112.9 (mean SE). According to the Two Sample T-Test, T=4.26, which yielded a p value less than 0.05.
After conducting the Phototaxis experiment, the means and standard error were found. In the illuminated half, the mean was found to be 192.7 seconds 88.5 (mean SE). In the sleeved half, the mean was found to be 107.3 seconds 88.5 (mean SE). The T value given by the Two Sample T-Test was T= 9.2 which yielded a p less than 0.05.
Discussion
After analyzing the data given by the individual Two Sample T-Test, statistical significance can be determined. In the geotaxis experiment, p<0>s pipetted up, squirted into a test tube, shaken up to line it up to the middle, and yet we still expect it to act as it would in nature. To correct this, possibly allow the planarian to be in the test tube for a longer amount of time, say 1,800 seconds as apposed to 300 seconds. More time would allow for the planarian to be more adjusted and act as it would in the wild. To expand upon the current experiment, I would test other stimuli, such as current or the presence of food, predator, or prey to observe how planaria react to it.
Literature Cited
Agata, K., Inoue, T., Kumamoto, H., Okamoto, K., Makai, M., Sanchez Alvarado, A., and Umesono, Y.. 2004. Morphological and functional recovery of the planarian photosensing system during head regeneration. Zoological Science. 21: 275-283.
Beauchamp, R.S.A. 1932. Rheotaxis in Planaria alpine. J. Exp. Biol. 10: 113-129.
Pongratz, N., Gerace, L, and Michiels, N. K.. 2002. Genetic differentiation within and between populations of hermaphrodititic freshwater planarian. Heredity. 89: 64-69.
Wisenden, B. D., and Millard, M. C.. 2001. Aquatic flatworms use chemical cues from injured conspecifics to assess predation risk and to associate risk with novel cues. Animal Behaviour. 62:761-766
