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Substratum preference in the genus Dugesia Dirk, Justin 2012

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Substratum preference in the genus Dugesia     Justin Dirk University of British Columbia May 30th, 2012 Biol 140      ABSTRACT The purpose of this study was to determine if Dugesia had a preference of substratum among sand, small rocks, and pebbles. This was determined by a randomized choice experiment. Dugesia were placed in a petri dish with water and equal portions of each substratum. There were 20 replicates, with each replicate consisting of 3 minutes for the organism to make a choice. At the end of three minutes, the substratum at which the Dugesia spent the most time constituted a choice. Two trials were conducted, and the data were subjected to the chi-squared test. In both trials, Dugesia chose sand 5 times. In trial 1, they chose pebbles 6 times and small rocks 8 times. In trial 2 they chose pebbles 8 times, and small rocks 7 times. The trend shows an avoidance of sand, however the chi-squared test showed that the results of both trials were not statistically significant and therefore we fail to reject the null hypothesis. The results can be explained by the experimental protocol, and the fact that Dugesia constantly scan for food in the environment via ciliary gliding, which renders it more difficult to climb elevated surfaces. Our experimental protocol was set up such that the most time spent in a specific substratum constituted a choice. Since small rocks and pebbles are elevated with respect to Dugesia, it takes them longer to scan for food in those substrata when compared to sand thus explaining their preference for small rocks and pebbles according to our protocol.    INTRODUCTION Dugesia, commonly known as planaria, are a type of freshwater flatworm. They can be found in freshwater bodies all around Africa, Europe, Middle East, Asia, Australia and North America (Sluys et al. 1998, Harrison 2008). They inhabit all types of freshwater bodies, such as lakes, rivers, streams, and ponds, however they tend towards dark places, such as under logs, rocks, plants and other debris to stay cool and hide from danger (Harrison 2008).  Some of the specific requirements of their habitat include higher concentrations of oxygen in water, as Dugesia do not have a cardiovascular system and only absorb oxygen and release carbon dioxide through diffusion across their epidermis (Freeman 2008).  Dugesia are 10 mm long, 1 mm wide, and 0.1 mm thick. The body has an arrowhead- shaped head with a cigar-shaped body starting at the caudal end and continuing until just before the head. On the underside of the body near the middle there is an opening known as the pharynx (Columbia Electronic Encyclopedia 2007). This is where food and waste is ingested and expelled. (Freeman 2008). The head has two white circular eye structures, called ocelli (eye spots), which detect light intensity in the environment (Smales and Blankespoor 1978). The ocelli have black circular irises on the medial side that absorb light. Dugesia also have auricles, which are ear-like structures with flaps, which detect the strength of the current around them, as well as certain chemicals present in the water (Smales and Blankespoor 1978).  Dugesia are predators on the larvae of Aedes albopictus (Malo and Andrade 2001). Dugesia scan the water for chemical signs of the prey using their auricles, and if they are near, they move towards them (Smales and Blankespoor 1978, Harrison 2008). Once there, Dugesia extends its muscular pharynx and attaches to the larvae, and slowly releases digestive enzymes and sucks up loose soft tissue (Smyth 2012). All the larvae can do is attempt to escape by moving away; however, that is seldom possible (Melo and Andrade 2001). It also plays the role of detritivore; Dugesia eat dead and decaying organic material that has sunk to the bottom of the body of water, and recycle it back to the environment (Pennak 1978).  There are three important interrelated abiotic factors that play a role in the behavior of Dugesia. These are water temperature, light intensity, and substrata. Dugesia can only inhabit waters from 0-28oC (Kawakatsu 1964). Temperatures below or above this range are lethal to the Dugesia (Kawakatsu 1964). In its natural environment, water tends to range from 4oC near the bottom up to 25oC towards the surface, however, they tend to move towards cooler temperatures near the bottom (Harrison 2008). In bodies of freshwater, the sun is the most important factor in temperature change of the water (Marietta College Department of Biology and Environmental Sciences 2011). Dugesia tend to avoid light intensity from the sun, which can range from 25000 - 10000 lux at the surface during the day to 0.0001 lux at night, and hide in benthic areas, underneath the substrata where the light intensity is low (Kawakatsu 1964, Schlyter 2006, Harrison 2008). Substrata in bodies of freshwater Dugesia inhabit can range from varying rock sizes, to mud, to organic debris such as fallen trees and dead animals (Harrison 2008).  The objective of the experiment was to determine the preferred substratum of Dugesia from a choice of sand, small rocks, and pebbles. Substratum is important, as mentioned above, because it allows the Dugesia to shelter itself from the sunlight to avoid warmer water temperatures. The substrata chosen allow us to test this by offering no shelter (sand), slight shelter (pebbles), and complete (small rocks) from light. The null hypothesis was that the type of substratum would have no effect on the site selection of Dugesia, and the alternate hypothesis was that the type of substratum would have an effect on the site selection of Dugesia. Knowing that Dugesia tend to avoid high light intensity, we predicted that Dugesia will have a preference for small rocks as they offer the most shelter from light.  METHODS  The experiment consisted of two separate trials, each consisting of twenty replicates. The experiment was set up according to Figure 1. A large (140 mm diameter) petri dish was sectioned into three equal partitions and the three substrata were placed into separate partitions, with water being added to within 1mm of the brim. Water was allowed to equilibrate throughout the experimental setup to minimize the effects of changing water temperature. The effect of light intensity was also minimized, allowing only the laboratory lights to illuminate the experiment and eliminating all shadows on the experimental setup. To randomize the orientation of the substrata during replicates, sand was assigned one and two, small rocks were assigned three and four, and pebbles were assigned five and six. A die was rolled and the associated substratum to the number rolled was pointed north. A replicate was then conducted by placing a Dugesia in the middle of the experimental setup and timed for three minutes. The substratum chosen by the Dugesia was the one it spent the most time in during the three minutes. The assumption was that this constituted a decision by the Dugesia. To minimize experimental timing error, each experimenter was given a timer and three timed the presence of the Dugesia in one substratum, while one timed three minutes. A new Dugesia was used for each replicate to eliminate the possibility of the Dugesia’s experience in the experimental setup impacting the results. The chi-squared test was used to determine weather the choices were statistically different or random, i.e., did Dugesia have a preferred substratum, or move around randomly.  No large modifications were done between the trials; however, the orientation of the first ten replicates in trial one was not randomized.       Figure 1. Experimental setup used in both trials to determine the substratum preference of Dugesia.   RESULTS  In trial one, the Dugesia tended to choose small rocks, and avoid sand during the experiment, as seen in Figure 2. Twenty replicates were conducted, however one replicate resulted in a Dugesia not moving from the center and was not included in the analysis. Out of the nineteen replicates, the Dugesia chose sand five times, small rocks eight times, and pebbles six times.  Figure 2. Number of Dugesia choosing sand, small rocks, or pebbles as their preferred substratum in trial 1. Calculated chi-square value = 0.74, n = 19.   The calculated chi-squared value was 0.74 and the critical chi-squared value at p = .05 is 5.99. Since 0.74 is less than 5.99, we failed to reject the null hypothesis, as the data were not significantly different from random choice. Although the data were not significantly different in this trial, there were some interesting qualitative findings. Three out of the four Dugesia classified as being smaller than the rest of the group chose the pebbles, four out of the five classified as large or long chose the small rocks, three out of the three classified as fast or quick moving chose the sand, and three out of four classified as average chose small rocks.  In trial two, the Dugesia tended to choose pebbles, and avoid sand during the experiment, as seen in Figure 3. Twenty replicates were conducted, and of those, the Dugesia chose sand five times, small rocks seven times, and pebbles eight times.  Figure 3. Number of Dugesia choosing sand, small rocks, or pebbles as their preferred substratum in trial 2. Calculated chi-square value = 0.70, n = 20.   The calculated chi-squared value was 0.70. The critical chi-squared value at p = .05 is 5.99. Since 0.70 is less than 5.99, we again failed to reject the null hypothesis, as the data were not significantly different from random choice. Again, although the data were not significantly different in this trial, there were some interesting qualitative findings. As in trial 1, small Dugesia chose pebbles as three of the five small Dugesia used in the trial chose pebbles. We also had very, very small Dugesia and two of the three in this category chose sand. Three of the six classified as average chose small rocks, similar to trial 1. A qualitative difference from trial one to trial two was that the Dugesia classified as large or long chose pebbles the most at three out of six times, instead of small rocks.    DISCUSSION  For both trials 1 and 2, based on the chi-squared statistical values, we failed to reject the null hypothesis. Originally it was predicted that the Dugesia would tend towards the rocks as it provided a dark area, which, according to Harrison (2008), the Dugesia prefer in their natural habitat. According to Figures 2 and 3, the experiment was unable to provide support for the prediction that rocks would be preferred in both trials; the Dugesia tended towards both rocks and pebbles, while avoiding sand.  The choices by the Dugesia of preferred substratum can be explained by three interrelated arguments. The first is that Dugesia are negatively phototactic, which means they avoid light (Inoue et al. 2004). They use their ocelli to sense light intensity and avoid high intensity by sending neuronal signals from two ganglia under the epithelium of the ocelli to neurons down its body (Sarnat and Netsky 2002, Harrison 2008). Kawakatsu (1964) stated that the main reason Dugesia avoid light is to seek colder environments, and since the sun is the main thermoregulator in their natural habitat (Marietta College Department of Biology and Environmental Sciences 2011), they  avoid light by moving under rocks to keep cool. However since the experimental set up kept the water temperature constant, the Dugesia did not have to avoid the light to keep cool, and thus did not remain under the rocks. Also the laboratory used fluorescent lights which may not mimic the exact wavelength of light that trigger an avoidance response by the Dugesia. The second argument to explain Dugesia’s behavior in both trials is that Dugesia constantly scan the environment using the auricles on the lateral side of the head. These contain chemoreceptors and are used to detect chemicals in the water, especially those of food, and transmit this information to both ganglia (Smales and Blanespoor 1978). The information is then sent to the rest of the body to make adjustments in its movement to orient itself towards the stimulus (McConnell 1967, Sarnat and Netsky 2002). As Dugesia are detritivores and predators, they must constantly scan the environment for food (Pennak 1978, Malo and Andrade 2001, Harrison 2008). Since there was only 3 minutes for each replicate to make a choice, with no initial adjustment time, it is possible that when placed in the new environment, Dugesia spent its time exploring its surroundings for food sources. This was observed as the Dugesia swam around randomly, circling the petri dish. Thompson and McConnell (1955) confirm this explanation as they also observed that Dugesia, when initially placed in a petri dish of water, tended to explore their surroundings. They even corrected for this by having a 10-minute buffer at the beginning of each replicate to compensate, something that may have improved our experiment.  The final explanation of Dugesias behavior in the two trials of the experiment has to do with their mode of locomotion, ciliary gliding. Dugesia secret a layer of mucus on the substratum in front of them and cilia beat throughout the mucus to create movement (Pavlova 2000, Sugimoto 2010). As the substrata are different sizes, it would be harder to move up a small rock, or pebbles when compared to sand. As seen in Figure 1, the sand layer is flat, while pebbles and small rocks have elevations. This means that the Dugesia would have to secret more mucus, and beat their cilia harder to climb over pebbles and small rocks.  If Dugesia were also constantly scanning for food, as argued in the previous paragraph, they probably passed through the sand region quickly, as locomotion was easy, while in the pebbles and small rocks they stayed longer as locomotion was more difficult, thus leading to a choice according to our protocol. The data show that Dugesia spent 74% and 75% of the time for trial 1 and 2, respectively, in small rocks or pebbles, offering support to the notion that the Dugesia spend more time scanning the environment in pebbles and small rocks.  Pickavance (1971) found that Dugesia are attracted to the mucus left behind by other Dugesia. This is a factor that could have played an important role in our data. As the Dugesia spent more time in the rocks and pebbles, and more mucus was used for locomotion, as the number of replicates increased, the subsequent Dugesia would be attracted to those regions, therefore increasing the number choosing those substrata. As well, biological variation also played a role in the results as it was observed that very small Dugesia tended towards sand in trial 2, while in both trials, small Dugesia tended towards pebbles, and as well in both trials, average to large size Dugesia tended towards rocks and pebbles. Smaller Dugesia may be unable to climb certain substrata.  A source of error was that the first 10 replicates of trial 1 were not randomized. Although it is hard to say if this had an affect on the results, it cannot be ignored as Dugesia may be oriented to the earth’s magnetic field, or the table may not have been level, adding an effect of gravity. Finally, for each trial, there were only 19 or 20 replicates, which may have also been the cause of lack of statistically significant results. To improve the results of similar experiments, the substratum should be cleaned in between each replicate, and more replicates should be tested.  CONCLUSION  We failed to reject the null hypothesis in both trial 1 and 2. Dugesia did not have a substratum preference in this experiment.   ACKNOWLEDGEMETS  The author would like to thank Modhita Sethi, Ethan Yeung, and Ellen Kim for their help in collaborating with setting up the hypotheses, conducting the experiment and brainstorming for the discussion. The author would also like to thank Junxia Zhang for her support in the laboratory by answering any and all of his questions. Finally, the author would like to thank Dr. Carol Pollock and the University of British Columbia for the opportunity and materials to conduct this experiment.  LITERATURE CITED Columbia Electronic Encyclopedia, 6th edition. 2007. Planarian [online]. Available from http://lycos.infoplease.com/ce6/sci/A0839279.html [accessed 29 January 2012].  Freeman, S. 2008. Biological Science, 3rd edition. Pearson Benjamin Cummings, San Francisco, USA  Harrison, N. 2008. A website with information on Dugesia [online]. Available from http://pioneerunion.ca.schoolwebpages.com/education/components/scrapbook/default.php?sec tiondetailid=2781 [accessed 29 January 2012].  Hiroko, T., and Kumiko, O. 1985. A temperature gradient apparatus and temperature preference of the thermally acclimated planarian, Dugesia japonica. Comp. Biochem. Physiol., 82A (4): 805-807.  Inoue, T., Kumamoto, H., Okamoto, K., Umesono, Y., Sakai, M., Alvarado, A.S., and Agata, K. 2004. Morphological and functional recover of the planarian photosensing system during head regeneration. Zoological Science, 21 (3): 275-283  Kawakatsu, M. 1964. On the ecology and distribution of freshwater Planarians in the Japanese islands, with special reference to their vertical distribution. Hydrobiologia, 26 (3-4): 349-408.  Marietta College Department of Biology and Environmental Sciences. 2011. A website on freshwater vs. Marine habitats [online]. Available from http://www.marietta.edu/~biol/biomes/lakes.htm [accessed 29 January 2012].  McConnell, J.V. 1967. A manual of psychological experimentation on planarians. Planarian Press, Ann Arbor, MI.  Melo, A.S., and Andrade, C.F. 2001. Differential predation of the planarian Dugesia tigrina on two mosquito species under laboratory conditions. Journal of the American Mosquito Control Association, 17 (1): 81-83.  Pavlova, G.A. 2000. Ciliary locomotion of the mollusk is different from that of the Planaria. Doklady Biological Sciences, 375 (3): 630-632.  Pennak, R.W. 1978. Fresh-water Invertebrates of the United States, 2nd edition. John Wiley and Sons, Inc., Hoboken, New Jersey.  Pickavance, J. R. 1971.  The diet of the immigrant Dugesia tigrina (Girard):I. feeding in the laboratory. Journal of Animal Ecology, 40 (3): 623-635.  Sarnat, H.B., and Netsky, M.G. 2002. When does a ganglion become a brain? Evolutionary origin or the central nervous system. Seminars in Pediatric Neurology, 9 (4): 240-253.  Schlyter, P. 2006. Radiometry and photometry in astronomy FAQ [online]. Available from http://stjarnhimlen.se/comp/radfaq.html#10 [accessed 15 May 2012].  Sluys, R., Kawakatsu, M., and Winsor, L. 1998. The genus Dugesia in Australia, with it phylogenetic analysis and historical biogeography (Platyhelminthes, Tricladida, Dugesiidae). Zoologica Scripta, 27 (4): 273-289.   Smales, L.R., and Blankespoor, H.D. 1978. The epidermis and sensory organs of Dugesia tigrina (Tubellaria: Tricladida). Cell and Tissue Research, 193 (1): 35-40.  Smyth, J.D. 2012. Encyclopedia Britannica Online, flatworms [online].Available from http://www.britannica.com/EBchecked/topic/209735/flatworm?anchor=ref529388 [accessed 29 January 2012]  Sugimoto, T. 2010. A theory for ciliary gliding in freshwater planarians. Journal of Aero Aqua Bio-mechanics, 1 (1): 57-63. Thompson, R. and McConnell, J.V. 1955. Classical conditioning in planarian, Dugesia dorotocephala, J. Comp. Physiol. Psych, 48 (1): 65-68.  Vowinckel, C., and Marsden, J.R. 1971. Reproduction of Dugesia tigrina under short-day and long-day conditions at different temperatures. Journal of Embryology and Experimental Morphology, 26 (3): 587-598.             APPENDIX A. Sample Calculations  Trial 1 Expected Values = (Total replicates)/(Number of choices) = 19/3 = 6.333333333 Table 1. Observed and expected results for substratum selected by Dugesia in trial 1.  Observed Expected (Obs.-Exp.) 2 ((Obs.-Exp.)2)/Exp. Sand 5 6.333333333 1.777777778 0.280701754 Small Rocks 8 6.333333333 2.777777779 0.438596491 Pebbles 6 6.333333333 0.11111111 0.017543859  (Observed-Expected)2 = 5 – 6.333333333 = 1.777777778 ((Observed-Expected)2)/Expected = 1.777777778/6.333333333 = 0.280701754 Chi-squared = 0.280701754 + 0.438596491 + 0.017543859 = 0.73684214 Rounded = 0.74 Degrees of freedom = 3 classes – 1 = 2  Therefore Critical X2 = 5.99 0.74 < 5.99 therefore the experimenters fail to reject Ho         


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