Isopods Armadillidium Vulgare and their taxic response to light and humidity

Experiment conducted by Augustine A. Zavala and Elizabeth M. Beck-Johnson

The purpose of this experiment was to determine whether isopods, in this case Armadillidium vulgare , would display a significant response to varying conditions of humidity and light. Based on the location and habitat of these crustaceans, we hypothesized that they would move away from the light and migrate towards moisture. This is known as positive hydrotaxis and negative phototaxis. In our experiment, we had six groups with ten isopods each. The apparatus used for the experiment were two pairs of fused petri dishes, one modified for studying photo taxis, the other for studying hydrotaxis. Each group measured the movements of the isopods over a period of ten minutes each for both modified Petri dishes. The data was averaged, and the chi-square statistical formula was used to determine whether the response to light and moisture was significant. The critical value of 3.8 was exceeded on both the hydro taxis and photo taxis portion of the statistical analysis, demonstrating that there was a significant response to these conditions.

 

 

The movement of organisms in response to changes in environment, response to stimuli has always been intriguing to man.  In our experiment, we studied a significantly smaller organism, isopods, and study their movements or taxis.  Taxis is defined as generated movement in response to a stimulus.  The stimulus could be light, humidity, temperature or food.  In our experiment we used light and humidity to observe the taxis of these Armadillidium vulgare . 

 

Isopods in Greek means “similar or equal foot” (Fossweb, 2006).  A. vulgare   is commonly known as roly poly, pill bug, sow bug.  It is related to other isopods like crabs, shrimps, and lobsters.  Isopods live in dark, damp environments and feed on decaying matter.  A wet environment is important because isopods use gills to breathe.  Based on the environment that A. vulgare lives in, our hypothesis was that isopods would demonstrate positive hydro taxis and negative photo taxis.  A study conducted on isopods by Waloff (1941) indicated that they are sensitive to humidity and light.  In that study the isopods demonstrated positive hydrotaxis and negative phototaxis.  

 

 

MATERIALS and METHODS

The class was divided into six groups with the groups randomly receiving ten A. vulgare each.  An experimental apparatus was constructed where one quarter of two Petri dishes was cut off and the dishes were glued together on a wooden board.  The two dishes together created an opening between two open areas.  The animals could move freely between the dishes.  The Petri dishes modified for phototaxis had one side lid covered with black tape.  The pair of petri dishes modified for hydrotaxis had a moistened paper towel placed on one end with the other side left dry.  The paper towel was moistened with ten drops of tap water.  When the isopods were introduced to each pair of dishes, care was taken to place the lids carefully so that no A. vulgare were crushed.

After the ten isopods were introduced to the petri dishes, one person from each group timed the movement of the crustacean ten times at one-minute intervals.  This was done to measure both hydrotaxis and phototaxis.  The results were posted on the chalkboard so that everybody could see the data from each other’s group.  The data from all six groups were broken down into columns for each of the conditions measured, wet/dry, and dark/light.  The results from all the groups were averaged and entered into the chi-square test statistical formula.  This formula was used in order to determine whether there was a significant response from the isopods according to their environmental conditions.

 

RESULTS

Six groups with ten A.vulgare apiece participated in the phototaxis and hydrotaxis experiment. In the phototaxis experiment, a class average of 42.6 out of 60 A. vulgare were found on the dark side of the apparatus. In the hydrotaxis experiment, 44.5 out of 60 A.vulgare were found on the wet side of the apparatus. Using the chi-square statistical formula, the critical values for the phototaxis results was 14.02. The critical values for the hydrotaxis results were 10.5. The degrees of freedom ( α = 0.05) were one with the critical value of X² being 3.8. The results in both experiments demonstrated a significant response to the stimulus, thereby rejecting Ho hypothesis. Table 1 shows the class data used for the experiment. Table 2 shows the results of the statistical analysis, with figure 3 graphing the results.

DISCUSSION

Our hypothesis before the experiment stated that the isopods used in our experiment would demonstrate positive hydrotaxis and negative phototaxis. I had based my presumptions on the environments where A.vulgare is normally found. They are normally found under rocks and logs where the conditions are moist and wet. I also knew that isopods use gills to respirate, indicating that they would need to live in a wet environment. The results of the experiment indicated that more isopods migrated towards the wet and dark parts of the Petri dishes. The calculated value for each of the experiments was higher than the critical value, indicating that the results were significant. This meant that we should reject the null hypothesis and accept the alternative hypothesis.

A similar experiment conducted by Waloff in 1941 measured the effects of humidity on Porcellio scaber, Dendrocoleum lacteum and Armadillidium vulgare . The experimentors used a rectangular dish with a plate of glass covering the dish. A sulfuric acid of a specific gravity was placed underneath the zinc floor on either side of the dish to raise the humidity inside the container. The experiment demonstrated that as the humidity increased, the isopods gradually stopped moving and stayed where it was the most humid. Waloff also came to the same conclusion while using Armadillidium vulgare and Oniscus asellus. All these isopods lived in similar habitats, around rotting wood, decaying matter in dark, moist conditions. They also demonstrated positive hydro taxis and negative photo taxis.

Our experiment was a lot simpler than the one done by Waloff. However, if more time was available we could have monitored the diet and age of isopods in order to determine whether these factors could have had an impact in the experiment. We could have also varied the distance of the light to the Petri dishes in order to expose the isopods to radiant heat.

©2010 Augustine A. Zavala