To conduct this experiment an invertebrate crustacean called Isopods were used. Isopods can live in the deep sea or ground habitats. The ones used for this study were terrestrial. Isopods have gills that need to be kept moist, which is why they thrive in moist environments.


Experimental Apparatus

Carolina Supply Company supplied the isopods used in this study. Before designing the experiment, they were kept in containers in the lab. One 12×7 uncovered plastic container was used to conduct the experiment. In the container was a small pile of dry soil on one side and a small pile of wet soil on the other. Enough water was used to moisten the soil, but not enough to run past the pile. There was a space with no soil between them. The walls were tall enough so the isopods could not climb over them.


Experimental Design

This experiment had a goal of isopod preference, either the wet or the dry soil. The subjects studied were five different groups with five different isopods in each group. The first group of five isopods were clumped together under a small cup in the middle of the container and then released all at the same time. They were observed for 3 minutes using a stopwatch. After the 3 minutes, the number of isopods in both types of soil was determined. If isopods were not directly in the soil after the 3 minutes it was determined by eye which side they were closest to. This 3 minute interval test was done five different times using all different isopods. Something that I observed conducting the experiment was that isopods would go to either side after they were released from the cup, and they would not migrate from one type of soil to the other.


Statistical Analysis

The Mann-Whitney U test was used to test which type of soil isopods prefer and if my prediction was correct. This was the best test to use because we were testing the significance between two sets (wet and dry soil) of unpaired data.




Trial Number Isopods In Wet Isopods In Dry Not In Either
1 2 3 0
2 2 1 2
3 4 0 1
4 2 3 0
5 5 0 0

Table 1.

Averages of the 5 trials were taken from both wet and dry soil. Based on results from figure 1, there was an average of 3 isopods in the wet and 1.4 in the dry after the five trials.

Screen Shot 2018-03-04 at 1.40.12 PM.png

Figure 1. Mean Number Of Isopods In Wet And Dry Soil

As part of the Mann-Whitney U test, each value was ranked shown in Table 2.


Dry Soil Rank Wet Soil Rank
0 1.5 2 5
0 1.5 2 5
1 3 2 5
3 7.5 4 9
3 7.5 5 10

Table 2. Ranked Isopod Data

The the sum of the ranks for each group were then calculated and the results show: Nwet = ∑ 5+ 5 + 9 + 10 =34 and Ndry = ∑1.5 + 1.5 + 3 + 7.5 + 7.5 =21. The larger number (34) is used to calculate the test statistic. The calculated test statistic was 19, and the critical value for a data set with 5 and 5 value is 21. If there was significance, the critical value is less then the test statistic. This is not what we saw. The results showed that the test statistic was 19 and the critical statistic was 21.

The P value for this data set was (p<0.05), which means there is no statistical significance and we can accept the null hypothesis that isopods do not have a preference between wet and dry soil.

If we wanted to test this prediction again and potentially change our significance, we could try using a larger sample size and change the experimental design. Many factors can contribute to the results we acquired, and there is always errors that could of occurred and room for improvement with our experimental design.



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