Moisture Preferences in Isopods

The Preference of Isopods to Non-Saturated, Partially Saturated,
and Fully Saturated Moisture Levels in Sand

Methods and Materials

Habitat Establishment

An artificial habitat was established in a 10.5” x 5.5” clear Tupperware container containing large granule beach sand. The substrate was sectioned into 3.5” x 5.5” areas using plastic dividers between sectors. The plastic dividers touched the bottom of the container to approximately ¼” under the substrate surface. This method of using plastic dividers was used to prevent transfer of water across the three regions.

Experimental Design

The three sectioned areas were each treated with different levels of increasing saturation. The first section did not have any water added to the substrate, the second section was partially saturated with 150 mL of water, and the last section was fully saturated with 300 mL of water.

Figure 1 The habitat set up and three treatments are shown. From left to right saturation levels; 0 mL, 150 mL, 300 mL, of water.

A total of six individuals were observed over 60 second time intervals. All individuals were initially placed in the middle area with the 150 mL saturated substrate. A acclimation period of 30 seconds was provided for the isopods to have a chance to move around before data was recorded.

Collection of Data

The six individuals were separated into two observation groups. One individual in each group was assigned to an observer to record movement data. The observer recorded how much time each individual spend on each substrate over the 60 second interval. The data set for all individuals were compiled and used in a statistical analysis to determine moisture preference. 

Statistical Analysis

The behavioral response of the isopods’ preference to moisture was tested using the non-parametric Kruskal-Wallis statistical test. The time spend on different substrates by individuals were ranked. To test the hypothesis that isopods prefer moist environments the longest time spend on a substrate was ranked as number one and decreased as time on substrate decreased. The calculated h-value from the test was compared to find significance level.


Observed data from each individual was arranged and ranked as shown in Table 1. The isopods overall spent more time on the substrate saturated with 300 mL of water. The R-values calculated for the Kraskul-Wallis test demonstrates the findings that isopods spent more time on the 300 mL saturated substrate. The R-values were calculated in a way that the higher the rank, the more time, in seconds, the isopods spent on the substrate. A high rank meant that the isopod spent the most time on that level of saturation. Table 1 shows that the R value of the substrate saturated with 300 mL had the highest ranking. This supports the hypothesis that isopods prefer moist environments.

Substrate Saturation Levels and Ranks
0 mL Rank 150 mL Rank 300 mL Rank

Seconds Individual Spent on Substrate

42 3 8 12.5 10 11
20 8 8 12.5 32 5
4 14 2 15 54 2
0 17 26 6.5 34 4
0 17 0 17 60 1
17 9.5 17 9.5 26 6.5
R1 = 68.5 R2 = 73 R3 = 29.5

Table 1 The ranking of time for individuals are established on the basis that the longer an individual is on a substrate the higher they are rank. The data points for each individual’s time on a different level of saturated substrate is listed with their ranks. The R- value relates to the ranking of time spend on one saturation level by the six individuals. The lower the value of R the longer the individuals stayed on that saturation level.

The substrate saturated with 0 mL and 150 mL of water were more closely ranked. However, the substrate with no water was ranked higher than the partially saturated substrate. A H-test was used to calculate the level of significance for these findings. The calculated h-value was 6.75. This value was compared to non-parametric ANOVA for samples with 6 individuals. The calculated value supported that the data was significant (p=0.05, when h>5.801, our h=6.75). These results show that there is a significant difference from the null hypothesis and there is a preference in isopods to different moisture levels.


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