Isopods: a Moisture Preference

Abstract:

Porcellio scaber, the sowbug were observed in a plastic container with two mounds of dirt at either end. One mound was moistened with water while the other was left dry. In five three minute trials the number of sowbugs in either environment or in between the two mounds were recorded. The averages were calculated and a one-way ANOVA single factor analysis was done. The data was significant at p=0.03704. There was a difference in the number of sowbugs in each environment. A comparison of the means indicated that the sowbugs were more often found in the moist environment.

Introduction:

The sowbug is small, dark colored terrestrial crustacean that is related to the roly-poly, of the species Armadillidium. Although the sowbug does not roll up as a defense mechanism against attack like its relative isopods, it has specialized glands to deter predators. These isopods typically feast upon decaying plants or animals and are often found hiding under rocks and rotting woods where it is moist. They have pseudotracheae which must remain moist and therefor isopods are more frequently found in wet environments. Therefor it was predicted that if isopods prefer moist environments then there would be more sow bugs found in the wet mound than the dry mound at the end of each three minute trial. Twenty-five sowbugs placed randomly into groups of five were placed into a container and the number of sowbugs in each environment was recorded after three minutes.

Methods:

Sowbug collection: Twenty-five sowbugs of about the same relative sizes were collected  from #KSCAnimBehav provided by Dr. Karen Cangialosi at Keene State College and placed into a tub for holding.

Environment Preparation: A shallow plastic container of 6in X 4in was obtained and two mounds of the same size of the mixed dirt substrate were placed at either end. One of the mounds was moistened with water from a spray bottle.

Trials: Five of the sowbugs from holding were randomly selected and placed into a beaker upside in the plastic container. The beaker was removed and the sowbugs were allowed to roam for three minutes. At three minutes the number of sowbugs in the wet mound, dry mound, or between the mounds was recorded. These five sowbugs were placed into a different holding container to avoid reuse in subsequent trials. This was repeated four more times for a total of five trials that each had new randomly selected sowbugs for observation.

Data Analysis: Averages for each environment were calculated. A Kruskal-Wallis Test was done on the data to determine significance.

Results:

There was a greater average number of isopods that were found in the wet environment as compared to the dry and the between averages.

Figure 1: Mean number of isopods found at each location with standard errors. 

Standard deviations were larger in the between and dry groups suggesting a greater variance for these environments compared to the smaller variation that was calculated for the wet environment. The wet environment had the greatest average of sow bugs found during this experiment. The Kruskal-Wallis test found H=-44.47 which is not significant at p=0.009.

Discussion:

The sowbug is one of few terrestrial crustaceans that must keep their respiratory surfaces moist. It was predicted that more isopods would be found in a moist environment. There were more isopods found in the wet environment in the five trials than in either the dry or between the two mounds. However, the results were not significant at p=0.009 indicating that there is  no moisture preference. In Figure 1, the wet environment has the highest mean and smallest variance whereas the were few to none found in between. The dry environment was frequented by only by 28% of the sowbugs whereas the wet environment had 60% of the sowbugs. It is possible that more scans were needed with more sow bugs to show a preference.

Appendix:

Trial (3min intervals) Number in Wet Number in Dry Between Environments
1 2 3 0
2 2 1 2
3 4 0 1
4 2 3 0
5 5 0 0

Table 1: Number of isopods in the different environments for each of the five 3-minute trials. 


Environment Mean
wet 3
dry 1.4
between 0.6

Table 2: Average number of isopods in the different environments. 


 

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