Isopod Behavior: Omnivorous Scavengers' Dietary Preferences

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Isopod Behavior: Omnivorous Scavengers Dietary Preferences (Meat versus Vegetation)

Introduction:

Isopods are known to be omnivorous scavengers and belong to the class Crustacea. They have an exoskeleton that is composed of calcium carbonate and have jointed appendages. They breathe through their pleopods on their underside and are limited to places with high humidity, such as under rocks or logs. (1)Terrestrial isopods have been known to be useful in testing the relative toxicities of chemicals present in an area and can also approximate the amount of contamination. (Drobne, 1997) They can accumulate heavy metals in their body tissues without suffering toxic effects. (Miyazaki, et al. 2014) Their diets consist of dead plants or dead animals and they take part in the process of decomposition and are beneficial and essential component of a healthy ecosystem. (Wagler, 2013) Isopods eat mostly vegetable or animal matter and their food is usually already dead and decaying and they feed where waste accumulates and moisture is present.(2) They can get eaten and move up the food chain and their feces can contain dissolved nutrients and digestive enzymes that can get routed through the food web. (Wieser, 1978)

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The experiment we designed, test the isopods preference to either vegetation or meat. They are omnivorous so we were interested to see which they preferred more. We plan to set up an arena that tests their preference of meat or salad leaves under moist sponges. The controls are wet and dry sponges. Their preference of moist to dry was proven in Lab 5 and most isopods prefer humid and moist environments since they dry out easily. (Sfenthourakis, Skouras, & Anastasiou, 2012) My hypothesis is that the isopods have a stronger preference for the vegetation or lettuce because it is more like their usual diet. Raw meat hasn’t decayed yet and under the conditions we will be experimenting in it may not be the most preferred food. Isopods are used to plants and vegetation and the raw meat may not attract them as much as the salad leaves. Lettuce has been preferred in many other experiments over peas and water weeds so I believe that that will be the case is this experiment as well. (Hassall, Zimmer, & Loureiro, 2005) Lettuce is more readily consumed by isopods according to an article that studied food preference of terrestrial isopods.(Hassall et al., 2005)

Materials and Methods:

In this experiment an arena was set up in an eight inch culture dish with sandpaper at the bottom. Above the arena was an 85W flood light on a ring stand to serve as a light source. (see Figure 1) The light was kept 20 cm above the arena. (Lab Manual, Miyazaki, et al. 2014) Then 4 sponges were placed at equal distance from each other in the arena, three were moistened with 6 ml of RO (reverse osmosis) water and one was dry. We also measured the sponges using Vernier calipers and kept them standing vertical in the arena, since the heights were mostly constant among the sponges. (see Table 2) The variables we tested included the meat versus vegetation diet. To experiment this we cut up raw chicken meat and salad leaves and placed them under one sponge each. After each trial we replaced the meat and the leaves and also changed the sponges. We used 4.5 grams of meat and 1 gram of salad leaves each time. The controls of this experiment are the dry and moist sponge with no food underneath. After Lab 5, which was done during the class before this experiment we were able to conclude that the isopods prefer a moist environment over a dry one. The arena was set up similarly, except there was no food, there were only 2 moist sponges and 2 dry sponges. The dry sponge is the negative control while the moist sponge is the positive control. Before we added the isopods to the arena we measured the temperature, in Celsius, in the center of the arena. We tried to maintain room temperature throughout the trials to allow for a safe environment for the isopods. The isopods used in the experiment included two isopod species, Porcellio scaber (sowbug) and Armadillidium vulgare (pillbug). We conducted 5 trials and used 20 isopods each time and we used different isopods after each trial. So in total we used 100 isopods to conduct the experiment. Each trial lasted 5 minutes. We measured the temperature before placing the isopods in the arena and we took measurements after 2.5 minutes and at 5 minutes, during each trial. (See Table 1) We randomly chose the isopods for each trial and placed them in the center of the arena to allow for more accurate results. After 5 minutes we counted the number of isopods under each sponge by lifting the sponges. After counting them we placed them in a container and obtained new isopods for the next trial. After 5 trials we conducted a Chi-squared test and used it at a 5% level of significance. We prepared an observed and expected table for each trial and summed up the amount of isopods under each sponge to get a Chi-squared value. During our third trial we had 2 dead isopods in the arena, but when we did the calculations we made the expected for the other column zero, so it cancelled out in the equation and did not affect our data analysis greatly.

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Results:

We conducted a Chi Square Test to analyze the data we collected Table 3 shows the values after we summed them up from each trial. Figure 3 shows the number of isopods under each sponge in a bar graph. The chi square equation is Æ© (observed-expected) 2/ expected. The equation yielded a chi square value of 30.24. The null hypothesis was that the isopods have no preference and after five trials the total expected amounts of isopods under each sponge would be 25. The p value at the 5% significance level was P< .001. The degrees of freedom were n-1 so it was 3 in our experiment. Our controls were based on the data from Lab 5 which is shown in Table 4. The Chi square value for the Class came out to be 31.25 and for our group it was 18.8. The degree of freedom was 2. Figure 4 shows the arena as it was set up in Lab 5. The P value for both the class and the groups was P < .001 at the 5 % significance level. For the experiment we conducted in Lab 6 we observed the majority of the isopods were under the salad leaves and for Lab 5 most of them were under the moist sponges. The amount of isopods varied from the expected values, especially for the dry sponges for both Lab 5 and 6.

Number of Isopods

Sponge 1 (dry)

Sponge 2 (Salad Leaves)

Sponge 3 (Moist)

Sponge 4 (Meat)

Other

(dead)

Total

Observed

5

43

29

21

2

100

Expected

25

25

25

25

0

100

Sponge 1 (moist)

Sponge 2(dry)

Sponge 3 (moist)

Sponge 4 (dry)

Total moist

Total Dry

Observed(Class)

38

6

27

9

65

15

Expected (Class)

20

20

20

20

40

40

Observed (Group)

7

0

12

1

19

1

Expected (Group)

5

5

5

5

10

10

Discussion:

The experiment yielded a higher preference to the salad leaves over the meat, and there were a higher number of isopods under the sponge with the salad leaves after each trial. The two controls, the dry and moist sponge without the food yielded what we predicted. After conducting the chi-square test for the summed up values from the five trials, we got a chi-squared value of 30.24. At the 5% significance level this value can be used to reject the null hypothesis that the isopods do not have a preference. We can accept the alternative that they do have a preference. The p value is less than .001 which also allows us to reject the null hypothesis and our experiment was significant. The p value is also less than the significance level .05. For lab 5 the chi-squared value for the class and our group also leads to rejecting the null hypothesis that the isopods have no preference. After this analysis, I can conclude that the experiment supported my hypothesis that the isopods had a preference to the lettuce and referring back to Table 3, we can see that that was the case. For lab 5 the hypothesis is also supported that the isopods prefer the wet sponges over the dry ones and we can refer to Table 4 for the data. The positive control in Lab 6 was the wet sponge, because we knew from Lab 5 that the isopods would prefer that over the negative control which was the dry sponge, which we knew they would avoid. Some reasons the isopods preferred the salad leaves could be that they are used to plant matter and dead animal decay. Although they are omnivorous, I think the heat from the light source could’ve been a factor causing the isopods to go to the meat less. The lettuce was probably cooler and we noticed that they were consuming the meat more than the leaves but they still were under the salad leaves more. Lettuce is more readily consumed and promotes more growth in the isopods. (Calh, ocirc, a, Soares, & Mann, 2006) Our controls also could have caused the isopods to just go under the wet sponge for the moist environment and ignore the variables we were testing. Some things that could have been improved are using less meat so the isopods aren’t so overwhelmed and less leaves to see more accurate results of which they prefer. The fact that they are omnivorous would lead us to think that they would be equally distributed under both but they weren’t and they preferred the salad leaves over the meat. Also after each trial when we lifted the sponges the isopods would run in another direction so that could have caused an error in the counting. We should’ve just counted them from outside the glass to get more accurate data.

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Generally, terrestrial isopods feed on dead organic matter and prefer dead vegetation over any other food. (Paris, 1963) So this experiment led me to support my hypothesis and conclude that isopods prefer vegetation over meat. Since I can conclude that now, I would further investigate the types of vegetation they prefer, and many articles I read say they prefer decaying plants over fresh plants. (Paris, 1963)

Works Cited:

  1. http://insected.arizona.edu/isoinfo.htm
  2. http://wwx.inhs.illinois.edu/resources/inhsreports/sum-2000/isopods/
  3. Calh, ocirc, a, Carla Filipa, Soares, Amadeu M V M, & Mann, Reinier M. (2006). Cadmium assimilation in the terrestrial isopod, Porcellio dilatatus--is trophic transfer important? (Vol. 371).

4. Drobne, D. (1997). Terrestrial isopods -- a good choice for toxicity testing of pollutants in the terrestrial environment. Environmental Toxicology & Chemistry, 16(6), 1159.

5. Hassall, Mark, Zimmer, Martin, & Loureiro, Susana. (2005). Questions and possible new directions for research into the biology of terrestrial isopods. European Journal of Soil Biology, 41(3–4), 57-61. doi: http://dx.doi.org/10.1016/j.ejsobi.2005.09.002

6. Miyazaki, Joan M., O’neal, Marvin H., Spikes, Deborah A. (2014) Fundamentals of Scientific Inquiry in the Biological Sciences I. (7th Edition ) Plymouth: Hayden-McNeil Publishing

7. Paris, Oscar H. (1963). The Ecology of Armadillidium vulgare (Isopoda: Oniscoidea) in California Grassland: Food, Enemies, and Weather. Ecological Monographs, 33(1), 1-22. doi: 10.2307/1948475

8. Sfenthourakis, Spyros, Skouras, Dimitris, & Anastasiou, Yiannis. (2012). A comparison of terrestrial isopod communities among different habitat types on Mt. Chelmos (Peloponnisos, Greece). Journal of Biological Research, 18, 198-204.

9. Wagler, Ron rrwagler utep edu. (2013). THE WONDERS OF TERRESTRIAL ISOPODS. Science Scope, 37(2), 59-67.

10. Wieser, Wolfgang. (1978). Consumer strategies of terrestrial gastropods and isopods. Oecologia, 36(2), 191-201. doi: 10.1007/BF00349808