Temperature Acclimation in the Terrestrial Isopod Porcellio Scaber D. F. Blake Hopkins Marine Station 17511 June 7, 1973 Introduction Acclimation' with respect to temperature is a widespread, species-specific phenomenon, and has been studies extensively by many previous researchers. (Edney, 1964, Edney, 1961, Prosser, 1961). In a study of Porcellio Laevus by Edney (1964), no sig- nificant acclimation was found with respect to respiration measurements. Significant results were obtained for P. Laevus when lethal temperature measurements were carried out, however, and the differences were thought to be attributable to temper- ature acclimation. In the isopod Armadillidium Vulgare, presumably a more terrestrial species, acclimation was demon- strated in both metabolic rate and L.T. 50 (lethal temperature at which 50% die) determinations. A good synopsis of the literature may be found in Prosser (1961), as well as theories concerning the biological bases for such physiological adapt- ations. In my brief study, no significant metabolic rate differ- ences were obtained between P. Scaber populations kept at five degrees C, Fifteen degrees C, and ambient air temperatures for a period of two weeks. No significant differences were found for L. T. 50 measurements either, but due to small sample sizes and a stringent assumption of normality for the analysis used, conclusions cannot be made with any certainty. 1. According to Edney (1964), Acclimation is defined as any compensatory change in response to temperature resulting fron events in an individual's lifetime. Methods For the habitat study, relative humidity was measured with a portable Honeywell relative humidity indicator. The probe was placed under the iceplant, where the population of isopods was found. Humidity measurements were recorded hourly for the two day study period. This data was used to ascertain a mean humidity value to be used for the lab populations. Acclimation studies were carried out on animals subjected to a constant temperature of either 5, 15, or 25 degrees C. for a period of two weeks. Edney (1964) indicated that for the genus Porcellio this length of time was sufficient for total acclimation to occur. The animals were placed in small glass habitats, one habitat per temperature and 100 animals per habitat. Constant high humidity was ensured by placing water resevoirs in the bottom of each enclosure. Other methods used in the experimental technique and analysis will be discussed in the appropriate sections. Part I: Field Studies In order to determine the habitat of the P. Scaber populat- ion I studied, hourly temperature and humidity measurements were taken in the field for a period of two days. The results are shown in fig. 1. "Within habitat" temperatures were taken with a thermistor probe implanted where the aggregation of isopods was found. "Above habitat" temperatures were taken ten cent- imeters above the location of the population. The mean "in habitat" relative humidity was 93% with a range of 87% to 96%. A study of the size differences between males and females indicated that a large dimorphism was present. From these results (fig. 2) I decided to study males only, to exclude respiration differences brought about by sex alone. Part II: L.T. 50 Measurements The apparatus used consisted of a constant temperature bath, accurate to within .2 degrees centigrade in which a test chamber containing the isopods was immersed. A moist- ened piece of filter paper placed in the chamber ensured a high humidity during the tests. Each test was carried out with ten randomly selected animals of one acclimation group, for a period of one hour. For each group, three temperatures were found which produced mortalities between 10% and 90%. The results are plotted on probit paper with a log temperature scale. (fig. 3). The control group was found to have signif- icantly higher L.T. 50 levels than the other two groups, but S FIG. 1 24 22 20 A HABITAT TEMP. O TEMP. 10cm. ABOVE 16 14 - 12: 10 —0— W 4 — 2- 4 8 12 16 20 24 4 8 12 16 20 24 time in hours 45 40 30. 20 15 10- FIG. 2 SIZE DISTRIBUTION male A temale 1 12 8 9 10 11 LENGTH (mm.) Sex ratio: N= 269, 119 male 150 female 5=.444 95% conf. limits are .37 and .61 this cannot be substantiated due to possible non-normalities present in the data. Part III: Respiration Measurements Respiration measurements were carried out using a Warburg constant volume respirometer. Each run consisted of hourly measurements of respiration for a six hour period, and a mean hourly rate was calculated for each of ten animals. Regression lines were fitted to the data and the results are shown in figs. 4,5, and 6. 910 values for each acclimation group were not computed because the data indicated that this varied consider- ably with the weight of the animal. To compare the acclimation groups, a composite of figs. 4, 5, and 6 was made for a standard animal of 30 mg. weight. This data is presented in fig. 7. Differences between the five degree and fifteen degree groups were not significant, but differences between the control group and the other two groups were significant at the .05 level. This difference can be attrib- uted to several confounding factors, none of which involve acclimation. Firstly, mean environmental temperature of the control group's habitat was 10.2 degrees centigrade, and an elevation of this curve with respect to the 15 degree group is to be expected. Secondly, behavioral differences exist which were not controlled for between the control population and the laboratory population. 2 FIG. 3 L.D. 50 80 60 50 — — — — — — — — — — — — — — 4 — — 40 30 20 2 10 A CONTROL 15° GROUP E 5° GROUP 30 31 32 33 34 35 36 31 38 — 39 40 temp. 0 8 .7 6 5 4 0 09 .08 07 .06 05 FIG. 4 control acc. group Respiration curve 2 2 — 20 30 40 50 60 log bodywt. (Me.) 25 1 —.4 0 — .09 .08 07 06 05 FIG 5 5 acc. group Respiration curve 150 20 30 40 50 60 log bodywt. (Me.) — o 1. 8 09 .08 .07 06 05 FIG.6 5 acc. group Respiration curve 50 20 30 40 50 60 log bodywt. (Me.) — 5 3 .2 1 A FIG. 7 0, consumption for 30 mg. avg. animal 8 ter 15 A CONTROL Q 15° GROUP E 5° GROUP 25 4 Part IV: Discussion Many error terms were present in the experimental method which could have been eliminated. Larger vessels, perhaps of aquarium size, could have been used for the lab populations to ensure that crowding did not cause behavioral modifications. The control population should have been enclosed in a vessel identical to those used for the lab population. More data should have been collected for the L.D. 50 calculations. References Edney, E.B., Oct. 1964, "Acclimation to Temperature in Terrestrial Isopods," Phys. Zool. vol. 37, no. 4. pp.364-394 Physiology, W.B. Comparative Animal Prosser, C.L., 1961, Saunders C. (1961). Acknowledgements would like to thank Dr. Frederick Fuhrman for his help and also his forgiveness after I broke two 820.00 micro-syringes.