AN ORIENTATIONAL RESPONSE TO WEAK GAMMA RADIATION

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois

1. The common planarian worm, Dugesia dorotocephala, displays a significant orientational response to increase in Cs¹³⁷ gamma radiation when the increase is no greater than 6 times background.

2. The worms are able to distinguish the direction of the weak gamma source, turning away from it, whether it is presented on the right or left side. The response sign is, therefore, the same as that of the response of these negatively phototactic worms to visible light.

3. There is a clear compass-directional relationship of the responsiveness to the experimental gamma radiation. A conspicuous negative response is present when the worms are traveling northward or southward in the earth's field with the gamma change in an east-west axis. No statistically significant mean turning response to the gamma radiation is found when the worms are traveling eastward or westward in the earth's field with the gamma change in a north-south axis.

4. The previously observed annual fluctuation in the character of the monthly orientational rhythm of north-directed worms has been confirmed in an additional year of study. During colder months, the rhythm is monthly; during warmer months it is semi-monthly.

5. There is a semi-monthly fluctuation in the response of Dugesia to weak gamma radiation during mid-morning hours, the worms turning away from the source for four days prior to new and full moon, and toward it for two days following new and full moon. The stronger the field strength, up to 9 times background, the larger the amplitude of the rhythm.

6. There is a direct relationship between intensities of gamma radiation between that of background and 9 times background, and the strength of the negative response of the worms.

7. Evidence suggests that the negative response of Dugesia to a gamma source may be modified by experimental alteration of the natural ambient electrostatic field.

8. Some possible biological significances of this remarkable responsiveness to gamma radiation, and its particular properties, are discussed briefly.